Maternal effects on weight and ultrasonically measured traits of lambs in a small closed Suffolk flock

Early growth and ultrasonically measured muscle and fat depths (FD) are commonly used as selection criteria for the, genetic improvement of meat species. Maternal effects on these traits have been recognised. This paper investigates how direct additive, maternal additive and maternal environmental effects influence early growth (n = 892) and carcass traits (n = 829) in lambs from a closed 100-ewe flock by fitting a series of mixed models, using a derivative-free maximum likelihood algorithm. These models included different combinations of the direct and maternal effects and some interactions between them. Maternal effects were found to influence all traits; birth weight (BWT), adjusted 8-week weight (8WT), adjusted 16-week weight (16WT), weight at scanning (SWT), muscle depth (MD) and FD. For the weight traits, the size of the maternal genetic effect declined with increasing age of the lamb. The direct genetic and maternal environmental effects remained constant at all ages studied. Maternal effects on FD and MD were environmental in origin. A high (negative) correlation was found between the direct and maternal genetic effects for lamb weight. This was particularly high (-0.95) for 8WT. These results indicate that genetic selection schemes for early weight and carcass traits should utilise models which account for more than just the direct additive effects. Failure to do so will lead to an overestimation of the genetic progress and will increase the chances of making sub-optimal selection decisions. (C) 2002 Published by Elsevier Science B.V.