The effect of sward characteristics upon grazing behaviour, intake and animal production from tropical pastures

Abstract

The effect of sward characteristics upon eating behaviour and animal production from tropical pastures was studied. Five experiments appraised the validity of techniques for measuring the quantity and quality of the herbage consumed by cattle. A further four experiments investigated pasture-animal relationships on monospecific grass pastures of varying yield and structure.Tropical pasture swards studied generally had an open habit of growth, and were markedly heterogeneous with large differences in the distribution of leaf and nutrients within the sward profile. These pastures were grazed selectively.Eating behaviour of cattle grazing these pastures was shown to be associated with the harvesting of leaf, and three-dimensional samples of the swards showed that cattle usually selected leaf from the uppermost layers. Leaf yield, leaf bulk density and the proportions of leaf and stem in the swards were shown to be the most important factors influencing selective grazing.An accurate technique for estimating leaf and stem components of extrusa samples was developed. However percentage leaf in extrusa samples did not provide a reliable indicator of animal production since these diets were often associated with a low intake per bite and a low daily intake of digestible nutrients.Feed intake (I) is a function of grazing time (T), rate of biting (R) and bite size (S), and techniques based on this principle (I = T x R x S) were developed for measuring herbage consumption of grazing cattle. Compared with intensive cutting, this technique proved to be fairly reliable.Bite size was shown to be the most important factor controlling daily intake of herbage.Bite size eaten varied according to pasture conditions from 56 mg OM bite -1 to 390 mg OM bite -1. Bite size was shown to be little affected by overnight fasting. Although there were large differences between animals in bite size, by using the same group of fistulated cattle, significant differences between pastures could be detected by sampling during a single period of intensive grazing provided that sward characteristics did not alter greatly throughout the day.When bite size was small cattle generally increased grazing time to above 650 min -1 24 hr -1 (compared with 400 to 500 min -1 24 hr -1 when larger bites were harvested), and mean daily rate of biting also increased during eating to 62 bites min -1 (compared with a mean of 50 bites min -1 24 hr -1). However when animals had extreme difficulty or a lack of desire to harvest feed, grazing time and number of bites decreased. These short grazing times and low number of bites on intensively grazed pastures were shown to be unrelated to a deficiency of protein or minerals. Since short grazing time and low number of bites were found with both high and low yield swards both measurements were poor predictors of animal performance.Animals masticate herbage to a similar degree of fineness before swallowing (mean modulus of fineness of 4.5) and with fibrous feeds more jaw movements per unit of intake'were recorded. The number of rumination bites was independent of the proportion of stem in the diet, but the number of rumination bites per kg of drymatter intake increased with increasing fibrosity of the diet.It was concluded that herbage yield, particularly yield of leaf and green herbage, had a dominant effect on grazing behaviour and animal performance. Numerical analysis overcame the problem of correlation between individual sward characteristics and showed that sward structure (leaf and green bulk densities, leaf to stem ratio and sward height) and the nutritive value of herbage within the sward markedly influenced eating behaviour and animal production.Practical applications of these findings are discussed in relation to better utilization of tropical pastures.