Association between days post-conception and lactation persistency in dairy cattle.

Determining the optimal insemination time for individual cows is not always easy, as amongst others, pregnancy also has an effect on absolute milk yield; the latter which has been reported in several studies. There is however, limited quantitative information between the association between days-post-conception (DPC) and lactation persistency, based on a lactation curve model, and, specifically, how persistency changes during pregnancy and relates to the days in milk at conception (DIMc). Understanding this association might provide indicators towards determining the optimal insemination time. In the study cited, the aim therefore was to investigate the association between DPC and lactation persistency, with an additional focus on the influence of DIMc.

Milk production data of 23 908 first lactation and multi lactation cows from 87 herds were used. Persistency was measured by a lactation curve characteristic decay, representing the time taken to halve milk production after peak yield. Decay was calculated for eight DPC (0, 30, 60, 90, 120, 150, 180, and 210 days after DIMc), which served as the dependent variable. Independent variables included DPC, DIMc (≤60, 61–90, 91–120, 121–150, 151–180, 181–210, >210 days), parity group, DPC × parity group, DPC × DIMc, and variables from 30 days before DIMc as covariates. 

The results showed an increase in decay, which is to say, a decrease in persistency, during pregnancy for both first and multi lactation cows, albeit in different ways. Specifically, from DPC 150 to DPC 210 days, multi lactation cows showed a greater decline in persistency compared with first lactation cows. Furthermore, a later DIMc (cows conceiving later) was associated with higher persistency. Except for the early DIMc groups (DIMc <90 days), DIMc did not affect the change in persistency by gestation.

It was concluded that the findings from this study contribute to a better understanding of how DPC and DIMc during lactation influence lactation persistency. This should enable more informed decision-making by farmers who wish to take persistency into account in their reproduction management.