Customized voluntary waiting period before first insemination in primiparous dairy cows: Effect on milk production, fertility, and health.

A customized voluntary waiting period (VWP) before first insemination was tested in 18 commercial dairy herds in Sweden to assess milk production, fertility, and health in primiparous cows expected to be suited for extended VWP. Cow selection for extended VWP was based on 3 criteria in early lactation: (1) the 10% of cows with highest genomic persistency index, (2) cows with a difficult calving or disease during the first month of lactation, and (3) cows with higher yield during d 4 to 33 after calving than the herd average for primiparous cows. Cows meeting at least one of these criteria were randomly assigned to either treatment with an extended VWP of at least 175 d (ExtExt; n = 174, calving interval [CInt] = 16.3 mo) or treatment with a conventional VWP of a maximum of 100 d; (ExtConv; n = 173, CInt = 12.4 mo). Cows not meeting any of the criteria were assigned to the conventional VWP treatment (ConvConv; n = 183, CInt = 12.0 mo). We found no differences in milk yield per day in the CInt between treatments, although 305-d and whole-lactation (WL) milk yields were higher for ExtExt cows (10,371 and 13,803 kg) than ExtConv cows (9,812 and 10,257 kg). Milk yield at the last test milking before dry-off was lower in ExtExt compared with ExtConv cows (24.9 vs. 28.3 kg), but the results showed no difference in dry period length between the treatments. Regarding reproductive performance, the ExtExt cows had a higher first service conception rate (FSCR; 60% vs. 45%) and lower number of inseminations per conception (NINS; 1.67 vs. 2.19), compared with the ExtConv cows. As expected, ConvConv cows had the lowest milk yield (305-d, WL, and per day) in the CInt; however, FSCR and NINS did not differ between ConvConv cows and cows in the other 2 VWP treatments. Disease incidence was higher for cows in the ExtConv compared with the ConvConv treatment, but there was no difference between ExtExt and the 2 other VWP treatments. Further, no difference in the proportion of cows with good udder health or culling rate was detected between any of the treatments, though due to low prevalence, the study lacked power to draw major conclusions on these results. Thus, prolonging VWP for suitable primiparous cows can produce benefits such as improved fertility in the form of higher FSCR and lower NINS, as well as lower dry-off yield, without compromising milk yield or prolonging dry period length.