Impact of season on birth weight, growth, and average daily gain of conventionally raised Holstein heifers in the Midwestern United States

The objectives of this study were to determine the effects of season on calf birth weight, growth, and ADG of Holstein heifers. This retrospective analysis included data from July 2018 to February 2022 and included 216 calves from the University of Illinois Dairy Research Unit. Calves were housed in individual hutches until weaned at 56 d. After weaning, calves moved to group housing until 175 d of age, at which time they moved to heifer dry lots. Feeding rates remained consistent throughout the year, and weaning took place from d 50 to 56. Body weight data were collected at birth, weaning, and when moved to heifer lots. Calves were assigned a season based on month of birth: winter (December, January, February), spring (March, April, May), summer (June, July, August), and fall (September, October, November). In this analysis, season was the fixed effect, and calf was a random effect and subject of the repeated effect. Data were analyzed using the MIXED and UNIVARIATE procedures in SAS. Winter-born calves had greater birth and weaning BW when compared with summer calves. Birth BW of winter calves tended to be greater and weaning BW were greater when compared with all nonwinter calves. Similarly, preweaning ADG of winter calves was greater than summer calves and all nonwinter calves. On the other hand, postweaning BW and ADG were greater for summer calves in comparison to winter calves. Likewise, summer calves were observed to have greater BW and tended to have greater ADG in contrast to all nonsummer calves. When combining the pre- and postweaning ADG through the first 175 d of life, there were no differences between winter and summer calves, winter and nonwinter calves, or summer and nonsummer calves. These results indicate calf birth BW and growth performance are greater through weaning during colder seasons. Conversely, calves born during winter do not perform as well after weaning once the seasons get warmer. Summer calves were observed to have decreased birth BW and performance through weaning, but as the seasons became cooler, they excelled postweaning to 175 d of age.