Animal preference and external plant attributes: application of principle

Introduction

A common denominator of the animal-plant relationships is that every grazing animal selects its food from the wide range of plants in natural vegetation, notwithstanding the fact that some animals eat various kinds of foods. Animal Preference (AP) is reserved for selection by the animal which is essentially behavioral and relative preference indicates proportional choice among two or more foods (Foresters, 1958; Ivins, 1952). AP is a useful term in understanding (1) vegetation changes, (2) formulating better animal management practices, (3) planning vegetation improvement programs, and (4) determining food intakes (Heady, 1964). Indeed, calculating rangeland grazing capacity resulted from multiplying the amount of yield by allowable use and/or AP to obtain the amount of forage available for grazing animals (Ebrahimi et al., 2010; Ebrahimi, 2007). Therefore, if AP is not determined accurately, the figure for grazing capacity will be incorrect. The wrong number of grazing animals on the land will ultimately lead to the pasture destruction or wasting forage resources. There are many factors influencing relative AP such as palatability, associated species, topography climate, soil, animal type and animal physiology. Among these, palatability and animal type are the most important factors influencing AP (Heady, 1964).

Palatability is defined as the plant characteristics or conditions, which stimulate a selective response by animals (Heady, 1964; Cowlishaw and Alder 1960; Young, 1957, 1948). As commonly used, the term implies acceptability but not necessarily desirability. Thus, a food stuff that is palatable may be essentially neutral with regard to preference, being neither attractive nor repellant to the taste. Palatability is extremely difficult to define in terms of the biological processes involved in food selection. It is also important to note that palatability differs from the external plants attributes (EPA) (Ganqa and Scogings, 2007; Scheidel and Bruelheide, 1999; Cronin, 1998; Hay et al., 1994; Frost and Ruyle, 1993; Hendry and Grime 1993; Rumbaugh et al., 1993; Russel et al., 1992). AP is probably related to EPA including presence of awns, spines, hairiness, position of leaves, stickiness, texture (Heady, 1964), thorns (Frost and Ruyle 1993; Russel et al., 1992), tissues, trichome, and toughness (Ganqa and Scogings, 2007; Scheidel and Bruelheide, 1999; Cronin, 1998; Hay et al., 1994; Hendry and Grime, 1993; Rumbaugh et al., 1993). According to Raufirad et al., (2013), and Arzani (1996), it can be concluded that EPA is the most important factor of palatability influencing relative AP as EPA is one of the first characteristics of the plant that animals face with when grazing in rangelands.

Since AP is directly related to the animal's characteristics, animal type is, therefore, considered as the other important factor affecting relative AP. Grazing animals can noticeably reduce the vigor of palatable species. Biomass losses at the grown-up stage can decrease seed production and vegetative extension, and differential grazing can consequently alter the dominance of the different plant species (Gross et al., 2001; Fraser and Grime, 1999; Van et al., 1998; Piper, 1996). It is perhaps not surprising that the most palatable species are generally restricted to habitats with low herbivore pressure (Elger et al., 2002; Fraser and Grime 1999). Furthermore, various kind of animals like sheep or goats significantly differ in their food habits as each species showing innate preferences for certain plants, some parts of plants, or plants within particular growth stages. Interpretation of differences would be most difficult because grazing animals exhibit variations in preferred foods from one location to another (Dasmann, 1949), in different seasons (Heady, and Torell, 1959), over a period of a few days (Nichol, 1938)), within the same day (Van Dyne, 1963) and among individuals (Van Dyne, 1963; Heady Torell, 1959). Since many species of grazing animals inhabit the same area, additional knowledge of food habits, including preferences, is needed so that vegetation may be controlled to provide preferred foods for desirable animals (Heady, 1964).

The most common way in determining AP is to observe whether they eat the plant. In general, the methods utilized for measuring preferences are those employed primarily for other purposes such as determining grazing capacity, effects of grazing on vegetation, forage production, food intake, animal nutrition, and range utilization (Heady, 1964) However, the question of what morphological characteristics precisely appeal to or repel livestock is crucial for effective rangeland management (Heady, 1964). Therefore, there is a need for developing a method that considers external features of the plants in order to evaluate the AP.

The relationship of AP with EPA and animal types is, however, exceptionally complex. Very little research (Scheidel and Bruelheide, 1999; Frost and Ruyle 1993; Rumbaugh et al., 1993) has been conducted to correlate the EPA with the acceptability of the plant as a food source for mammalian herbivores (Russel et al., 1992). Therefore, little information is available on EPA while it may be an important factor affecting AP. Although there is a large body of research on the food habits of different animals, EPA is not usually investigated (Heady, 1964).The above discussions make it clear that EPA and animal type play an important role in AP. Although the relationship between these factors (i.e., some EPA and animal type) and AP has been previously studied (Arzani, 1996), there is a lack of research on the relationship of AP with all external attributes of the plant and animal type. Thus, the main objective of this study was to understand the relationship between EPA (including leaf position, branch density, leaf trichome, leaf spininess, stem spininess, height, leaf-stem ratio, prehensile resistance, stem trichome, inflorescence spininess, awns and succulence) and animal type with AP.