Relationships between birth or calving seasons and first-lactation performance of Holstein cows in the Midwestern United States.

The purpose of this study was to establish the effects of birth season on performance of first-lactation Holstein cows. Further analysis was conducted to determine if the impact of calving season was a more appropriate indicator. This study analyzed data from 2009 to 2022 and included 524 primiparous Holstein cows from the University of Illinois Urbana-Champaign Dairy Research Unit (Urbana, IL). Cows were managed per farm protocols. Seasons were assigned by birth and calving months: winter (December, January, February), spring (March, April, May), summer (June, July, August), and fall (September, October, November). The main comparison was winter versus summer seasons of birth and calving, but summer versus non-summer and winter versus non-winter were also used. Data were analyzed using the MIXED, UNIVARIATE, and MEANS procedures in SAS v. 9.4. Based on the first-lactation data, average test day milk yield (kg) for d 5 to 35 was less for summer-born cows in comparison to winter and non-summer-born cows. Average first test day fat and protein percentages were less for summer-born cows in contrast to winter. The length of pregnancy was less for summer-born cows compared with non-summer. The age at first calving (AFC) was lower for summer-born cows in comparison to winter and non-summer, whereas winter was greater than non-winter. No differences were observed for the number of times bred, number of lactations, or 305-d milk yield. Summer-calving cows tended to have less first test day milk yield (kg) compared with non-summer. First test day fat and protein percentages were lower for summer in comparison to winter and non-summer, whereas winter was greater than non-winter. Days pregnant were fewer for summer-calving cows relative to winter. Summer-calving cows stayed in the herd for fewer lactations than winter and non-summer, however winter-calving cows stayed for a greater number of lactations. Cows that calved in winter and summer tended to have lower AFC relative to non-winter and non-summer-calving cows, respectively. No differences were observed in times bred or 305-d milk yield. Overall, an association between birth and calving seasons was observed, suggesting their influence on first-lactation performance. Birth season data were more closely related to first test day milk yield and AFC, whereas calving season had a greater effect on first test day milk components and productive life, implying greater focus should be placed on calf husbandry and first-lactation cows calving during the warmest season in the Midwestern United States.