Grazing Behavior of Ruminants and Daily Performance from Warm-Season Grasses

The various ingestive behavioral measurements have seldom been used to estimate long-term DM intake or to An estimate of the animal-production potential of pastures can be explain differences in intake on daily animal responses assessed by knowing the daily dry matter (DM) intake of the grazing animal and the digestibility of the DM consumed. The objective of among grazing management strategies in a production this paper is to examine the relationships between pasture canopy setting. This is also true relative to predicting daily anicharacteristics, ingestive behavior, and daily animal response from mal response. This need has been previously noted warm-season pastures. Of daily DM intake and digestibility of the (Hodgson, 1982a; Hodgson et al., 1994) and attributed, DM consumed, the former is the most variable and the most difficult in part, to a failure to see the role of ingestive behavior to determine. One approach to estimating daily DM intake has been measurements in production systems (Cosgrove, 1997). to use the components of ingestive behavior to determine a shortSeveral examples of the usefulness of grazing behavioral term intake rate (g min 1 ), which can be scaled using grazing time data were provided by Cosgrove (1997). One is the (min d 1 ) to give a 24-h DM intake (kg d 1 ). This approach has been importance of canopy height of ryegrass-based (Lolium used experimentally with some success, but has not found application perenne L.) pastures in maximizing daily forage intake in production settings. While aspects of ingestive behavior, including ingestive mastication, are common to all grazing ruminants, literature of the grazing animal and an understanding of why it indicates that differences occur among ruminant species and that is important. Another is the importance of leaf area animals ingest different pasture species differently. This results in index, green leaf mass, and associated stem height of plant-animal interactions. Frequently these dynamics are not clearly the canopy in understanding daily forage intake among addressed for cool-season and warm-season pastures in literature regrazing systems. views, which adds undue confusion to the general area. Ingestive The general lack of application of grazing behavior behavior is discussed relative to animaland pasture-generated bounds measurements to production systems is neither a critiwhich operate within paddocks and can greatly alter ingestive behavior cism of previous research nor of this general area of estimates. Also presented are relationships between diet particle size, research. It does beg, however, for the incorporation of associated with ingestive mastication, and steer daily gains. ingestive behavior measurements into long-term animal response studies to assess which of the numerous measurements have utility in aiding the producer in achievT utilization of pastures by the grazing animal ing greater efficiency in the animal enterprise. The focus remains a complex biological process that is not of this study is to delineate important components of well understood. This general state exists in spite of grazing behavior already identified for warm-season ongoing grazing behavioral research since the initial perennial grass pastures, to examine linkage between studies on tropical grasses in the early 1970s (Allden ingestive mastication and animal performance, and to and Whittaker, 1970; Stobbs, 1973a,b). Since this early discuss important boundaries that alter the grazing enviwork, much of the continuing research on grazing beronment. havior has shifted to the utilization of temperate pastures (Hodgson, 1982b; Hodgson et al., 1994). In the Complications development of grazing behavior research, the reductionist approach has emerged in which small segments Grazing behavior research on both tropical and temof the soil–plant–animal complex (namely, the plant– perate pastures has resulted in valuable data, unique to animal interface) have been examined in intensive, a specific plant species-animal type within each experishort-term experiments (Cosgrove, 1997; Ungar, 1998). ment. Generally, each experiment is conducted to test a These short-term studies have identified the imporspecific hypothesis. In the literature, however, ingestive tant ingestive behavioral components of animal intake behavior data from different experiments are frequently and the influential interacting components of the pasintermingled without regard for plant type (tropical or ture canopy. This has led to considerable knowledge temperate) or animal type [cattle (Bos spp.), sheep and understanding about how animals graze. Recent (Ovis spp.), or goats (Capra spp.)] and occasionally the comprehensive reviews addressing animal grazing bespecific identity of the data are lost. This has probably havior are available and will be left to the reader (Coleresulted from the perceived need to explain relationman et al., 1989; Gordon and Lascano, 1993; Hodgson ships and has been accomplished by inserting related, et al., 1994; Cosgrove, 1997; Ungar, 1998; Sollenberger but not necessarily the best, data to possibly fill a void and Burns, 2001). in the understanding of the process and has thus complicated interpretation. Clearly, while the elements of grazing behavior and J.C. Burns, USDA-ARS, Dep. of Crop Science, and Dep. of Animal Science, North Carolina State Univ., Raleigh, NC 27695; and L.E. Sollenberger, Dep. of Agronomy, Univ. of Florida, Gainesville, FL Abbreviations: ADG, average daily gain; BR, bite rate; BV, bite 32611. Received 29 May 2001. *Corresponding author (joe_burns@ volume; DM, dry matter; HD, herbage bulk density; IB, intake bite; ncsu.edu). IR, intake rate; IVDMD, in vitro dry matter disappearance; NDF, neutral detergent fiber. Published in Crop Sci. 42:873–881 (2002).