The effect of plant structure on the intake of tropical pastures. I. Variation in the bite size of grazing cattle

Abstract

To determine the effect of sward canopy structure on bite size a technique was developed for measuring the size of bite prehended by grazing cattle. Almost complete recovery (a mean of 95%) of ingested material was achieved by inserting a foam rubber plug in the lower oesophagus of oesophageal-fistulated animals, whilst the number of bites taken during grazing was automatically recorded. Theoretical calculations showed that a mean bite size of less than 0.30 g OM/bite can seriously limit daily intake of herbage by grazing cattle.

Four experiments were conducted to measure the variation in bite size prehended by Jersey cows when grazing tropical pasture swards of varying canopy structure. Bite size on a nitrogen fertilized Setaria anceps cv. Kazungula sward averaged 0.39 g OM/ bite, whereas an adjacent sparse unfertilized sward averaged 0.13 g OM/bite. Mean size of bite of cows grazing 5-week-old regrowths of the trailing legume (Macroptilium atropurpureum cv. Siratro) was 0.24 g OM/bite, compared with 0.34 and 0.38 g OM/bite for nitrogen fertilized Digitaria decumbens and Setaria anceps pasture respectively.

Growth regulators, gibberellic acid (GA) and 2-chloroethyl-trimethylammonium chloride (CCC), were successfully used to alter the sward canopy structure of Digitaria decumbens (pangola) and Chloris gayana (rhodes) grass pastures. GA increased herbage yield, and sward height, and produced erect stemmy plants with long internodes, whereas CCC reduced plant height and internode length, and resulted in a denser sward which was higher in nitrogen and in vitro digestibility due to a higher leaf content. Measurement of bite size prior to any changes in sward structure being manifest showed that bite size was unaffected by the growth regulators per se. However, bite size on the CCC treated pastures sampled after 6 weeks was larger (0.52 g OM/bite) than from the same cows grazing pastures treated with GA (0.40 g OM/bite).

It was concluded that the size of bite prehended by cattle grazing some tropical pasture swards can limit intake of herbage, and that sward bulk density, a low stem content, and a high leaf/height ratio have a major influence upon the size of bite taken by cows grazing these pastures.