Genetic parameters of body condition score (BCS) and effects of BCS and BCS change on ewe performance : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Animal Science, Massey University, Palmerston North, New Zealand

Abstract

Body condition score (BCS) is an important management technique that can be easily learnt and implemented on farm to determine the body condition of ewes. The industry recommended BCS is 3.0 to 3.5 at mating to ensure optimal production. Currently the average industry BCS is less than 3.0 and recent research suggests that the change in BCS may be more important for determining the subsequent production of the ewe. The objectives of this thesis were to examine the relationships between BCS and production through exploring the effects of genetic and phenotypic BCS and BCS change on productive performance. Records of BCS were obtained from Focus Genetics and New Zealand Merino flocks to determine the effect of BCS change on phenotypic production and estimate the genetic parameters of BCS. There was no increase in production for ewes above a BCS of 3.5, therefore, BCS of 3.0 to 3.5 should remain the target BCS for phenotypic production. Ewes that decreased phenotypic BCS between lambing and weaning were associated with greater production indicating these ewes had utilised their stored body fat to achieve high milk yields. The estimated heritabilities of BCS change were low indicating limitations in the ability to alter the shape of BCS profiles by selection. Heritability, genetic and phenotypic parameter estimations of BCS and production traits were performed on 9,585 dual-purpose ewes and 2,007 Merino ewes. The heritability of BCS in New Zealand dual-purpose sheep was found to be moderately heritable (0.16-0.30) and had a high genetic correlation between BCS measurements across the production year. The best time to record BCS for genetic selection was confirmed to be mating. Live weight and BCS are highly genetically correlated, therefore, it may be relevant to explore the inclusion of BCS in the selection criterion to ensure that BCS does not exceed the optimal range of 3.0 to 3.5. The results of this thesis indicate that observing BCS is a valuable tool and it would be possible to change the genetic potential for BCS with genetic selection. This information can be used to develop selection criteria for BCS.