In order 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.
The onset of puberty is primarily influenced by age and weight within breed. 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 does not appear to be a primary regulator of puberty, as puberty does not occur at a constant percentage of body fat.
The 1996 NRC estimations of Mcal and metabolizable protein requirements 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 will be reached by the start of the breeding season .
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% of mature weight. Dual purpose breed heifers such as Braunvieh, Gelbvieh, or Red Poll tend to reach puberty at about 55% 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% of mature weight.
However, in well-managed herds, opportunities may exist to lower heifer development costs by lowering traditional target breeding weights. Some research from the past five years has indicated that a group of composite heifers reaching 50 to 55% of mature body weight at breeding had similar reproduction and first-calf production traits to heifers that reach 60–65% of mature weight. The key to making a lower target weight work is probably to start with a uniform set of heifers and to monitor them closely so that all reach the desired target.
Heifer nutrition needs
Target daily weight gains from weaning to breeding are usually around 1.5 lbs. per day. This level of gain does not require that heifers be housed in a dry lot and fed a high concentrate diet. Heifers can meet this level of gain on high quality forage. But often, available forage will not allow this level of growth without supplementation. Therefore, many heifer development diets are based on moderate- to good-quality forage and a supplemental energy (and possibly protein) source.
Several studies have indicated that heifers fed a higher starch diet (i.e. grain) reached puberty at a lower body weight compared to a higher-fiber diet with the same amount of calories and protein — even though the two diets resulted in the same body weight and fat reserves. How-ever, other studies have shown 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.
Fat supplementation of heifer diets is generally restricted to less than 5% of the total dry matter intake (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.
Nutrition influences fertility
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 rates compared with a low plane. This difference in reproductive performance 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.
While it has been shown that undernutrition can delay puberty in heifers, short-term fasting is generally less disruptive to reproduction in cattle than in swine and other monogastrics. But, recent research showed that cycling heifers that abruptly received a lower energy diet had dominant follicles that grew slower and reached a smaller maximum diameter than adequately fed heifers. In long-term studies with chronically nutrient-restricted heifers where the heifers lost 17 to 18% of their body weight, the heifers became anestrus.
Overfeeding heifers before breeding has also been demonstrated to have detrimental effects on pregnancy percentages. In one study, heifers that gained 1 to 1.5 lbs./day were more likely to become pregnant 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 for a period of time followed by a period of more rapid rate gain, but that reached the same target weight and body condition score pre-breeding as heifers 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 weight. This difference is probably related to the fact that increase in efficiency during a re-feeding phase is not constant — the efficiency of gain is higher during the early periods of the re-feeding phase and decrease over time.