Replacement heifer development is a critically important area for veterinarians to offer production medicine advice to their beef-producing clients. In order for replacement heifers to calve at approximately 24 months of age and to reach puberty the equivalent of three heat cycles before the start of the mature cow breeding season, heifers must become puberal by 11 to 13 months of age.
Replacement heifer development is a critically important area for veterinarians to offer production medicine advice to their beef-producing clients. In order for replacement heifers to calve at approximately 24 months of age and to reach puberty the equivalent of three heat cycles before the start of the mature cow breeding season, heifers must become puberal by 11 to 13 months of age. Once puberty is attained, nutrition must be at a level that allows the heifer to continue cycling, ovulate a viable oocyte, and establish pregnancy. Nutritional demands of heifers during pregnancy exceed that of mature cows because the heifer is partitioning nutrients for her own growth as well as fetal growth and development. This increased demand for nutrients continues through early lactation, when the beef female has her highest nutritional requirements. Deficiency of energy or protein for extended periods of time during any production phase during the first two and one-half years of life will have a negative impact on: fetal development, calf viability, milk production, and/or rebreeding for the next pregnancy.
During the early preweaning phase (first 90 days of life), the heifer calf's requirements are met primarily by her dam's milk production, but starting early in life, forage plays a role in supplying nutrients for the calf. By the time a calf is 60 days of age, she is consuming 1.5% of her bodyweight as forage drymatter. As the preweaning phase progresses, forage becomes an increasingly important nutrient source. As long as nutrient intake (milk and forage) is adequate for growth, no additional energy is needed in most heifer development systems. However, some investigators speculate that nutritional plane during the first two to three months of life influences the timing of puberty and the effectiveness of later dietary manipulations to affect age at puberty. Increased plane of nutrition early in life can be due to superior forage quantity and quality or high dam milk production. The use of creep feeding in replacement heifers should be avoided if the additional energy is used for fat deposition, most importantly into the udder parenchyma. Fat deposition in the udder of immature heifers has been shown to decrease lifetime milk production and offspring's weaning weights.
Puberty in the beef heifer is reached when she is able to express estrous behavior, ovulate a fertile oocyte, and obtain normal luteal function. The maturing of the neuroendocrine system that induces maturation and ovulation of the first oocyte as well as the hormonal changes that induce the first expression of behavioral estrus are the result of a gradual increase in gonadotropic (luteinizing hormone; LH, and follicle stimulating hormone; FSH) activity. This increased gonadotropic activity near the time of puberty is due to a decreased negative feedback of estradiol on the hypothalamic secretion of gonadotropin-releasing hormone (GnRH). As puberty approaches, the gradually increased frequency of LH pulses results in increased secretion of LH which enhances development of ovarian follicles that produce enough estradiol to induce behavioral estrus and a preovulatory surge of gonadotropins.11 Wave-like patterns of follicular development can be detected as early as 2 weeks of age in heifer calves, and the duration of follicular waves increases and the maximum diameter of dominant follicles increases with age through puberty.
The onset of puberty is primarily influenced by age and weight within breed. Other factors can also have some influence on the onset of puberty and include: exposure to bulls, time of year, and exposure to progestogens. Age of puberty in other species such as humans and rats is influenced by percent body fat or by body fat distribution, however, in cattle, fatness is not the sole regulator of puberty, as puberty does not occur at a constant percentage of body fat, and age and breed appear to be important contributing factors.
High starch diets appear to influence the age and/or weight of puberty. Ciccioli et al. (2003) reported that heifers with a higher starch intake had lower weight at puberty compared to a isonitrogenous-isocaloric diet with higher fiber even though the two diets resulted in the same body weight and fat reserves.30 Similarly, Gasser et al. (2006) found that heifers fed a higher starch diet were younger and lighter at puberty than heifers fed a lower NE control diet when the treatments were started at 99 days of age. In contrast, Marston et al. (1995) reported that heifers fed a high-concentrate diet reached puberty at the same weight but at a younger age than heifers raised on lower energy diets. Some investigators have hypothesized that increased propionate production is associated with the positive effects of starch supplementation on puberty; however, Lalman et al. (1993) demonstrated that supplementing heifer diets with propionic acid did not hasten puberty.
It has been shown that under-nutrition can delay puberty in heifers, short-term; and in long-term studies with chronically nutrient-restricted heifers where the heifers lost 17 to 18% of their body weight, the heifers became anestrus.
The latest edition of the NRC Nutrient Requirements of Beef Cattle expresses protein requirements as absorbed protein, also known as metabolizable protein (MP). Metabolizable protein replaces the earlier use of crude protein (CP) and is defined as the true protein absorbed by the intestine, supplied by microbial protein and undegraded intake protein (UIP). The MP system separates and accounts for the two components of protein nutrition of importance to the animal - the needs of the rumen microorganisms and the needs of the beef animal. Lalman et al. (1993) showed that feeding UIP in excess of NRC requirements may improve energy utilization of heifers fed mature forage, but may delay the onset of puberty compared with heifers fed monensin. Kane et al. (2004) found that in cycling beef heifers supplemented with high levels of UIP, anterior pituitary gland synthesis, storage, and secretion of gonadotropins was decreased, and they suggest that these changes may impair follicular growth and development.
Fat supplementation of heifer diets is generally restricted to less than 5% of the total DMI due to potential negative effects of higher inclusion on fiber digestibility and reduction in DMI. In a review of fat supplementation and its affect on beef female reproduction, Funston (2004) reported that nutritionally challenged replacement heifers may experience reproductive benefits from fat supplementation, but there is limited benefit of fat supplementation in well-developed heifers.
Some researchers have reported that supplemental fatty acids had positive effects on ovarian function and reproductive performance that was independent of energy source. In contrast, Howlett et al. (2003) reported that adding oilseeds or soybean hulls to corn silage based diets did not affect reproductive performance of heifers; and Lammoglia et al. (2000) found that a high fat diet fed for 162 days to beef heifers did not affect age at puberty, AI services per pregnancy, or final pregnancy percentage. A potentially negative consideration for feeding oilseed sources of fat is that phytoestrogens, which have been shown to negatively affect reproduction in cattle, can be present.
The major minerals that need supplementation in heifer diets are sodium, calcium, and phosphorus. Magnesium and potassium require supplementation under certain circumstances. Because salt is deficient in most natural feeds, it should be supplemented by either including it with the concentrate or feeding it free-choice. The level of salt needed in the diet can vary depending on the diet, type of cattle, and environmental conditions, but a general rule is to supply 0.25 to 0.5% of the diet on an as fed basis (1 to 2 oz) per day.
Calcium metabolism and phosphorus metabolism are interrelated and complex. Controlling factors include: vitamin D, parathyroid hormone, thyrocalcitonin, and the dietary levels of calcium and phosphorus. The absorption of calcium is regulated to a large extent by calcium intake. The higher the intake of calcium the less that is absorbed. The extent of dietary phosphorus that is absorbed depends not only on the source of phosphorus, but also vitamin D levels, and the level of other minerals such as aluminum, manganese, and potassium in the diet.
There are 15 trace minerals required by cattle. Of these, six may be deficient in forage-based diets. These are: copper, cobalt, iodine, selenium, zinc, and manganese. Some researchers have seen a positive reproductive effect associated with trace mineral supplementation while others have not. Saxena et al. (1991) found a correlation between serum copper and zinc concentrations and age at puberty in heifers And, DiCostanzo et al. (1986) reported improved first-service conception percentage in heifers fed corn silage diets either with manganese, or manganese, copper, and zinc compared to unsupplemented controls. In contrast, others have failed to observe a response to trace mineral supplementation on reproductive performance in cattle.
Ionophores were originally cleared for use to improve the feed efficiency of feedlot cattle on high-concentrate diets and to improve pasture cattle gains. Inclusion of ionophores in heifer diets has been shown to increase the number of heifers that reach puberty by the start of the breeding season, decrease age at puberty, decrease weight at puberty, increase corpora luteal weight, and increase the amount of progesterone produced. The decrease in age at puberty was independent of improved average daily gain and increased body weight.
The 1996 NRC estimations of Mcal and metabolizable protein (MP)requirements for British-type heifers from weaning through early pregnancy should be used as a guideline in formulating rations for developing heifers but adjustments may need to be made to achieve the desired gains. Factors such as amount of activity required for grazing, environmental temperature, breed and compensatory gain may decrease or increase the actual animal requirements when compared to the NRC estimates. Using NRC estimates plus any adjustments, one can calculate requirements to meet a desired "target-weight" at a specific time during development. If the target-weight is not met, adjustments can be made so that the desired weight at the start of the breeding season is achieved.
The target-weight concept is based on reports that Bos taurus breed heifers such as Angus, Hereford, Charolais, or Limousin are expected to reach puberty at about 60 percent of mature weight. Dual purpose breed heifers such as Braunvieh, Gelbvieh, or Red Poll tend to reach puberty at about 55 percent of mature weight, and Bos indicus heifers, most commonly Brahma or Brahma-cross, are older and heavier at puberty than the other beef breeds; about 65 percent of mature weight.
Meeting but not grossly exceeding the target weight 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, improved udder development, and increased conception percentage compared with a low plane. This difference in pregnancy percentage is probably at least partially due to differences in pituitary function of heifers fed a low-energy versus a high-energy diet.
Adequate gains during the weaning to breeding phase are also necessary for proper udder development and future milking ability. For heifers fed to gain 0.5 kg/d (1.1 lbs/d), 0.59 kg/d (1.3 lbs/d), or 0.64 kg/d (1.4 lbs/d) postweaning, an advantage was seen in milk production compared to slower gaining controls.
Overfeeding heifers before breeding has also been demonstrated to have detrimental effects on pregnancy percentages. Heifers that gained 0.45 kg to 0.68 kg/d (1 to 1.5 pounds/day) had higher (P<.01) pregnancy percentages during a 45 day breeding season than did heifers with gains above or below this range. Body condition scores in the same group of 1,863 heifers showed the same result with improving first-service conception rates as body condition increased up to a score of 6 and then declining in fat heifers. In addition, excessive supplemental feeding of beef heifers before puberty has been shown to reduce lifetime calf weaning weights due to impaired milk production. This impaired milk production appears to occur in heifers that exceed energy intake needed for optimal postweaning gain and subsequently deposit fat in the udder.
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. Researchers have shown that heifers that are fed to gain slowly followed by a period of more rapid rate gain but that reached the same target-weight and body condition score pre-breeding as heifer fed to gain at a consistent rate from weaning to breeding had the same reproductive performance. Some studies have indicated that less feed was used to develop heifers that were fed for compensatory gain than was used by heifers that had a steady rate of growth, but others report that a similar amount of feed is required to raise heifers to a common body weighte. This difference is probably related to the fact that increase in efficiency during a refeeding phase is not constant – the efficiency of gain is higher during the early periods of the refeeding phase and decrease over time.
The target-weight concept can be continued for planning nutritional requirements through pregnancy. A heifer should weigh 80 to 85% of her mature weight at the time of calving as a 2-year-old. Overfeeding protein during the breeding season and early gestation, particularly if inadequate energy is supplied to the rumen, may be associated with a decline in fertility. The mechanism for this decline may be by decreasing uterine pH during the luteal phase in cattle fed high levels of degradable protein. The combination of highly digestible protein and low energy concentrations on an as-fed basis in early growth cool-season grasses may explain the lower than expected fertility seen in females placed on such pastures near the time of breeding.
The nutritional demands of pregnancy increase as gestation progresses. These demands occur not only due to fetal growth, but also due to uterine/placental growth and metabolism involved with the fetal/maternal interaction and exchange of nutrients and waste.
Heifers calving in BCS of 4, 5, or 6 respectively, had calves with progressively heavier birth weights, but dystocia score was not influenced by BCS at calving. Heifers with greater weight gains prepartum had calves with heavier actual and 205-d adjusted weaning weights than did heifers with moderate weight gains. Greater BCS at calving resulted in more heifers in estrus and more heifers pregnant by 40 and 60 d of the subsequent breeding season. Thin females should be fed levels during the last third of pregnancy to achieve a targeted body condition score of ≥ 6 at calving, whereas those in moderate-high to high-body condition at 90 d prepartum should be fed levels to maintain body reserves. Heifers that calve in poor body condition have lighter birthweight calves, a longer postpartum interval to return to estrus, and lower pregnancy rates during the following breeding season.
During the first 80 to 100 days following parturition, a heifer must: continue to grow at about 0.23 kg per day (0.5 lbs/d), support lactation for a suckling calf, resume estrous cyclicity, and conceive for its second pregnancy. The maintenance requirement for lactating heifers average about 20% higher than that for nonlactating heifers, but requirements are greatly affected by milk production potential. In beef cattle, peak lactation occurs at approximately 60 days postpartum and maximum yield has been reported to range from 4.1 kg/d to 13.6 kg/d (9 to 30 lbs/d).
It is clear that energy and protein requirements post-calving greatly exceed that of mid-gestation heifers and even late gestation heifers. These higher demands make it difficult to add body condition to heifers once they begin lactation. Because post-calving condition score and energy balance control ovulation, and condition scores of 6 or greater are required for high conception rates in heifers, both body condition at calving and level of nutrition postpartum are critical control points affecting pregnancy rates. Ciccioli et al. (2003) showed that primiparous cows fed to gain more weight for the first 71 days postpartum had a shorter interval to first postpartum estrus and ovulation, a larger dominant follicle at first estrus, and higher pregnancy percentage at the first estrus compared to cows fed to gain less weight. 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 lactation cow. Lalman et al. (2000) found that feeding high-energy diets postpartum to thin primiparous cows reduces the negative effects of prepartum nutrient restriction but does not completely reverse those effects. However, increasing dietary energy intake was associated with a curvilinear increase in milk yield and percentage milk fat and a linear increase in energy available for milk production; consequently a high-energy diet, rather than a moderate- or low-energy diet was necessary to improve the energy status of thin heifers.
For postpartum diets deficient in protein, additional dietary protein will increase intake and total diet energy. In addition, Wiley et al. (1991) reported that UIP fed to primiparous 2-year-old beef cows in early lactation increased postpartum weight gains and increased reproductive efficiency of those cows, regardless of prepartum nutrition. Dhuyvetter et al. (1993) concluded that when adequate protein is provided to lactating mature beef cows for optimal rumen function, the addition of UIP decreases weight loss. However, they also reported that cows fed less UIP had a shorter postpartum anestrus period than cows fed higher UIP diets. Other studies have supported the use of UIP to meet MP requirements in primiparous beef heifers in early lactation. Patterson et al. (2003) reported that while dietary supplementation with UIP had little benefit to heifer performance during gestation, weight gain was improved during lactation and pregnancy percentage as 2-year-olds was higher compared to heifers fed to meet CP requirements. In contrast, Strauch et al. (2001) reported that UIP supplementation did not affect body weight, body condition score, calf weight, milk production, or postpartum interval when MP was adequate.
Social interaction within beef herds dictates a lower status to smaller, younger animals such as replacement heifers. If harvested forage or supplements are fed to groups that contain both mature cows and replacement heifers, the intake of heifers is negatively affected by dominance aggression displayed by mature cows. In addition, mature cows are able to consume 27% more alfalfa and 50% more brome hay per unit of metabolic body weight than 10-month-old heifers. Improved forage utilization in cows seems to be partially due to increased digestive function, with cows have a faster rate of in situ NDF degradation and digestibility than heifers. These constraints illustrate that higher-quality diets are required for heifers than for cows, and the need to feed heifers separately from mature cows.
The nutritional development of heifers from birth to the time they become pregnant for their second calf is a critical component of cowherd management. Veterinarian can use targeted body weights and condition scores to monitor progress throughout heifer development. Meeting NRC recommendations for net energy and MP are consistently supported in research studies targeting heifers during periods from birth through their second breeding season. Supplementation with fat, minerals, and additional UIP has not been consistently reported to enhance reproductive function of heifers.
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