The goal of most heifer replacement programs is to raise high-quality, healthy heifers in an efficient and economical manner. These heifers have the genetics, frame, body condition and management background to conceive in a timely manner, reach their potential for high first-lactation milk yield and become profitable replacements to the herd. Heifer rearing costs are the second- or third-largest cost on most U.S. dairy farms today. First-lactation heifers make up between 30% to 50% of the milk cows in most dairy herds. First-lactation performance also sets the stage for future production. Here are a few factors impacting first-lactation milk production. In utero period Recent research has explored the conditions calves face in utero and the long-term performance impacts they may have. The term epigenetics is used to describe phenotypic stressors on pregnant cows that impact the in utero calf and its future performance. Perhaps the best example of this is heat stress on dry cows. University of Florida research has repeatedly shown significantly lower birth bodyweight and decreased future production for calves born from dams under heat stress. This reduced production continues when these cows are followed through the second and third lactation. Continued research at the University of Wisconsin-Madison took this a step further and showed granddaughters born of these heat-stressed dry cows also had a significant reduction in milk production through 35 weeks of lactation. Heat stress in dry cows is truly a generational problem. Preweaning period Feeding 2 to 4 quarts of high-quality colostrum to calves within a few hours of birth continues to be a key component in calf health and performance as well as first-lactation milk production. Serum total protein analysis is a simple and repeatable way to evaluate passive transfer of immunoglobulins in dairy calves up to 5 days of age. Researchers in Po-land in 2021 reported that calves with serum total protein concentration greater than 6 g/dL produced 3,400 pounds more milk through their first lactation. Preweaning growth rates are associated with increased milk production in first lactation as shown in studies both from Cornell University in 2012 and 2013 and the University of Minnesota in 2017. Penn State research also shows a positive correlation between starter intake preweaning and first-lactation milk production. Increased starter intake in the first two months of life also results in greater rumen development and a more consistent growth pattern through weaning. Dry matter intake from liquid and dry feed has a synergistic effect and both are needed to increase first-lactation milk production. Prepubertal growth Some refer to the prepubertal time between weaning and breeding as a forgotten period; yet, this 4- to 10-month age is a critical period to maintain heifer growth to carry them into the milking string. Target growth rates from 1.8 to 2 pounds per day are necessary to bring heifers through this prepuberty period in a timely manner and set up their future success as cows. Bodyweight at first calving There is a philosophical disconnect between age at first calving and average daily gain on many dairies. Bodyweight of a heifer at the time of first calving is a proxy for growth and size. If heifers have not reached the desired size at calving, they will continue to grow during lactation. This is much less efficient and happens at the expense of milk production. Dr. Tom Overton, Cornell University, estimates that 1 pound of growth during lactation cost 7 pounds of milk. How big should heifers be at the time of calving? To answer this question, a farm must know the average weight of mature cows in the herd. Cows typically do not reach their mature size until their third to fourth lactation. A subset of these cows between 80 and 120 days in milk should be used as the predictor for mature size. Overton cautions not to use the last load of cull cows that were long in days in milk with excess body condition. Overton said fresh heifers should weigh 85% of their mature bodyweight post-calving. If springing heifers are easier to weigh, they should be closer to 95% of this mature bodyweight shortly before calving. To reach these goals, farms must maintain an ambitious daily rate of gain during the rearing process. If your first-lactation heifers do not meet production expectations, assess these different stages of your calf and heifer program. As we learn more about epigenetics, in utero environmental conditions, such as heat stress, may also need to be evaluated. Barry Visser is a nutritionist for Vita Plus.