Adjusting lactation milk yield of Zimbabwean Holstein dairy cows for the joint effects of age at calving and month of calving
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Milk yield is influenced by both genetic and non-genetic factors. For accurate genetic evaluations there is need to account for these factors and quantify their effects on milk yield. The main objective of this study was to quantify the joint effects of age at calving and month of calving on lactation milk yield and to compute appropriate adjustment factors for milk yield. A total of 29 315 edited lactation records of Zimbabwean Holstein dairy cows collected from 1979 to 1994 by the Zimbabwe Dairy Herd Improvement Association (ZDHIA) were used. The data were divided into two sub-sets. The first set of 16 073 records was used to develop multiplicative age-month adjustment factors and for determination of genetic parameters. The second data set with 13 242 records was used for verification of the adjustment factors. This was done to test the effectiveness of multiplicative age-month adjustment factors which were developed in this study. An Animal Model was run in the Average Information — Restricted Maximum Likelihood (AIREML) software package. The multiplicative adjustment factors for lactation milk yield varied with age at calving and month of calving. The factors were highest in young cows (1.21), decreasing with increasing age at calving and were lowest (0.89) at mature age (72 to 84 months). After 72 to 84 months the adjustment factors started increasing. The adjustment factors were also highest (1.21) in summer (December, January and February) and lowest (0.89) in winter (May, June and July). Heritability estimates were highest for first lactation cows and were low for later lactation cows. The heritability estimates were similar for both adjusted and unadjusted records. The heritability estimates for the unadjusted records were 0.38, 0.26, 0.24 and 0.21 for first, second, second and over, and all combined lactations, respectively. The corresponding estimates for the adjusted records were 0.45, 0.25, 0.25 and 0.24. This study showed that age-month adjustments were effective. The local Holstein dairy industry is recommended to use these age-month adjustment factors developed using local data to eliminate biases in genetic evaluations. Heritabilities are high enough to practise an effective progeny test and selection scheme for milk yield.