Birth weight responses to shearing ewes in early to mid gestation

This study compared the effect on foetal growth of shearing ewes in very ea rly pregnancy (day 50) with the effect of shearing later in pregnancy (day 70 or 100). One hundred and sixty ewes were allocated at day 49 of pregnanc y to four 'time of shearing' treatments - pregnancy day 50 (P50), P70, P100 (no. = 45 ewes per treatment) and unshorn (shearing of this group occurred 43 days after weaning) (no. = 25); two methods of sheaving (by standard co mb and cover comb); and two levels (1 or 2) of pregnancy/rearing rank. All ewes had been mated over a 28-day period and grazed pasture throughout the trial. The mid point of lambing was 27 August 1996. Live weights of ewes (c orrected for fleece weight) were not influenced by sheaving time at any sta ge except at day 45 of lactation when ewes shorn at P50, P70 or P100 were a bout 3.0 kg lighter than unshorn ewes. Shearing method had no effect on ewe live weight. Time of shearing and sheaving method had no effect on ewe gre asy fleece production or wool tensile strength. Shearing significantly (P < 0.05) influenced the birth weight of single- (by 0.7 to 0.8 kg) but not tw in-born lambs. Maternal plasma triiodothyronine concentration increased pro portionately by 0.47 to 0.85 (P < 0.05) and lasted for at least 20 days in response to sheaving irrespective of time of shearing. No difference in pla sma insulin concentration was detected between sheaving groups but plasma g lucose concentration proportionately increased by 0.16 to 0.16 (P < 0.05) f or 10 to 20 days after shearing. These results indicate that shearing betwe en day 50 and 100 of pregnancy can increase lamb birth weights and that the response is associated with an increase in thyroid hormone concentrations in the maternal circulation.