The effect of different milk pricing schemes on a selection index for South African Holstein cattle

[摘要] English: A total of 150673 first lactation records of South African Holstein lactating cows wereused to estimate the genetic parameters of milk (MY), butterfat (BFY), and protein(PRY) yields, and butterfat (BFP) and protein (PRP) percentages. The data comprising113 056 dams and 1 429 sires, distributed over 1 205 herds, were collected over a periodof 21 years, 1980 to 2000.The study was aimed at the following aspects: to determine the non - genetic factorsinfluencing milk traits for use in the model for a subsequent genetic analysis; to estimatethe variance-covariance components and heritabilities of all traits. Finally, to calculatethe weighting factors for a selection index and predict the expected response to selectionwhen using different milk pricing systems.Five fixed effects were defined, viz: milking frequency, age at calving, herd, month ofcalving and year of calving. Analysis of variance indicated that all effects were highlysignificant (p < 0.0001) explaining on average 64.60%, 62.96%, 64.36%, 15.66% and23.86% of the total variation for MY, BFY, PRY, BFP and PRP, respectively.Considering the R2 and RMSE as an option, month of calving for all traits, milkingfrequency for BFP and PRP, and age at calving for PRP showed a minor contribution tothe variation and were, thus, excluded from the model for the subsequent analysis.Heritability estimates varied from medium for BFY and PRY to high for MY, BFP andPRP. This indicates that faster genetic progress will be possible through selection of MY,BFP and PRP. The highly positive genetic correlation among yield traits indicatesselection for milk yield will result in a favourable response with butterfat and proteinyield. However, it will lead to an unfavourable response to selection for the percentagetraits. This is due to a negative genetic correlation between milk yield with percentagetraits.Using the Desire program, weighting factors and expected response per generation formilk, butterfat and protein yield were calculated under three milk buyers (A, B and C)pricing systems. The standard deviations of alternative breeding objectives and selectionindices were also calculated.The weighting factors for milk, butterfat and protein yields were 0.16, 2.41 and 3.24 formilk buyer A, 0.04, 3.43 and 5.70 for milk buyer Band 0.03, 3.48 and 4.37 for milkbuyer C, respectively. While the expected response for the corresponding traits, when theintensity of selection equals one, were +546kg, + 16.6kg and + 15.6kg for MY, BFY, PRYrespectively. in the ease of milk buyer A, +509kg, +17.1kg and +15.7kg for milk buyer Band +504kg, + I7.3kg and + I5.5kg for milk buyer C.The correlations of indices of B and C with respect to index A were 0.97 and 0.84,respectively. This indicates that the loss of efficiency when selection index Band C wereused is 3 % and 16 %, respectively. Rankings of Holstein cows under the three milkpricingsystems were different and this indicates that depending on the choice of theirbreeding goals South African Holstein breeders should follow different selectionprograms.

[发布机构] University of the Free State