Effects of grazing sequence and condensed tannins on ingestive behaviour, herbage intake, and performance of lambs grazing Yorkshire fog pasture

Abstract

A grazing experiment was conducted using Yorkshire fog (Holcus lanatus cv. Massey Basyn)/white clover (Trifolium repens cv. Grasslands Tahora) pasture to assess the grazing behaviour, diet selection, herbage intake, and performance of lambs as affected by low concentrations of condensed tannin (CT) in the grass, and grazing sequence, under rotational stocking management in summer 1994/1995, in New Zealand. Sixty spring‐born castrated male lambs were used in the experiment. Twelve lambs were slaughtered at the start of the experiment to measure carcass weight. Another 48 lambs were allocated to two groups in sets of twenty‐four to graze eight paddocks rotationally in a Leader/Follower sequence. Half of the lambs within each group were drenched with 10 g polyethylene glycol (PEG: mol. weight 3350) twice daily at 0830 and 1630 hours and the remaining lambs were drenched with water as a control. There were higher proportions of grass leaf and white clover and lower proportions of grass stem and dead material in the Leader sward than in the Follower sward, and correspondingly higher proportions of green grass and white clover and a smaller proportion of dead material in the diet samples selected from Leader swards. Leader lambs had greater intake per bite (79 versus 64 ± 3.9 mg OM per bite, P < 0.001), higher total N concentration (2.67 versus 2.35 ± 0.065%, P < 0.01) and in vitro OMD (78 versus 72 ± 0.7 %, P < 0.001) in the diet, and spent more time grazing (660 versus 620 ± 7 min, P < 0.001) than Followers, and their herbage DOM intake was significantly greater (56 versus 43 ± 0.8 g kg−1 W0.75 d−1, P < 0.001). Leader lambs had greater liveweight gain (107 versus 37 ± 5.0 g d−1, P < 0.001), greater carcass weight (15.0 versus 12.5 ± 0.2 kg, P < 0.001), and more wool growth (109 versus 80 ± 2.9 mg 100cm−2 d−1, P < 0.001) than Followers. There was no significant difference in CT concentrations between the diets selected from the Leader and Follower swards (2.59 versus 2.26 + 0.165 g kg−1 DM). Consistent increase in rumen ammonia concentration by PEG administration indicated that the low concentrations of CT in Yorkshire fog reduced the rate of protein degradation in the rumen. However, there were no significant effects of CT on herbage intake, body growth, and wool growth rate of lambs.