Genetic relationship between lactation curve traits in the first three parities of dairy cattle
In Kenya, selection of dams for the national contract mating scheme is based on 305-day lactation milk yield (TMY) performance across parities. At farm level, only cows that had good TMY performance in previous parity are retained for the subsequent lactation. This strategy has resulted in improved milk production performance, although increased incidences of physiological disorders, mastitis and reduced fertility have been reported in high-yielding cows. Records on functional traits relative to milk yield are scant. Properties of lactation curves could be used indirectly to explain the productive and functional efficiency of dairy cows. This study assesses the relationship between lactation traits, based on the lactation curve, to evaluate the effects of selection for TMY on lactation curve properties and to assess the effects of selection decisions based on first lactation on performance in later lactations. A repeatability analysis revealed negative genetic correlations between peak milk yield (MYmax) and persistency (S); MYmax and days in milk at peak (DIMP); and TMY and DIMP. Genetic correlations were positive between DIMP and S, and TMY and MYmax. This implies that selection for high TMY would result in high MYmax and a reduction in DIMP. This alters the shape of the lactation curve, shifting production pressure to early lactation, which aggravates a negative energy balance, thus compromising the physiological integrity of the cow. Continuous selection for high TMY could be responsible for the reported decline in reproductive efficiency and lactation physiology. Correlation estimates between traits in different parities were bidirectional (correlation estimates changed signs (+/-) in different parities), implying that selection decisions made in lactation 1 may not have similar outcomes in lactations 2 and 3. Selection at farm level should be optimised by using records from three parities, as is done in the contract mating scheme.
Keywords: Correlation estimates, reproductive performance, test-day milk yield