Converting spring-born heifers into summer-calving herds
Selection decisions can increase subsequent cow longevity and productivity
By Travis Mulliniks
Weaning is an opportunity to determine or select replacement heifers. For each ranch operation, the selection decision may be decided by one or the combination of factors such as disposition, genetic merit, age of dam, performance of dam, body weight of heifer, date of birth, etc. Nevertheless, these selection decisions should be made with long-term profitability in mind.
For me, I like to look at it as their net present value. Just like any capital investment, retained or purchased heifers are only worth the sum of all the income over their lifetime, including salvage value minus production costs that occurred during development.
Developing replacement heifers is one of the most expensive and complex management decisions for a cow-calf producer, which also has long-term implications for profitability. In my career, I can’t tell you the number of times I have had producers tell me their selection for heifers was to sell the heavy and light ends at weaning while keeping the middle. Or some kind of similar selection process with the mindset that heavy heifers would make big cows and light weight heifers would be less productive.
Selecting heifers in this manner, however, tends to sell the older, more mature heifers rather than selling heifers that may had more genetic potential for growth. Heifers born early in the calving season are older and tend to be heavier body weight at weaning. These early-born heifers also have greater rates of puberty attainment prior to the start of their first breeding season. This results in early born heifers conceiving earlier in the breeding season and therefore calving earlier in the calving season.
Therefore, we set up a study to determine the impact of converting spring-born heifers into a summer-calving herd on growth, reproductive performance, longevity and productivity compared to early or late-born May heifers. Over a three-year period, 273 Red Angus/Simmental crossbred heifers were utilized to determine the impact of converting March-born heifers (Convert; n = 90) to a May-calving herd compared to May-born heifers on reproductive performance, body weight and calf performance from a yearling to five years of age.
May-born heifers were retrospectively grouped into two different groups: heifers born in the first 21-day of calving as a heifer calf (Early; n = 123) or heifers born after the 21-d of calving as a heifer calf (Late; n = 60). Heifers were exposed to bulls for a 45-d breeding season with a bull-to-heifer ratio of 1:20. Average birthdate for each heifer group was March 14, May 12 and June 4 for Converts, Early May and Late May; respectively.
Convert heifers were larger in body weight during heifer development; however, by calving as a three-year-old, there were not differences in cow body weight among the three different heifer groups for the rest of the study. Selecting older heavier heifers at weaning didn’t cause a shift in long-term cow size or just increasing cow size of the cow herd.
After five years of age, retention rate was increased in Convert cows compared to Early and Late May-born cows with no difference between the two May-born heifer groups (Figure 1). Increasing cow herd longevity is critical for long-term profitability due to the number of calf crops required to pay for development costs, which can be anywhere from 3 to 10 years of age to pay off heifer development and opportunity costs depending on cost of production and initial heifer costs.
In breeding seasons with limited forage quality, selecting for older heifers born in earlier calving seasons may allow for increased forage intake capacity. The early increase in body weight in Convert heifers (25-50 pounds increase) may have helped increase intake capacity during those critical stages of early lactation as a two-year-old to benefit reproductive performance as shown in the differences in retention rate between a yearling heifer and two-year-old in Figure 1. The inability of young cows to consume enough energy especially during early lactation is a driver of delayed resumption of estrus and ultimately decreased pregnancy rates. This may be advantageous during periods when forage quality is approaching 7% crude protein or less.
Similar to long-term retention rate, Convert heifers weaned an additional 244 lb of total calf weaned per original heifer over the five years (Figure 2). This increased pounds of calf weaned was driven by two things: a slight increase in calf weaning weight as a young cow (2- and 3-yr-old) and the increase in retention rate in the herd for Convert heifers compared to Early May and Late May born counterparts.
An effective strategy for many cow-calf producers is to select early born, older replacement heifers. As shown above, this may result in increased longevity in the cow herd while increasing total production of more calves weaned and lifetime profitability of the replacement heifer. If selection is done prior to weaning, this can allow for increased flexibility in managing the non-replacement heifers such as calfhood implants that may provide a cost-effective strategy to increase weaning weight of non-replacement heifers.
Mulliniks is the Glenn & Mildred Harvey Professor of Beef Cattle Management, Oregon State University.
