The effect of tree shade on ambient conditions and heat stress indicator traits of new-born South African Mutton Merino and Dormer lambs: Preliminary results

This preliminary study investigated the provision of shade on heat stress indicators of South African Mutton Merino (SAMM) and Dormer lambs shortly after birth, during the autumn 2017 and 2018 lambing seasons. Newborn lambs were assessed to determine whether welfare, as assessed by respiratory response and rectal temperature as heat stress indicators, survival and early growth benefitted from the provision of shade. Groups consisting of 4-17 pregnant SAMM and Dormer ewes were randomly allocated to 5-10 paddocks with natural shade from trees and 5-9 paddocks that were directly in the sun with no shade available. The lambs were recorded within 24 h of birth at noon. Climate data were obtained from a nearby weather station. The lambs were also weighed at 12 (SD = 2) days of age at tail-docking. Tree shade had a moderating effect on temperature, resulting in lower maximum daytime and higher minimum night-time temperatures. There was an interaction between a temperature-humidity index (THI) and the treatments (access to shade or no access to shade) for respiration and rectal temperature (P < 0.01). Both traits were relatively unaffected by the THI at values below 77. Unshaded lambs exhibited a pronounced upwards trend following a THI-threshold of 77-78. Tailing weight tended to be higher while lamb survival of live-born lambs to tail-docking was lower in lambs born in shaded paddocks but these trends did not persist to weaning. Shade is needed to enhance animal welfare by alleviating the effect of high THI-values on hot days in an autumn lambing season.

Automated summary

This study found that providing shade for South African Mutton Merino and Dormer lambs can help alleviate the negative impact of high temperatures on the newborn lambs, including reducing the severity of heat stress indicators. There was an interaction between temperature-humidity index and shade access on respiratory and rectal temperature, with unshaded lambs showing a pronounced upward trend above a THI threshold of 77.