Heritability of feed intake in grazing Merino ewes and the genetic relationships with production traits

Abstract

The feed intake of 1279 mature non-pregnant and non-lactating Merino ewes grazing pasture was estimated by faecal marker dilution methods using rumen controlled-release devices containing chromium sesquioxide capsules. The ewes were from two genetic resource flocks (QPLU$ flock at Trangie in NSW and a Western Australia flock at Katanning) that are representative of the major bloodlines and strains in the Australian Merino population. Pedigree information over several generations was used to genetically link other animals from the resource flocks that had additional production data to allow estimation of genetic correlations between feed intake and production traits with greater accuracy. Data were available for over 16 000 records for growth, wool and reproduction traits from the ewes and their relatives, together with carcass and meat quality traits from over 4000 rams that were slaughtered at ~18 months of age. Feed intake of the ewes was significantly affected by the reproductive status of the ewes at their previous lambing and feed intake, expressed as relative digestible dry matter intake (rDDMI), had an estimated heritability of 0.32 ± 0.08. The genetic correlations between rDDMI and growth traits were positive and larger than their standard error for birth (0.24 ± 0.12) and hogget (0.20 ± 0.09) weight, although inclusion of ewe liveweight as a covariate in the model reduced the correlations to close to zero. Generally, the genetic correlations between rDDMI and the wool, reproduction, carcass and meat quality traits were close to zero and smaller than their standard errors, with little effect of including ewe liveweight as a covariate. The results suggest that feed requirements of ewes could be reduced by selection, although ewe weight and growth would also decline unless accounted for in the model. Practical and cost effective methods of measurement of intake under grazing conditions need to be developed.