The relationship between level of sward height reduction in a rotationallygrazed sward and short-term intake rates of dairy cows

The objective of this study was to investigate the relationship between lev el of sward height reduction (SHR) and short-term intake of herbage by lact ating dairy cows offered swards differing in initial bulk density (BD). Thr ee experiments were carried out in which cows were presented with swards re presenting different levels of SHR (nominally described as ungrazed, low, m oderate and high). Experiments 1 and 2 differed with respect to initial swa rd ED [ungrazed sward 1.7 vs. 2.5 kg dry matter (DM) m(-3) respectively]. E xperiment 3 investigated the interaction between ED and SHR. In each experiment, sixteen Holstein/Friesian cows (fitted with excreta col lection bags) grazed for a 1-h period in 200-m(2) plots that had been graze d on the previous day to predetermined sward surface heights (SSH) (= level s of SHR). Herbage intake rates were assessed from changes in live weight, with a correction for insensible weight loss (IWL). Biting rates were recor ded from visual observation. high levels of SHR were associated with a sign ificant reduction in SSH, herbage mass and leaf fraction, and a significant increase in sward ED, stem and dead fraction, and DM concentration. Herbage intake, expressed either as DM intake per bite or DM intake per hou r, declined as level of SHR increased from low to high. The level of SHR ge nerally had no effect on biting rate. Intake rates varied from 1.9 to 4.4 k g DM h(-1), whereas DM intake bite(-1) ranged ire from 0.5 to 1.3 g. Pooled regression analysis identified SSH (P < 0.001; r(2) = 0.94) as the princip al determinant of DM intake bite(-1). The regression equation was not signi ficantly improved by the addition of terms for leaf fraction ED, or herbage mass. In Experiment 3, a significant interaction between level of SHR and sward ED was observed. It is concluded that the principal factor controllin g intake (g DM bite(-1) or kg DM h(-1)), as swards are progressively grazed down, is SSH, but at a high level of SHR sward ED also influences intake b ite(-1).