The influence of shearing on growth performance of feedlot lambs under high temperatures

Abstract

With temperatures being forecast to increase over southern Africa, producers are increasingly likely to have to mitigate the impacts of heat stress on production animals. Shearing offers a potentially simple method of increasing the heat tolerance of sheep, but no conclusive study has yet been performed on the impact of shearing on feedlot production under South African conditions. To quantify the effect of shearing on the production efficiency and heat tolerance of Dohne Merino lambs under feedlot conditions, fifty-six ram lambs were randomly allocated to be shorn or not and provided with either a high (11.06 MJ/kg ME) or low energy (9.29 MJ/kg ME) finishing diet. The lambs were finished off for 42 days during summer (29˚C ± 4) and then slaughtered. Weekly measurements of weight, back fat depth and feed intake were made while respiration rate and surface and rectal temperature were recorded on the hottest day of each week. Shearing affected mean weight gain (312 g/day vs. 347 g/day; P = 0.022) and feed conversion (5.43 vs. 4.70; P = 0.026) over the full growth period while daily feed intake (1.62 kg vs 1.59 kg; P = 0.457) was unaffected. Feed intake was higher in the low energy group (P = 0.033), but growth and feed conversion were unaffected by diet. Unshorn lambs had higher average respiration rates (160 breaths/minute vs. 142 breaths/minute; P < 0.001) but similar surface (38.35˚C vs. 38.09˚C; P = 0.095) and identical rectal temperatures as shorn lambs, indicating that thermoregulation through increased respiration was sufficient to prevent heat stress from occurring more frequently than in shorn animals. Shearing of feedlot animals therefore provided only a small improvement in thermal tolerance and resulted in a significant decline in production performance, indicating that shearing does not provide an economic benefit to producers under feedlot conditions.