Both genetic improvement and milk yield of dairy cows are dependent on a high reproductive performance. Under normal situations, oocyte fertilisation rates following insemination at the appropriate time are high (>90%) and gross genetic defects in embryos are estimated to be around 10%. This means that the biological limit of conception rate should be around 80%. Such a performance has rarely been realised in practice, with 60% being a normal target for lactating cows in New Zealand dairy herds. At a herd level, reproductive performance is best measured by the percentage of cows pregnant within a period after start of the breeding season and this is determined by the combination of submission rate and conception rate. Major factors that affect reproductive performance in New Zealand dairy herds include anoestrus, errors in heat detection, fertilisation failure and embryo mortality. Given adequate nutrition, herd management is undoubtedly the major determinant of reproductive efficiency. The low reproductive efficiency in dairy cows creates a bottleneck to genetic improvement and is a major factor affecting milk production efficiency.
Proceedings of the New Zealand Society of Animal Production, Volume 56, , 34-37, 1996
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