Influence of sward characteristics on grazing behaviour and growth of Hereford steers grazing tropical grass pastures

Abstract

Relationships between sward characteristics, grazing behaviour and growth of Hereford steers were studied on nitrogen-fertilized (378 kg ha-1 yr-1) Setaria anceps cv. Nandi and Digitaria decumbens swards continuously stocked at 4.3, 6.2 and 8.0 beasts ha-1 during five periods over 2 years. Availability of herbage, as measured by bite size, and nitrogen content and in vitro digestibility of herbage in the top of the swards were the two most important factors influencing the performance of steers. The relative importance of these factors varied between seasons, between stocking rate treatments and to a lesser extent between pasture species. The highest growth rates were measured on the leniently stocked pastures because steers were able to harvest feed easily (a large bite size) and could more readily select the more nutritious plant parts from the top of the sward. The swards were highly heterogeneous, and numerical analysis of data showed that at the same herbage yield, spatial distribution of herbage (leaf bulk density and leaf to stem ratio) and the nutritive value (in vitro digestibility and nitrogen content) of herbage greatly influenced the growth of steers. Cattle were unable to satisfy their feed requirements on some high quality but low-yielding swards, despite increasing grazing time to compensate for the small bites prehended (up to 707 min in 24 hr in spring). Consequently nitrogen and digestibility contents of extrusa samples were at times poor indicators of performance.