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Post-weaning pasture feeding of the breeding ewe

Thompson, K. F.
Fields of Research
ANZSRC::070202 Animal Growth and Development
Effects of various pasture feeding levels on ewe fasted weight gain during summer were assessed over two years. Two levels of pasture feeding (lactation high and lactation low) from parturition to weaning (10 weeks) produced differences of 7 to 8 kg fasted weight and 0.99 and 1.27 body condition score units in ewes. Groups of ewes (n=40) were offered combinations from a range of pasture allowances (0.8, 1, 1.4,2 or 5 kg DM/ewe/day) from weaning for 15 or 13 weeks for the first and second years respectively. Pasture mass ranged from 2468 to 3906 kg DM/ha, and was composed of 39 to 54 % grass leaf, 16 to 36 % dead material and 9 to 15 % legume. Pasture intake increased significantly with pasture allowance (P<0.05). There was no significant difference in pasture intake between lactation low and high groups. Ewes on lower allowances tended to reach a stable fasted weight and those on 5 kg DM/ewe/day gained throughout. Other groups offered a low pasture allowance during summer followed by 5 kg D/lewe/day for the last 5 weeks had high rates of weight gain over the later period. Change in fasted weight (FW) were associated with changes in body condition score (CS): CS = 0.101 (se=0.0040) * FW - 2.71 (se=0.22) Regression of weight gain on the reciprocal of pasture allowance accounted for 74.4% of the variance with further variance accounted for sequentially by fasted weight 11.6% percentage grass leaf 3.8%, pasture mass 1.3%, percentage legume 0.9% and lactation feeding level 2.5%. The above values were very similar if body condition score was substituted for fasted weight. These variates and factors accounted for the between year and within year variation in weight gain response to pasture allowance. The regression coefficients indicate that in general terms: ewe weight gain decreased by 20 g/day in light and 15 g/day in heavy ewes, or decreased by 82 g/day for each unit increase in condition score; increased by 5 g/day for each % unit increase in grass leaf or legume content of the sward, or for every 100 kg DM/ha in pasture mass; or decreased by 5 g/day for each % unit increase in dead material content in the sward, and increased by 36 g/day for lactation high groups compared with lactation low. Independent effects of condition score and fasted weight on weight gain response to pasture allowance were not identified. Pasture allowance per kg fasted weight did not adequately account for higher weight gain of light compared with heavy ewes. There was a greater rate of weight gain in lactation low groups but they only recovered between 16% and 43% of the initial fasted weight difference between the lactation low and high groups. The carry over effect of lactation feeding level on ewe weight gain after weaning suggested that the lactation high group had either a lower maintenance requirement or require a less intake per unit of gain, or both. At the end of pasture allowance treatments, ewes from the lactation high group were significantly (p<0.05) heavier than those from the the lactation low group. The use of compensatory gain in the recovery of weight in a ewe flock is discussed.
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