Developing selection indexes & estimation of genetic parameters for traits of economic importance in dairy cattle under once-a-day milking : a thesis presented in partial fulfillment of the requirements for the degree of Doctor of Philosophy in Animal Science, Institute of Veterinary, Animal Biomedical Sciences, Massey University, Palmerston North, New Zealand
In New Zealand, about 5% of dairy herds are milked once-a-day (OAD). The cows are inseminated using sires from the twice-a-day milking system (TAD) evaluated on breeding worth (BW) or an OAD selection index. Testing for sire-by-milking frequency (MF) interaction (S×MF) could reveal if developing an OAD-specific selection scheme is justified. In this thesis production records were analysed from herds milked OAD and an equivalent TAD population provided by Livestock Improvement Corporation. Across MF, heritabilities (h2) and genetic correlations were similar for milk yields (h2: 0.21-0.36), although they tended to be greater in TAD. Genetic correlations were 0.35-0.40 between milk and fat yields, 0.85 between milk and protein yields and 0.54-0.60 between fat and protein yields. Observed rank correlation between OAD and TAD EBVs of the sires were moderate to high for milk yields, being greater in Jersey (J) (0.74-0.84) sires compared to Holstein-Friesian (F) and F×J crossbred (0.55-0.77) sires. Those values were greater than their critical values of the expected correlations (5th percentile), indicating that S×MF was not significant. Data from a university herd indicated that J cows were more efficient at production of milk solids (MS; fat + protein) per 100 kg of live weight than F or F×J cows milked OAD. In comparison, data from commercial herds milked either OAD or TAD indicated that F cows milked OAD had 19%-25% lower milk yields, whilst the reduction in yields from F×J and J cows was around 15%-19%. Breed effects (F-J) were lower on OAD compared to TAD systems, but heterosis effects were similar across MF (4.1%-7.6%). Under a progeny testing selection scheme for herds milked OAD, estimated genetic gains ranged from 3.3 to 3.7 kg/year for MS. Nevertheless, genetic gain resulting from the selection of bulls generated in TAD systems and dedicated to OAD herds would results in a similar increase compared with a separate scheme (only 11%-13% less of MS), indicating that there is little advantage in the implementation of a separate selection scheme. The main conclusion was that the S×MF interaction was not significant and farmers operating under OAD milking achieve similar genetic gain using sires from the TAD milking selection scheme but ranked on an OAD-selection index.