Comparing cows - including dry period and lactation length in a yield measure

To assess economic and environmental consequences of dry period length in dairy cows, milk yields have to be compared. Milk yield is mostly compared as yield in the 305 days after calving (305d yield), but this measure ignores extra milk yield in the previous lactation when cows are milked longer. In addition, it does not adjust for actual lactation length, while a shorter dry period may go together with a shorter calving interval. We aimed to 1) develop a just measure of milk yield that includes calving interval and dry period length and 2) apply this to compare 217 cows with a conventional, short or no dry period before 2nd calving. The data was a convenience sample from 14 commercial farms; herd and dry period length were confounded. An ‘effective lactation’ was set from 60 days before last calving to 60 days before the next calving and expressed in ‘effective yield’ in kg of fat-and-protein-corrected milk per cow per day. Thereby, it included possible extra milk in the previous lactation and excluded milk that depended on the next dry period choice. Cows without a dry period had a 22% lower 305d yield than cows with a conventional dry period. Adding the extra milk yield in the previous lactation and correcting for the shorter calving interval (355 vs. 408d), the effective yield was only 9% lower. The 305d yield of cows with a short dry period did not differ from cows with a conventional dry period, but the effective yield was 7% higher. When 1st lactation 305d yield was used as a covariate of genetic merit, the effective yield of cows with a short and conventional dry period did not differ, while it was 2 kg per day lower (across production levels) for cows without a dry period. The difference between results from 305d and effective yield emphasizes the importance of method choice when comparing cows. Effective yield enables a just comparison of milk yield when dry period length and calving interval vary.