Response to selection for open face and greasy fleece weight in Romney sheep : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Animal Science at Massey University
Selection responses and genetic parameters were estimated on wool, body and reproductive traits in a New Zealand Romney flock. In 1956 the flock was divided into 3 sub-flocks, each of which has been a closed breeding group (consisting approximately 80 ewes and 4 rams) since 1958. In one group the most open-faced yearlings were retained for breeding, in another, those with heaviest yearling fleece weights, while in the third replacements were chosen at random. Traits examined included post-shearing live weight (LW), greasy fleece weight (GFW), clean fleece weight (W), quality number (QN), character grade (CHG), staple length (SL), total crimp number (TCN), crimp frequency (CF), clean scoured yield (Y) and mean fibre diameter (MFD) for the ewes and ram and ewe hoggets; number of lambs born (LB) and number of lambs reared (LR) per ewe joined; date-of-birth (DOB), birth weight (BW), weaning weight (WW), medullameter index (MI) and face-cover grade (FC) for the ram and ewe hoggets and the standard deviation of the fibre diameter (SFD) and percent medullated fibres (PMF) for the ewe hoggets. The average inbreeding coefficient increased by approximately 0.10 in all 3 flocks over the 21 years evaluated. Within years, sheep with the highest inbreeding coefficients generally suffered a depression in the level of performance. Heritabilities (h2) and genetic correlations (rg) were calculated using the paternal half-sib correlation approach. Most h2 estimates were similar to already published values. Estimates of the h2 of ram hogget GFW were substantially smaller than corresponding ewe hogget values (0.07 to 0.15 cf 0.28 to 0.34). Face-cover grade h2 in the face-cover flock appeared to be much reduced, suggesting a possible decline in genetic variation for this trait. Phenotypic and genetic correlations were calculated amongst all hogget traits and between ewe hogget and ewe average lifetime performance traits. Very high genetic and phenotypic correlations were found between hogget GFW and hogget W. Ewe hogget GFW tended to be positively associated with LB and LR, both genetically and phenotypically. Hogget FC was generally unrelated to other hogget and ewe traits. Ewe hogget performance in LW, GFW, W, QN, SL, TCN, Y and MFD were generally moderately to strongly related with the performance of the same ewe in corresponding traits at older ages. There was often wide variation in the three (one from each group) genetic correlation estimates for pach pair of traits. This variation was attributed to either the small number of observations available (about 80 sires per flock with 5 to 7 progeny per sire) or changing genetic variance and covariance components in the selection flocks. Realized heritability (h2r) estimates for FC and hogget GFW ranged between 0.39 to 0.54 and 0.06 to 0.19, respectively. The h2r estimates of hogget GFW were in good agreement with the paternal half-sib correlation estimates of h2 derived from the ram hogget data but were markedly less than equivalent estimates obtained from the ewe hogget data. The h2r estimates of FC were generally higher than paternal half-sib estimates, but were in good agreement with estimates derived by other workers. In the face-cover flock correlated responses were generally small. Positive correlated responses of about 10% were recorded in LB and LR over the 21 years evaluated. In the fleece weight group the components contributing toward increased GFW all showed positive correlated responses. Lamb production (LB and LR) of the fleece weight group showed an increase of about 30%, relative to the control flock, over the 21 years studied.