Feeding the replacement heifer from weaning through first calving needs to be systematic, with defined goals. A targeted growth approach is based on defining the desired age at first calving, generally between 22 and 24 months of age, and knowing the mature size of cows in the herd. With these two values, one can determine the needed rate of gain from weaning to breeding to achieve 50%–55% of mature weight at 13–15 months of age based on desired age at first calving. From breeding to first calving, a desired rate of gain can be determined to achieve a postcalving body weight of >85% of mature weight.
After weaning, calves are not yet fully functional ruminants with a capacity to ferment lower quality forage sufficiently to achieve requirements. Weaned calves should receive the same starter grain fed prior to weaning to minimize digestive adjustment or reduced intake due to feed novelty. Grain should be provided up to a dry-matter intake of 2.5–3 kg/day; thereafter, additional high-quality forage should be provided. Energy requirements for adequate growth in calves 3–15 months old may be derived largely from good-quality forage. Meeting protein requirements during this period is challenging because protein requirements are high relative to energy requirements. Use of corn silage without restricting energy intake will result in greater fat accumulation but not optimal frame growth. Replacement heifers who have not reached breeding age require more protein to support lean body weight gain.
For sample diets for large-breed dairy heifers 3–15 months of age, See table: Example Dietary Characteristics for Growing Replacement Heifers Example Dietary Characteristics for Growing Replacement Heifers . The crude protein concentrations of these example diets are high, because there is excess RDP, most likely due to an underprediction of RUP potential in younger animals as a result of faster rate of passage. More typical prebreeding heifer diets would contain at least 15% crude protein, and up to 18%. Feeding a TMR is a reasonable approach to feeding weaned calves, although grain and hay diets can also be used.
After breeding, requirements for protein (13%–14%) and energy during gestation may be met with good-quality forages. In general, the feeding of corn silage during this period should be limited to no more than one-half of the diet dry matter, preferably less. This is to prevent fattening, which increases disease risk at calving. There are many options for feed ingredients in these diets, provided that nutrient intake meets requirements for the desired rate of gain. Because feed is the single most expensive cost of raising a heifer, use of low-cost ingredients and feed efficiency should be considered. Two approaches to feeding heifers are to provide ad libitum intake of a lower-quality diet or precision feeding of a higher-quality diet. Precision feeding entails limited intake to meet requirements. A lower intake results in slower rate of passage and greater feed digestibility. Reduced feed intake and greater digestibility results in less fecal waste to manage.
Insufficient growth rates result in either an older age at first calving, which increases the cost of heifer rearing, or smaller dams at first calving, which limits milk production and conception rates during the first lactation. Conversely, excessive growth rates, especially those associated primarily with fattening, can adversely affect subsequent milk production and also increase the risk of metabolic problems at calving. Target growth rates between weaning and calving will vary based on mature weight and age at first calving. Typical daily gain will range from 500 g/d for smaller breeds to 800 g/d for larger breeds. Weight and height should be measured frequently to assess whether the feeding program is achieving desired growth rates and frame development. The growth response of heifers to apparently similar diets varies greatly among farms, making monitoring of growth rates particularly important. For target body weights at a range of age increments for heifers of various mature body sizes, See table: Recommended Target Body Weights (kg) for Heifers of Different Ages Recommended Target Body Weights (kg) for Heifers of Different Ages .
In addition to diet, environment for the replacement heifer can greatly impact performance and outcomes. The different dietary requirements of calves at different ages dictate that they be kept in separate pens based on their age and size. Calves between weaning and 5 months of age should be kept in groups of 6 or fewer. Older calves may be kept in larger groups; however, the size of the animals should be fairly uniform. Sufficient bunk space and lying area should be provided for each animal within the group to ensure adequate heifer comfort and minimize stress.