CHAPTER 59 Heifer Development: Nutrition, Health, and Reproduction
Replacement heifer development is a critically important area of beef production in which veterinarians can provide valuable production medicine advice to their clients. Development costs for preparing a heifer to calve at 24 months of age average about 31% of her lifetime expenses, whereas delaying first calving to 30 or 36 months increases the cost of development to 42% or 46% of her lifetime costs, even if her lifetime productivity is extended by 6 to 12 months.1
Not only does the group of replacement heifers need to calve at a mean of 24 months, but the distribution of calving should result in most if not all of the heifers calving early in the calving season.1 In order to reach this goal, the heifer development program should ensure that most heifers in the replacement pool reach puberty at least 42 days before the start of breeding, because the conception rate to first service is lower on the pubertal estrus than on the third estrus.2,3
Many producers put additional pressure on heifers to reach puberty at a young age by breeding them 3 to 4 weeks earlier than the mature cowherd. The stress of calving is greater on heifers than on older cows and calving difficulty is more likely in this group. Thus, breeding replacement heifers essentially one heat cycle earlier than the mature cows allows the producer to concentrate on the heifers at calving. In addition, the length of time from calving to the resumption of cycling is longer in heifers than in cows.2 Therefore, calving heifers earlier than mature cows gives the heifers the extra time they need to return to estrus and be cycling at the start of the subsequent breeding season.
The beef heifer has reached puberty when she is able to express estrous behavior and ovulate a fertile oocyte. The onset of puberty is influenced primarily by genetic factors governing age and weight at onset specific to the breed.2,3 The age at puberty can be decreased by selecting for breeds with a younger age at puberty, selecting within a breed for younger age at puberty, or crossbreeding with another breed that has a similar or younger age at puberty. Other factors also can have some influence on the onset of puberty and include exposure to bulls, time of year, and exposure to progestogens.2–4
A SYSTEMATIC APPROACH TO HEIFER DEVELOPMENT
Weaning to Breeding Nutrition
The target weight concept is based on the fact that Bos taurus breed heifers such as Angus, Hereford, Charolais, and Limousin are expected to reach puberty at approximately 60% of mature weight.2 Dual-purpose breed heifers such as Braunvieh, Gelbvieh, and Red Poll (selected for both meat and milk production) tend to reach puberty at about 55% of mature weight. Bos indicus heifers, most commonly Brahma or Brahma-cross, are older and heavier at puberty than the other beef breeds, reaching puberty at about 65% of mature weight.3 The target weight for heifers can be based on the average mature weight for the herd, or it can be determined by using the frame score to predict mature weight4 (Table 59-1). Once the target weight is known and the number of days until the start of the breeding season (or until a mid-development ration change) is determined, the rate of gain needed is a simple calculation.
Meeting the target weight, but not grossly exceeding it, is important for heifer fertility and production. Developing heifers on a high plane of nutrition (both energy and protein) from weaning to breeding results in earlier puberty,3 improved udder development,3 and increased conception rates4 compared with a low plane.
Although hitting the target weight at the start of the breeding season is important for fertility and future productivity, weight gains do not need to be consistent throughout the weaning to breeding period. Freetly and associates showed that so long as replacement heifers grow to meet a minimal body weight before mating, a period of limit feeding, followed by full feeding to capture compensatory gain, may be used to decrease the amount of feed required for heifer development without a decrease in the ability of the heifer to conceive, or a decrease in her calf’s growth potential; however, first-calf survival may be affected.3
Last 60 Days of Gestation Nutrition
The nutritional demands of pregnancy increase as gestation progresses. These demands increase not only as a result of fetal growth but also because of uterine/placental growth and metabolism involved with the fetal-maternal interaction and the exchange of nutrients and waste.
In one study, heifers calving in body condition scores (BCSs) of 4, 5, or 6, respectively, had calves with progressively heavier birth weights, but dystocia score was not influenced by BCS at calving.4 Heifers with greater weight gains ante partum had calves with heavier actual and 205-day adjusted weaning weights than did heifers with moderate weight gains.18 Greater BCS at calving resulted in more heifers in estrus and more heifers pregnant by 40 and 60 days of the subsequent breeding season.18 Thin females should be fed levels during the last third of pregnancy to achieve a targeted BCS of 6 or higher at calving, whereas those in moderate-high to high body condition at 90 days before calving should be fed levels to maintain body reserves.
First 80 Days of Lactation Nutrition
During the first 80 to 100 days after parturition, the heifer must continue to grow at about 0.5 lb per day, support lactation for a suckling calf, resume estrous cyclicity, and conceive for her second pregnancy. The maintenance requirement for lactating heifers averages about 20% higher than that for nonlactating heifers, with actual maintenance requirements being greatly affected by level of milk production. Marston and colleagues3 illustrate the importance of adequate body condition at calving in that supplementation of energy or protein after calving had little effect on subsequent pregnancy rate. The period of time between calving and rebreeding is fairly short, only 82 days to maintain a 365-day calving interval, and during this time the cow has her highest nutritional demand due to lactation. Because of these factors, weight gain or body condition increase is difficult in the early postpartum cow. Lalman and co-workers4 found that feeding high-energy diets post partum to thin heifers reduces the negative effects of prepartal nutrient restriction but does not completely reverse those effects.
Use of Progestogens
Progesterone and synthetic progestogens induce puberty in heifers, and management systems that capitalize on this result have been developed. Short and colleagues showed that more prepubertal heifers (8.5 months of age and weighing 249 kg) given a progesterone implant for 6 days plus an injection of estradiol-17β 24 hours after implant removal showed estrous behavior and ovulated within 4 days than heifers treated with estradiol-17β alone.5 A commercially available synthetic progestogen is melengestrol acetate* (MGA). Studies have demonstrated the ability of MGA to induce puberty in heifers, especially heifers near the age and weight requirements for spontaneous induction of puberty. Conception rate at first service for heifers that attained puberty while being treated with MGA administered orally for 14 days, followed by prostaglandin F2α given as an intramuscular injection 17 days after the final day of MGA feeding, was not different from that for control heifers that attained puberty during the same period.5
Stay updated, free articles. Join our Telegram channel
Full access? Get Clinical Tree
