Genetic and environmental causes of variation in milk production traits of Sahiwal cattle in Pakistan

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Abstract
Data from about 4000 Sahiwal cows from eight large herds in Pakistan were used to study the influence of genetic and environmental factors on some milk production traits. First-lactation mean values were 1363 kg, 1395 kg and 252 days for milk yield up to 305 days after calving, total lactation yield and lactation length, respectively. Second and third lactation yields were proportionately 0·12 and 0·18 higher, respectively, at 305 days. The effect of herd-year at calving was by far the most important source of variation for all traits. Heritabilities estimated in uni- and trivariate analyses, using restricted maximum likelihood (REML) with an expectation maximization algorithm for an animal model, ranged from 0·14 to 0·17 for first-lactation traits. The estimates were generally lower for second lactation and higher for third lactation traits. Genetic correlation between lactations for the same trait were close to unity, whilst the phenotypic were considerably lower. Repeatabilities for milk yield traits were 0·42 and for lactation length 0·31. Genetic and phenotypic correlations between first-lactation 305-day milk yield and lactation length were 0·83 and 0·71, respectively. Genetic trends for all traits were close to zero but a substantial deterioration in performance, caused by negative environmental factors, was observed. Although the heritabilities were low, the prospects for genetic improvement are good, as indicated by a rather large additive genetic variation. A multivariate animal model, including the first three lactations of 305-day milk yield, was recommended for the most accurate prediction of breeding values for milk production.