Effect of sward height on short-term intake by steers grazing winter oat pastures

Abstract

Sward structure and animal grazing behaviour are key variables in determining herbage intake. The present study was carried out with the objective of evaluating the effect of different sward heights (SH) on the short-term ingestive behaviour variables, in steers grazing winter oats (Avena sativa cv. Calén). Six Holstein-Friesian steers (196 ± 4 kg LW), grouped into three pairs, grazed on swards with different surface heights (SH): 40 cm (SH40), 50 cm (SH50), and 60 cm (SH60). Short-term intake rates (IR) were estimated during grazing sessions of 1 h by the double-weighing technique with correction for insensible weight losses. The number of grazing jaw movements (GJM) and bite jaw movements were measured using the acoustic recorder technique. Measurements were conducted along three consecutive days. On each day, the pairs of steers grazed on one of the SH (1 pair per SH per d). Sound files were analysed visually and aurally. Number of GJM and BJM were determined during three 5-min periods, from minute 5 to 10, from minute 27 to 32 and from minute 50 to 55 of the 1 h grazing session. Bite mass (BM) was calculated as the quotient between IR and the number of bites. Variables of grazing behaviour were analysed by ANOVA according to a 3 × 3 Latin Square design. Classes included in the model were treatment, pair of animals, day, observation time window, and the interaction between treatment and observation time window. Unexpectedly, BM was not affected by the SH (P = 0.97; 0.59, 0.61 and 0.60 g DM bite−1, for SH40, SH50 and SH60, respectively). Similar BM were the result of a numerical, non-significant, decrease in IR (P = 0.65; 30.2, 29.6 and 26.3 g DM min−1, for SH40, SH50 and SH60, respectively), together with a decrease in bite rate (BR; P< 0.001; 51.8, 49.1 and 44.3 bite min−1, for SH40, SH50 and SH60, respectively) with increasing SH. The rate of GJM was similar among treatments (P = 0.50; 88.4, 90.4 and 89.7 GJM min−1, for SH40, SH50 and SH60, respectively). Similar rates of GJM and different BR resulted in increasing numbers of GJM per bite as SH increased (P< 0.001; 1.73, 1.88 and 2.05 GJM per bite, for SH40, SH50 and SH60, respectively) showing than more manipulative grazing jaw movements were needed to form a bite when the steers grazed on the taller swards.