Jerry W. Stuth
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Foraging Tactics of Animals-the Plant-Animal Interface
- The Landscape Level of Diet Selection
- Plant Community and Patch Level of Diet Selection
The Feeding Station Level of Diet Selection
Dietary Plasticity and its Implications for Interspecific Competition
List of Figures
Optimum livestock production on grazing lands is in part a function of the ability of the animal species employed to harvest nutrients in an effective and efficient manner. An understanding of the temporal and spatial dynamics of the grazing process in a complex environment is critical for optimizing livestock production.
Each pastoral situation offers a unique environment from which the animal must garner nutrients, maintain thermal balance and interact socially with other individuals of the herd both to sustain itself and the species. Each population employs an evolutionary strategy directed toward maintenance of fitness. Reported scientific studies indicate that both domestic livestock, and most economically important wild ungulates, forage optimally and are energy maximizers. That is, they maintain fitness by feeding optimally to consume the greatest amount of energy and/or other nutrients (Schoener 1969, 1971, 1983, Charnov 1976, Pyke et al. 1977, Krebs and Davies 1978, Belovsky 1978, 1981a, 1981b, 1984, 1986a, 1986b, Whitman 1980, Hixon 1982, Owen-Smith and Novellie 1982, Black and Kenney 1984, Kenney and Black 1984, Belovsky and Slade 1986, Horner and Staddon 1987). Thus, grazing managers must develop an understanding of the grazing patterns employed by the animals they are managing.
The strategic view involves understanding the genetic evolution and predisposition of a given species to forage. Mechanisms employed by the animal have their roots in the history of the evolution of the species, (Westoby 1974, Ellis et al. 1976, Rosenzweig 1981), and are largely inherent in the animal. The inherent nutritional aspects were discussed in detail in the previous chapter (see Chapter 2). The objective of this chapter is to provide an overview of the inherent behavioral aspects of grazing.
Perhaps the single most important aspect of understanding plant-animal interactions is developing an appreciation of the foraging process (Crawley 1983). When an animal grazes a plant, a hierarchy of instinctive responses and behavioral actions have been taken by him that leads to the point of prehension and consumption (McNaughton 1987, Senft et al. 1987, Senft 1989) (Fig. 3.1). Each landscape unit (pasture, paddock, block, allotment) is composed of a complex of different habitats or distinct groupings of plant species in communities (Fig. 3.2). Habitats are delimited by the type of plant species present, their spatial arrangement and structural configuration, e.g., a grassland community with scattered trees less that 1 m in height in contrast to a shrubland of dense shrubs over 3 m high with some grassland occupying the interspaces. Habitats can be further delineated into patches which contain more homogeneous grouping of species. In the example above, a patch is represented by the shrub aggregation or the grassland interspace.
When the animal has oriented itself in a habitat it must decide when to lower its head and establish a feeding station along its grazing path. Within the feeding station, the animal must then select from among the individual plant species those it will consume and beyond that which of the plant parts will be eaten. Therefore, the diet selection process has two major levels that must be clearly distinguished, spatial choice and species choice.
The Landscape Level of Diet Selection
Spatial choices position the animal in a landscape prior to selecting plant species or parts from among an aggregate of available plants. The landscape level of diet selection is characterized by those physiognomic and thermic features of a management unit which influence animal movement patterns. A given landscape unit (pasture) is characterized by boundaries, distribution of plant communities, degree of accessibility and distribution of water, thermal and mineral foci (Table 3.1).
The animal must first come to understand the nature of its landscape by locating the boundaries, routes of access and escape, plant communities and contacts, and the seasonality of desirable species. Therefore, the more experience the animal has with the variety of habitats and plant species available to it in a variety of years, the greater its ability to optimize grazing tactics to survive in a management environment (Smith 1984, Senft et al. 1987, Senft 1989). When first introduced into a pasture, animals seek the boundary of their environment (e.g., fence or home range), and once their experience base is sufficiently high, typically 24-72 hours for livestock, they begin the search for the source of the most vital consumption item, water. Free-standing water is the principle focus around which most of the larger ungulates such as cattle, buffalo and impala, orientate their foraging strategies. They seek the most energy efficient sources of forage as referenced to known water sources (Coleman et al. 1989). Large herbivores are "central place foragers", with the central or home place centered on water. The observed probability distribution of foraging around water (Senft et al. 1985, Smith 1988) resembles that expected for theoretical central place foragers where foraging activity is universally related to distance from a central point of habitation.
Optimum grazing area is defined by an approximate circle whose radius is generally not over 0.8 km from the water source. About 1.6 km is considered the maximum outer limit for a herd of cattle (Bos spp.) or flock of sheep (Ovis spp.) to balance their forage and water needs, (Valentine 1947). However, during drought, the effective grazing area is increased as forage supply diminishes (Squires 1982, Walker et al. 1987, Smith 1988). When calculating stocking rates for a given area, the manager must consider not only the spatial distribution of the animal population distribution in relation to water but also frequency of drought and reduce stocking rates accordingly (Fig. 3.3). Smith (1988) has defined the form of this relationship as an inverse log equation., while Andrew (1988) referred to it as a sigmoid logistic curve.
Rough terrain, such as gullies, steep slopes and/or rocky outcrops, restrict animal movements even when water sources are within otherwise acceptable distances. Heavy concentrations of shrubs have been found to create access problems to potentially grazable areas especially when the animal encounters high stem densities and overlapping canopies. Animals prefer to use established trails, roads, cut paths/openings and pipeline/utility right-a-ways rather than attempt to penetrate thick brushy areas or traverse difficult terrain. Animal movement in rough terrain is determined by the agility of the species in question. Goats (Capra spp.) and large ungulates inhabiting mountainous terrain have adapted foraging skills which minimize terrain impact on movement patterns.
Spatial-use patterns of livestock can be regulated by mixing experienced animals with inexperienced animals. This matriarchal system of experience training can be a vital means for reducing the learning curve of replacement animals in a herd so that if extended drought occurs experienced animals can show others those sites which improve their chances for survival. Large herd liquidations during a harsh drought have a significant long-term impact on the experience base or living memory within the herd. By contrast, because most domestic animals are creatures of habit they may establish landscape-use patterns that do not optimize animal distributions. Thus, the introduction of new animals may, in certain instances, enhance distribution patterns.
Smith (1988) proposed that ungulates have a hierarchy of physiological needs which exhibit thresholds for altering activities and subsequent movement within pastures (Figure 3.4 and Figure 3.5). This hierarchy determines the probability that a given site or patch will be frequented by grazing animals. The greatest need is that of water. If visual cues, (e.g., windmill, trees, etc.) are prominent near water sources, the strength of the water's attraction is increased. Distribution of thermal foci which allow animals to maintain homoiothermy in a landscape relative to water location, may interact with prevailing winds during the growing season to affect the amount of potential grazing pressure a site will receive. Small domestic stock, particularly goats and sheep, drift against winds resulting in noticeable disproportionate use of those portions of pastures where the prevailing winds enter the pasture (Smith 1988).
The primary orientation to water and thermal balance causes animals to forage away from these foci to meet their nutritional needs. Most ungulates first harvest food, then move either to loafing and bedding sites to ruminate and digest the food ingested in a previous grazing bout (meal), and/or to areas for predator avoidance. If the starting point for grazing is a water or thermal focus, the subsequent distance covered by the animal is determined in part by digestive capacity or rate of food passage through the animal and in part by the potential harvest rate of forage encountered, potential grazing velocity and level of satiety of the animal (Walker et al. 1989). Once satiated, the animal either returns to a thermal, water or strategic bedding site depending on thresholds of these various needs. The interaction of thermal regulation and digestive capacity is responsible for the noticeable "piospheres" or rings of utilization which diminish in area with distance from water sources.
Grazing time per day is a function of forage quality, thermal balance and short-term stability of forage supply. Animals reduce daily grazing time as digestibility of forage available declines and retention time of ingesta increases. When daytime temperatures are within the thermal neutral zone of cattle, most grazing, 90%, takes place during daylight hours (Table 3.2). During hot periods cattle reduce afternoon grazing and increase night-time grazing. Cattle have demonstrated little directional grazing after darkness, so as nighttime grazing increases it is in the neighborhood of termination points of grazing at dusk (Walker and Heitschmidt 1988). Evidence is mounting that cattle rely on vision to move about in their environment so as darkness sets in, they loose many of their visual cues so they do not venture far from nighttime bedding areas.
When winter temperatures are below the thermal neutral zone of cattle they limit evening grazing but increase afternoon daylight grazing substantially. Therefore, longer daytime grazing bouts with directed grazing occur during the winter.
When forage supply is restricted, the animal compensates by increasing grazing time. However, if in a severe caloric deficit, the animal tends to give up the search due to the high cost of travel relative to the energy garnered from edible forage located (Coleman et al. 1989).
Evidence suggests that the animal's selection of a given plant community is largely related to those attributes of a site which influence its ability to harvest nutrients. Table 3.3 provides a comprehensive summary of community attributes and the way they impact animal use of a site.
There have been several studies to isolate those community attributes which affect the selection of communities by a grazer. Senft et al. (1987) established that forage quantity and quality was closely related to the ratio of amount of time spent grazing in community relative to the area it occupied within the landscape. The abundance of seasonally preferred plant species have also been shown to influence the patterns of plant community use (Senft et al. 1985).
Preference for communities is usually measured either by determining the ratio of percent grazing time to percent of land area or percent of grazing capacity of given management unit or landscape of the animal. Implicit in this measurement is that as animals increase time in a site, the greater the quantity of nutrients harvested from the site. This assumption implies that communities which afford an animal species high harvest rates per unit of grazing time are preferred by that animal. Put in another way, plant community profitability can be valued by measuring the potential ingestive rates (g/min) of forage by the animals (Table 3.4). The greater the density of high quality food species, the slower the grazing velocity therefore the greater residence time and intake level attained relative to other communities available to the animal (Senft et al. 1987). If these communities lie between important water and thermal foci, site preference is magnified.
However, several studies have shown that grazing preference based on occupancy:area ratios can be misleading if assumed to reflect "food value" of a site (Butterfield and Stuth 1990). Figure 3.6 provides a conceptual view of the functional nature of landscape use categories when occupancy:area ratios are contrasted to utilization:herbage mass ratios for the same site. Site preference in this case results in four major preference categories:
(1) Grazing preferred.
(2) Grazing avoided.
(3) Terrain constrained or directed use.
(4) High impact grazing sites.
Grazing preferred sites are those sites with high occupancy:area ratios and high utilization:herbage mass ratios with the major bulk of the animal's forage derived from these sites. Grazing avoided areas contain low-value food or are inaccessible to the animal(s). Terrain constrained or directed use sites are unique in that these sites have high occupancy times yet little utilization relative to herbage mass in the pasture. Examples of preferred directed use sites include near-satiated grazing of these sites at access points to water sources, sites where normal animal movement causes herd concentration in pasture corners or against gullies, hills or roads and prevailing wind-directed grazing. Finally, there are those sites where limited occupancy relative to area in the pastures results in high utilization relative to herbage mass in the pasture high impact sites. Low potential sites occurring along directional grazing paths where animals are exhibiting high grazing velocities can result in limited occupancy time but high levels of use of available forage.
Generally, pastures in which grazing occupancy time and level of utilization are highly correlated, possess few terrain constrained or high impact sites. Pasture configurations which result in poor correlations of occupancy time and forage use offer opportunities to manipulate habitat to improve harvest efficiency. Animals apparently establish directed grazing paths which increases the probability of encountering more profitable sites. Highly profitable communities attract ungulates from neighboring foci. Memory of their level of profitability most likely establishes the direction from water, thermal or resting foci but directed grazing will occur between foci and preferred sites (Bailey et al. 1988). The duration of grazing in a community or patch relative to another site along a grazing path is largely determined by the relative differences in harvest rates. Low harvest rates (g/min) result in high grazing velocity and so short resident time in the community/patch. The rate of grazing velocity is dependent again on the interaction between level of satiety and distance from thermal/water foci.
Animals who have loafed and ruminated for extended periods leave foci at a higher velocity not slowing their travel pace until a highly profitable site is encountered (Smith 1988). As the gut is filled, the rate of ingestion slows. This behavior often leads to high residence time but low harvest rates due to increased selectivity. To understand this phenomena requires a greater understanding of feeding station behavior (Demment et al. 1981).
An animal's feeding station is established when it stops walking, lowers its head and bites a plant. At this point certain sensory cues have caused the animal to stop searching and to select a species or combination of species it perceives as profitable. The pattern of feeding stations is strongly related to the distribution and profitability of patches in a community, the size of the community and the geographical relationship of the community to the animals grazing path (Novellie 1978, Ruyle and Dwyer 1985).
Forage behavior at this level can be categorized as search time, time spent travelling between feeding stations, biting rates within feeding stations, and duration of biting while at a feeding station (Stuth and Searcy 1987). Recent studies in foraging behavior of cattle indicate that they have definite seasonal foraging strategies in response to changing plant phenologies and the availability of forage (Stuth et al. 1987). Cattle increase the amount of grazing time allocated to searching between feeding stations when forage conditions are more universally high across species and habitats (Fig. 3.7). The animals appear to select fewer plant species and focus their selection on plant species which offer the maximum amount of green forage per bite (bite size) within the primary food group. This in turn leads to an overall drop in bite rate which is a result of increased search time while actively grazing. If forage becomes limiting during these high quality periods, animals intensify searching to acquire an adequate daily intake until their preferred primary food group is depleted. However, as the season progresses and the amount of senescent material in the crown or canopy increases, cattle and sheep reduce search time between feeding stations and increase selection time at the feeding station. Each feeding station is more fully exploited during these times. In fact, when the animal stops to graze a feeding station, most of the available green forage is fully consumed before moving on to the next feeding station. During these periods, intraspecific (animal-animal) competition becomes most critical with respect to the nutritional well being of the individuals. Herding instinct causes the herd to fragment into smaller feeding groups and disperse over a wider area of the landscape when forage supply is low (Smith 1988). High grazing pressures during periods of high differential palatability between species available often cause nutritional problems for an individual. And as grazing pressure increases the number of unexploited feeding stations diminishes.
Searching between feeding stations occupies 20-30% of the grazing hour and appears to be an adjustment mechanism associated with forage quality. As stated previously, animals reduce overall grazing time as forage quality declines seasonally. However, less of this time is allocated to searching between feeding stations in a community or patch. The net result is reduced seasonal differences in actual grazing time at feeding stations.
Patchiness within communities has its greatest effect on distance between feeding stations (Fig. 3.8). Observed distances travelled between feeding stations can be up to 10 fold greater in distinct patchy communities as compared to communities with dense, continuous swards, 20-25 steps verses 2-3 steps between feeding stations, respectively.
Several studies have focused on the influence of plant communities on ingestion rates (Alden and Whittaker 1970, Chacon and Stobbs 1976, Arnold and Dudzinski 1978, Arnold 1981, Forbes et al. 1985). Most of these studies have analyzed foraging behavior under heavy grazing pressures and rapidly declining monospecific stands of forage. Basically, these studies have found bite size to diminish as herbage supply in a plant community increases while biting rate declines.
Once an animal establishes a grazing location, his experience with available forage is utilized in a plant species-to-species appraisal and selection process. This process is specific to the animal species (see Chapter 2). Herbivores, as noted earlier, exhibit an evolutionary adaption predisposition to feed on plant species from one or more of their primary food groups, grasses, forbs and browse (Provenza and Balph 1987 a,b). Therefore, the grazing value of a plant depends on the animal species in question (Demment and Van Soest 1981, Hanley 1982, Hanley and Hanley 1982, Owen-Smith and Cooper 1987).
At this point it is essential that the palatability of a plant and the preference for that plant be differentiated (Heady 1964). Palatability refers to those factors inherent to a plant species that elicit a selective response by the animal. Preference involves proportional choice of one plant species from among two or more species and is essentially behavioral. The preference status of a particular plant species is largely dependent upon its inherent abundance, its morpho/phenological characteristics, the array of species on offer and the species of animal in question. Preference constantly changes as abiotic factors (i.e., season and weather conditions) alter the nature of the plant community. Some species are selected only under specific conditions. Therefore, broad generalizations about species selection and preference should be tempered by the understanding that animal selectivity is a dynamic, situation-specific process. However, recent studies (Colebrook et al. 1987) have indicated that preference can be quantified for an animal species as well as selection order predicted based on the relative rank order of absolute preference values. Implicit in these findings is the concept that specialized or focused grazing on some plant species may relate largely to its relative preference ranking at the time of active growth.
Plants have been generally classified into five general selectivity categories (Table 3.5) which follow the functional categories outlined in Chapter 2. Plant species selected in greater quantities, as a percent of diet, than found in the landscape (percent composition), are referred as preferred or favored species (Fig 3.9). Such plant species do not generally dominate the diet unless they dominate the community. Instead preferred species enhance the diet nutritionally resulting in better than normal animal performance. These species have high handling time for the animal but high nutrient concentration and/or are low in floristic composition.
The more abundant species are generally consumed in proportion to their availability and are referred to as proportional or desirable species. When present in high percentages they dominate the diet and usually provide the basis for estimates of grazing capacity. These species are not generally as high in nutrients as the preferred species but afford the animal the opportunity to maximize instantaneous intake rates (gm/bite).
Species not readily consumed by animals generally make up a lesser percentage of the diet than the percentage available in the vegetation. Consumption of this undesirable, forced, rejected or avoided selection group is highly condition specific, i.e., incidental grazing when other preferred and proportional species are abundant, seasonal selection of specific plant parts (mast, pods, fruits, flowers, etc.) or major dietary components when the preferred and desirable species are limited. They allow the animal to survive in a subsistence situation. Incidental consumption is believed to be a response to animal sampling of the environment as conditions change.
Recent studies correlating animal selection ratios (%diet/%available) and a given plant species' inherent abundance in a sward have revealed four types of relationships (Fig 3.10). Particular plant species are preferred regardless of abundance and the presence of associated species; the preferred species are generally higher successional species. Secondly, there are those species which are consumed proportional to availability and consumption is highly correlated to their inherent abundance. A third group in Figure 3.10, variable, transcends all selection categories, their consumption of which changes from avoidance to preference as herbage mass declines. These plant species are referred to as variable or secondary preference species, and generally exhibit morphological constraints to consumption by animals. Finally, there is the last group or avoided species which is selected at levels below their availability. Selection ratios of avoided species are poorly correlated with their inherent abundance. Those species generally contain undesirable nutritional attributes (see functional group discussion in Chapter 2).
Nonconsumable species are generally not found in the diet of the animal except in extraordinary situations. Generally, only specific adverse conditions result in any consumption. Exceptions might include pods or fruiting bodies e.g. mesquite beans or prickly pear cactus fruit. These species generally affect the animal only indirectly by reducing the overall grazing capacity of the range but can have a positive effect on nutrition. This is particularly true of shrubs when cattle are the primary herbivore as shrubs create microclimates for certain species which are nutritionally richer than associated species or maintain green material longer into dry or cold periods of the year.
Finally, there are the detrimental or toxic species. When most of the favored species are reduced in the landscape, toxic species express themselves in the diet devastatingly. Cyclic poisonous plant problems in arid regions is a testament to this problem.
If we assume that most ungulates are energy maximizers and feed optimally, we can predict that plant species in the preferred primary food group of an animal which provide high instantaneous intake rates of nutrients without the negative effects of secondary compounds (e.g., phenolic acids) should receive the most selective pressure by the animal. In another words, plant species offering the highest bulk density of unmixed green foliage with the highest nutrient concentration and lowest content of secondary compounds has the greatest probability of being grazed. This conclusion must of course be understood and acted upon in terms of the inherent food group preference and nutritional requirements of the animal species and associated plants in the landscape. For instance, cattle have higher dry matter requirements, lower nutrient requirements, less precise prehensile organs and larger rumen volume/body volume ratios than goats (Demment and Van Soest 1981). Cattle's buccal/oral cavity is much larger leading them to form a larger bolus prior to swallowing. Therefore, grasses with their high canopy bulk density of green material are more profitable than a small forb with higher nutrient concentrations but smaller size. Goats on the other hand can "afford" to select smaller plants that are less profitable for cattle; to a goat a browse leaf presents a proportionally larger bite size than to a cow. Also, the ratio of nutrient concentration per bite to required nutrient concentration must be maintained at higher levels for a goat than a cow.
Work by Cooper and Owen-Smith (1986) on goats and several African ungulates indicated that plants bearing spines reduces the potential rate of harvest by animals. Shrubs from which large bites could be taken were preferred in this study if secondary compounds were not high.
It appears that ungulates focus their grazing activity on a few highly profitable species when overall forage quality of the landscape is high with the consequences that search time increases, biting rate declines and bite size increases (Coleman et al. 1989). When observing grazing strategies through time, one could hypothesize that animals would be attracted to plant communities during rapid growth periods based on the abundance of highly profitable species. As phenologies of plant communities become mixed, animals should reduce species selectivity and focus their attention on communities which offer the greatest harvest rates of green foliage regardless of species. Once herbage is dormant, the animal's only option is to graze on sites with more abundant plant material regardless of greenness.
Plant morphology also influences the probability of being grazed. If grasses elicit a selective response early in their growth cycle, selective pressures increase as relative abundance or phenologies change (Stuth et al. 1987). Therefore, communities with a high proportion of forage utilized in the early growing season, have a higher probability of being grazed when environmental conditions are conducive to plant growth.
What are the morphological attributes of primary food groups which influence the grazing decision? In grasses, it appears to be physical presentation of green leaf blade relative to its pattern of senescence and culm development (Fig. 3.11). Grasses with rapid culm development (determinant growth) and strong, midrib leaf structure are selected less frequently if allowed to develop long-standing, senesced leaf material. Degree of sheath development and angle of growth by tillers influences the height and position of leaf blade material relative to the soil surface so cattle have a much harder time selecting short or decumbant species than sheep.
Forbs are characterized by two temporal presentations, ephemeral-annual and perennial. Ephemeral-annual forbs grow rapidly and complete their life cycle quickly. Therefore, they present a unique problem for ungulates. They possess high value for short periods in the animal's annual production cycle. The concentration of nutrients in most forbs exceed the nutritional requirements of ungulates. So while they are a preferred group their distribution on the landscape and the standing crop available in various communities along with the bite size they afford affect animal foraging tactics from one landscape to another.
Perennial forbs tend to allocate more resources to structural components, thereby, creating greater differentials in quality between plant parts than is the case with annual forbs. Moreover, they do not generally accumulate previous years' growth as does shrubby browse. Because they are present through much of the grazing season, they are particularly vulnerable to over-use by forb-preferring ungulates. This over-use reduces the relative acceptability between plant parts causing the plant to become more attractive to the animal. That is, handling time is reduced and bite size/quality is increased.
Browse presents itself in many forms: deciduous or evergreen, spineless or spiney, single leaves or compound leaves, short or tall, single stemmed or multi-stemmed, etc. Selective pressures on this food group is again dependent on the associated animal species community. Browse-preferring ungulates (concentrate selectors and intermediate feeders, Van Soest 1982) have adapted prehensile and digestive organs to a level where height, spineyness and secondary compounds are the principle plant characteristics affecting selective pressure on browse species (Cooper and Owen-Smith 1986). Generally secondary compounds play a major role in suppressing selective pressures on evergreen browse species. Spineyness, leaf size and to a lesser degree, secondary compounds, are important morphological/physiological attributes of deciduous browse species which influence the selection response. Again, the relative importance depends on the attributes of the particular animal species as discussed in Chapter 2.
Whatever the situation, animals are continually making choices among plants at the feeding station level. Choice are influenced by plants in the animal's view, the animal's short-term memory of plants in previous feeding stations and the frequency with which positive reinforcement of that choice has been made while actively grazing. The kind of plant chosen is largely related to the kind of animal, the relative abundance of alternative food sources, and the complexity of the landscape relative to the water and thermal needs of the animal.
Most of the previous discussion focused on the plant/animal relationship with little regard for animal/animal relationships. There is an abundance of site specific information on dietary overlap between animal species. However, the intent of this
section is to focus on food group plasticity of the major groups of domestic grazing livestock.
Cattle have often been referred to as generalists, eating a wide variety of foods and plant parts. However, graze-out studies have indicated that they preferably select the grass food group unless the availability of grasses is so marginal as to severely limit daily intake requirements (Launchbaugh et al. 1990). Analysis has shown that cattle maintain a grass-dominated diet over a wide array of herbage standing crops, switching to browse only after severe restriction in dry matter intake (Fig. 3.12) or to forbs when temporal flushes of desirable species occur.
Goats on the other hand, show a high preference for browse regardless of availability and a negative to proportional response to grasses, i.e., they increase the amount of grass in their diets relative to its availability as the composition of browse and forbs decline (Fig 3.13). Like cattle, goats consume low quantities of forbs unless large flushes of highly desirable species emerge. Recent studies of Cashmere goats grazing cool-season, grass-legume pastures indicate that they consume mostly grasses with legumes comprising less than 10% of their diet. No alternative browse was available to the animals in this study.
Although sheep have a rumen:body volume ratio similar to cattle, their principal dietary preference is the forb food group and to a lesser degree, grasses (Hanley 1982, Demment 1982, Demment and Van Soest 1984). Browse is utilized more readily by sheep than cattle but generally does not comprise a major portion of their diet unless grass and forbs are in limited supply.
Increasing grazing pressure by one animal species can force another species into their less preferred food group resulting in reduced performance, decreased harvest efficiencies or both. Cattle, because of their high demand for grass, regulate the level of grasses consumed by the smaller goat or sheep (Rector and Huston 1982). If desirable browse species are maintained and stocking ratios are properly balanced, little competition occurs between cattle and goats resulting in an increased potential carrying capacity of a landscape (see Chapter 7). The same can be said for sheep and cattle or sheep and goats if an abundant and stable source of forbs are provided to the animals. However, since forbs comprise a low but constant portion of cattle and goats diets and are actively consumed by sheep, it would appear that the degree of competition for food between these species is determined by the availability and desirability of the forb component of a landscape. As forb supply declines under combination grazing by cattle, sheep and goats, the resulting impact depends on the time pastures are jointly shared by all species, the amount of alternative browse available and the level of utilization of the grasses.
The concepts covered in this chapter to this point, have focused on the grazing hierarchy as seen by the animal from the landscape level to the selection of individual plants. But the direct impact of grazing animals on physical properties of soils and growth of plants together with the indirect impacts on soil aggregate stability, plant food reserves, and effective precipitation, also combine to markedly affect not only plant competition and community composition, but also subsequent spatial configuration of forage resources. These small shifts and changes in plant community composition and the spatial relationships of these plants lead to long-term changes in the landscape. These changes in turn alter the grazing animal's grazing behavior, a classic feedback loop as explained by systems analysis developers. It must be noted that while the relationships discussed in this section are short-term in nature with only gradually shifting foraging tactics from one year to the next, a long-term reconfiguration of landscapes occurs which alters grazing behaviors over the same time frame.
Patch grazing (i.e., small areas of intense defoliation) is often the most obvious early sign of landscape reconfiguration. The rapid growth of new green foliage in a patch previously grazed clean of competing vegetation offers the animal a highly desirable food source which in turn facilitates redefoliation in a relatively large, distinct area. This kind of intense patch defoliation and redefoliation is in marked contrast to the more typical selective grazing of individual plants in a mixed plant community. Such frequent defoliation in turn alters the hydrologic condition of the patch/community, ultimately to the point where plant species composition shifts and forage production declines. Furthermore, repetitive defoliation results in the expansion of the size of the patch and in some cases leads to the loss of most plant material and consequent development of eroded areas.
Once soil loss accelerates to the point of erosion, a permanent reconfiguration of the grazing area occurs. If, despite these landscape and terrain reconfigurations, the essential physiological needs of the animal can in some measure be met, the outcome is typically a reduction in animal population levels. However, in many cases of landscape terrain degradation, the needs of the animal cannot be met resulting in population relocation or extinction.
Landscapes are not only altered by grazing animals but often by other agents including man. Such alterations quite commonly lead to change in animal use not only in terms of intensity of use but that of occupancy patterns. For example, logging of transitional areas between summer and winter range alter season of use by elk (Sheehy 1988). Such temporal shifts in landscapes typically
result in either intensification of interspecific competition for resources or greater niche separation depending on the behavioral flexibility of the animal species involved.
Landscape configurations are also altered by natural events. Annual variation in precipitation patterns regularly result in major changes in landscape configurations and subsequent alterations in animal grazing patterns. For example, sporadic rainfall events in regions marked by soils with varying moisture holding capacities or slope position, create areas of lush green growth adjacent to communities with senesced forage. Depending upon intensity and location of precipitation events, these highly contrasting landscapes can shift throughout the year. Drought can accentuate this dynamic pattern and causes animals to range further from normal grazing areas thereby exposing forage in distant sites to grazing pressures during periods stressful to the impacted plants. If drought persists, areas near water can be denuded resulting in an ever expanding ring of degraded forage resources around essential watering points.
In all of these various situations, hierarchical feedback shapes available plant communities and subsequent animal behavior. The processes are complex, and interdependent. If management chooses to alter vegetation or facilities, landscapes are inevitably altered so the animals must readjust their grazing tactics. Care and consideration must be given to ascertain potentially destabilizing consequences in forage composition and relationship before changes in habitat and grazing management programs are implemented.
The grazing animal possesses a unique prehensile morphology to gather and processes food in a digestive system adapted to the primary food groups ingested (see Chapter 2). The grazing process used to gather food can best be described as a hierarchical system of diet selection interacting with the animal's physiological needs (water, thermal, balance, food, etc.) resulting in a unique pattern of use across a given landscape. The configuration of forage resources, water locations, thermal foci and terrain constraints interact with animal's hierarchy of needs to determine the overall impact of animal populations. The reaction of forage relative to animal grazing pressure provides both a short-term feedback suggesting the need to alter grazing tactics, and a long-term feedback in terms of the successional trends of the plant community. This interactive hierarchical system of plant-animal-soil interactions reflects management inputs relative to manipulation of animal populations and the vegetation matrix (Fig. 3.14). As pointed out in this chapter, foraging behavior involves tactics animals employ within this manipulated hierarchical system. The reaction of plants to these grazing tactics as both individuals and in aggregate is the subject of the following two chapters.
Alden, W. G. and I. A. Whittaker. 1970. The determinants of herbage intake by grazing sheep: The interrelationship of factors influencing herbage intake and availability. Aust. J. Agr. Res. 21:755-766.
Andrew, M. H. 1988. Grazing impact in relation to livestock watering points. Trends in Ecol. Evol. 3:336-339.
Arnold, G. W. 1981. Grazing behavior, p. In: F.H.W. Morley (ed.), Grazing animals. Elsevier Scientific Pub. Co. Amsterdam, Oxford, New York.
Arnold, G. W. and M. L. Dudzinski. 1978. Ethology of free-ranging domestic animals. Elsevier Scientific Pub. Co. Amsterdam, Oxford, New York.
Bailey, D. W., L. R. Rittenhouse, R. H. Hart, and R. W. Richards. 1988. Cattle memory for food resource level and locations. Proc. West. Sec. Amer. Soc. Anim. Sci. 39:8-11.
Belovsky, G. E. 1978. Diet optimization in a generalist herbivore: the moose. Theor. Pop. Biol. 14:105-134.
Belovsky, G. E. 1981a. Food plant selection by a generalist herbivore: the moose. Ecology 62:1020-1030.
Belovsky, G. E. 1981b. Optimal activity times and habitat choice of moose. Oecologia (Berlin) 48:22-30.
Belovsky, G. E. 1984. Moose and snowshoe hare competition and a mechanistic explanation from foraging theory. Oecologia (Berlin) 61:150-159.
Belovsky, G. E. 1986a. Generalist herbivore foraging and its role in competitive interactions. Amer. Zool. 26:51-69.
Belovsky, G. E. 1986b. Optimal foraging and community structure: implications for a guild of generalist grassland herbivores. Oecologia (Berlin) 70:35-52.
Belovesky, G. E. and J. B. Slade. 1986. Time budgets of grassland herbivores: body size similarities. Oecologia (Berlin) 70:53-62.
Black, J. L. and P. A. Kenney. 1984. Factors affecting diet selection by sheep II. Height and density of pasture. Australian J. Agr. Res. 35:565- 578.
Butterfield, C. H. and J.W. Stuth. 1990. Landscape constraints on foraging area selection. J. Appl. Anim. Behav. Sci. (In press).
Chacon, E. A. and T. H. Stobbs. 1976. Influence of progressive defoliation of a grass sward on the eating behavior of cattle. Australian J. Agr. Res. 27:709-727.
Charnov, E. L. 1976. Optimal foraging: the marginal value theorem. Theor.
Pop. Biol. 9:129-136.
Colebrook, W. F., J. L. Black and P. A. Kenney. 1987. A study of factors influencing diet selection by sheep. p. 85-86. In: M. Rose (ed.), Herbivore nutrition research. Australian Soc. Anim. Prod., Brisbane.
Coleman, S. W., T. D. A. Forbes and J. W. Stuth. 1989. Measurements of the plant-animal interface in grazing research. p. 37-52. In: G.C. Martin (ed.), Grazing research methods. Amer. Soc. Agron., Anaheim, CA.
Cooper, S. M. and N. Owen-Smith. 1986. Effects of plant spinescense on large mammalian herbivores. Oecologia (Berlin) 68:446-455.
Crawley, M. J. 1983. Herbivory: the dynamics of animal-plant interactions. Blackwell Scientific Publications. Oxford.
Demment, M. W. 1982. The scaling of ruminoreticulum size with body weight in East African ungulates. Afr. J. Ecol. 20:43-47.
Demment, M. W. and P. J. Van Soest. 1981. Body size, digestive capacity, and feeding strategies of herbivores. Winrock International Livestock Research Pub. Morrilton, Ark.
Ellis, J. E., J. A. Wiens, C. F. Rodell, and J. C. Anway. 1976. A conceptual
model of diet selection as an ecosystem process. J. Theor. Biol. 60:93-
Forbes, T. D. A., E. M. Smith, R. B. Razor, C. J. Dougherty, V. G. Allen, L. L. Erlinger, J. E. Moore, and F. M. Rouquette, Jr. 1985. The plant-animal interface. p. 95-116. In: Watson, V. H. and C. M. Wells, Jr. (eds.). Simulation of forage and beef production in the Southern Region. USDA - Southern Cooperative Series. Bull. 308.
Hanley, T. A. 1982. The nutritional basis for food selection by ungulates. J. Range Manage. 35:146-151.
Hanley, T. A. and K. A. Hanley. 1982. Food resource partitioning by sympatric ungulates on Great Basin rangelands. J. Range Manage. 35:152- 158.
Heady, H. F. 1964. Palatability of herbage and animal preference. J. Range Manage. 17:76-82.
Hixon, M. A. 1982. Energy maximizers and time minimizers: theory and reality. Amer. Natur. 119:596-599.
Horner, J. M. and J. E. R. Staddon. 1987. Probalistic choice: A simple invariance. Behav. Proc. 15:59-92.
Kenney, P. A. and J. L. Black. 1984. Factors affecting diet selection by sheep. I. Potential intake rate and acceptability of feed. Australian J. Agr. Res. 35:551-563.
Krebs, J. R. and J. B. Davies. 1978. Behavioral ecology: an evolutionary approach. Sinauer Associates, Inc. Sunderland, Mass.
Launchbaugh, K. J. W. Stuth and J. W. Holloway. 1990. Influence of range site on diet selection and nutrient intake of cattle. J. Range Manage: (in press).
McNaughton, S. J. 1987. Adaptation of herbivores to seasonal changes in nutrient supply, p. 391-408. In: J. B. Hacker and J. H. Ternought (eds). The nutrition of herbivores. Academic Press. Sydney, Australia.
Novellie, P. A. 1978. Comparison of the foraging strategies of blesbok and springbok on Transvaal highveld. S. Afr. J. Wildl. Res. 8:137-144.
Owen-Smith, N. and S. Cooper. 1987. Palatability of woody plants to browsing ruminants in a South African savanna. Ecol. 68:319-331.
Owen-Smith, N. and P. Novellie. 1982. What should a clever ungulate eat? Amer. Natur. 119:151-178.
Provenza, F. D. and D. F. Balph. 1987a. Diet learning by domestic ruminants: theory evidence and practical implications. Appl. Anim. Behav. Sci. 18:211-232.
Provenza, F. D. and D. F. Balph. 1987b. The development of dietary choice in livestock on rangelands and its implications for management, p. 2356-2368. In: Proc. Forage Selection and Intake by Grazing Ruminants". 79th Ann. Meet. Amer. Soc. Animal. Sci., Logan, UT USA.
Pyke, G. H., H. R. Pulliam, E. C. Charnov. 1977. Optimal foraging: a selective review of theory and tests. Quart. Rev. Biol. 55:137-154.
Rector, B. S. and J. E. Houston. 1982. Composition of diets selected by livestock in combination. p. 320 In: Abst. Joint Meeting Amer. Soc. Anim. Sci., Guelph, Ontario, Canada.
Rosenzweig, M. L. 1981. A theory of habitat selection. Ecology 62:327-355.
Ruyle, G. B. and D. D. Dwyer. 1985. Feeding stations of sheep as an indicator of diminished forage supply. J. Anim. Sci. 61:349-353.
Schoener, T. W. 1969. Models of optimal size for solitary predators. Amer. Natur. 103:277-313.
Schoener, T. W. 1971. Theory of feeding strategies. Ann. Rev. Ecol. Syst. 2:369-403.
Schoener, T. W. 1983. Simple models of optimal feeding-territory size: a reconciliation. Amer. Natur. 121:608-629.
Senft, R.L. 1989. Hierarchical foraging models: Effects of stocking and landscape composition on simulated resource use by cattle. Ecol. Modeling 46:283-303.
Senft, R. L., M. B. Coughenour, D. W. Bailey, L. R. Rittenhouse, O. E. Sala, and D. M. Swift. 1987. Large herbivore foraging and ecological hierarchies. Bioscience 37:789-799.
Senft, R. L., L. R. Rittenhouse, and R. G. Woodmansee. 1985. Factors influencing patterns of grazing behavior on shortgrass steppe. J. Range Manage. 38:81-87.
Sheehy, D. P. 1988. Grazing relationships of elk, deer and cattle on seasonal rangeland in northeastern Oregon. Ph.D. Dissertation, Oregon State University, Corvallis. 269 p.
Smith, M. S. 1984. Behavioral ecology of sheep in the Australian arid zone. Ph.D. dissertation. Australian National University, Canberra.
Smith, M. S. 1988. Modeling: three approaches to predicting how herbivore impact is distributed in rangelands. New Mex. Agr. Exp. Sta. Reg. Res. Rep. 628.
Squires, V. R. 1982. Behavior of free-ranging livestock on native grasslands and shrublands. Trop. Grassl. 16:161-170.
Stuth, J. W., J. R. Brown, P. D. Olson, M. R. Araujo, and H. D. Aljoe. 1987. Effects of stocking rate on critical plant-animal interactions in a rotational grazed Schizachyrium - Paspalum savanna. p. 115-139. In: Plant-Animal Interactions. F. P. Horn, J. Hodgson, J. J. Mott, R. N. Brougham (eds). Winrock International. Morrilton, AK USA.
Stuth, J. W. and S. Searcy. 1987. A new electronic approach to monitoring ingestive behavior of cattle. p. 81-82. In: Proc. II Internl. Symp. Nutrition of Herbivores. Brisbane, Australia.
Valentine, K. A. 1947. Distance from water as a factor in grazing capacity of rangeland. J. Range Manage. 45:749-754.
Van Soest, P. J. 1982. Nutritional ecology of the ruminant.
O & B Books, Corvallis, Oreg.
Walker, J. W., J. W. Stuth and R. K. Heitschmidt. 1989. A simulation approach for evaluating field data from grazing trials. Agr. Syst. 30: 301-316.
Walker, B. H. and R. H. Emslie, R. N. Owen-Smith and R. J. Scholes. 1987. To cull or not to cull: lessons from a Southern African drought. J. Appl. Ecol. 24:381-401.
Walker, J. W. and R. K. Heitschmidt. 1988. Some effects of a rotational grazing treatment on cattle grazing behavior. J. Range Manage. 42:337-342.
Westoby, M. 1974. An analysis of diet selection by large generalist herbivores. Amer. Natur. 108:290-304.
Whitman, T. G. 1980. The theory of habitat selection: examined and extended using Pemphigus aphids. Amer. Natur. 115:449-466.
Figure 3.1 Hierarchical view of the diet selection process from the landscape level down to the individual plant.
Figure 3.2 Landscape configuration reflecting the unique set of forage resources, water locations and terrain constraints which effect use patterns by grazing animals.
Figure 3.3 Zonal impact of water and shape on adjustments in stocking rate of a grazing unit.
Figure 3.4 Hierarchy of large grazers' physiological and behavioral needs which affect patterns of landscape use. These threshold levels trigger initiation and velocity of movement and frequency of encounter of locals within a landscape.
Figure 3.5 Interactive effects of water and thermal foci as they affect energy maintenance and intake of ruminant animals.
Figure 3.6 Conceptual view of the functional nature of landscape use categories when considering the ratio of occupancy time to percent area occupied contrasted to the ratio of forage utilized relative to herbage mass a plant community contributes to a pasture.
Figure 3.7 Effects of seasonal quality of plant communities on feeding station behavior. High forage quality (e.g. spring) results in short feeding times at stations (FS) with longer search intervals between feeding stations.
Figure 3.8 Pattern of feeding stations along a directional grazing path as influenced by patch environments which vary in herbage mass (g/m2) and potential harvest rates (g/min).
Figure 3.9 Animals are faced with a choice of plants at a feeding station which offer different potential instantaneous intake rates, nutrient density and secondary compounds.
Figure 3.10 Classification of plant
species based on their relationship to herbage mass (hg/ha) and associated
electivity index (EI): (EI = % diet + % landscape)
% diet - % landscape)
Figure 3.11 Structural characteristics of grasses influencing grazing decision.
Figure 3.12 Preference status of the primary food groups of cattle as a function of their selection ratios in a grazed landscape.
Figure 3.13 Preference status of the primary food group of goats as a function of their selection ratios in a grazed landscape.
Figure 3.14 Hierarchical presentation
of the components of the plant-animal-soil interface. (Adopted from Coleman
et al. 1989).