Discipline: lactation; Keywords: extended calving interval, extended lactation, insemination, milk yield.
Time of dry-off, calving and the start of a new lactation are critical transition events for a dairy cow. Large changes in both physiology and management increase the risk for disease and culling. The cow faces these transition events every year, as a one-year calving interval is usually aimed for. A one-year calving interval is usually associated with a good average 305-day yield and better economic results compared with longer calving intervals. However, it is possible that by reducing the number of transition events by extending calving may be beneficial for fertility and health: (1) insemination results could be improved because of a better metabolic status at time of insemination, as less inseminations are needed when insemination is delayed from 40 to 120 days after calving; (2) by extending calving interval the number of cows that are dried-off with a high milk yield (e.g. more than 18 kg per day) could be reduced, which could improve udder health and welfare; (3) in some cases farmers aim to reduce the number of calvings and calves born to reduce the labour required for transition management and calf care. In this study by Dr E.E.A. Burgers and colleagues they investigated farms with customized calving intervals with respect to calving to first service interval, accomplished calving interval, services per conception, conception rate at first AI, peak yield, lactation persistency, 305-day yield and effective lactation yield. The results were published in the Journal of Dairy Science, Volume 104 of 2021, page 443 to 458, the title being: Fertility and milk production on commercial dairy farms with customized lactation lengths.
In total, 4 858 complete lactations of Holstein Friesian cows between 2014 and 2019 from 13 farms were grouped by lactation number (1 or, 2 and more) and calving to first service interval or calving interval. Cow inseminations were grouped by lactation number and calving to first service interval or calving interval class. The fertility and milk production characteristics were analyzed with generalized and general linear mixed models.
Calving to first service interval class was not associated with number of services per conception, but the extended calving interval class was associated with increased services per conception. More than 50% of cows in the calving to first service interval class of less than 84 days ended up in longer than expected calving intervals (more than 364 days), showing that these cows were not able to conceive in the desired calving interval. More than 50% of cows in higher calving interval classes of more than 420 days had an earlier first insemination before the successful insemination, showing that these extended calving interval classes consisted of both cows with an extended waiting period for first insemination and cows that failed to conceive at earlier insemination(s). On most farms, lactation persistency was greatest in the calving interval class of less than 364 days, probably related to the low peak yield in this class. When this shortest calving interval class was excluded, persistency increased with extended calving interval classes on most farms. Although at the majority of farms 305-day yield was greater in calving intervals of more than 532 days, the effective lactation yield at most farms was greatest in calving intervals from 364 to 531 days, especially for more lactation cows. Based on the results of this study, a calving interval of between 364 and 531 days seems most optimal for milk production, when high-yielding cows are selected.
The results of the study suggest that it will differ per farm what strategy in terms of waiting period for first insemination will be optimal for milk production. For heifers, a calving to first service interval of more than 196 days will result in the greatest effective lactation yield, when high-yielding heifers are selected. For cows, a calving to first service interval of more than 140 days will result in the greatest effective lactation yield, when high-yielding cows are selected.