To assure adequate passive transfer of antibodies, all calves should receive at least 2 L of high-quality colostrum within 6 hr of birth. Colostrum feeding should continue until calves are 3 days old, but the initial feeding of colostrum is critical for passive transfer of immunity.
After the period of colostrum feeding, the traditional nutritional strategy for raising dairy replacement calves has been to minimize liquid feed consumption, maximize solid feed consumption, stimulate early rumen development, and wean calves at a relatively young age (usually 4–8 wk). Although growth rates are less than maximal, feed costs are minimal. In addition, the risk of enteric disease after weaning is less than during the liquid feeding period, making early weaning beneficial in the management of enteric disease.
Under this system, targeted rates of gain for calves of the large dairy breeds are ~400–600 g/day for the first 3–4 wk of life. This requires a dry-matter intake of 600–750 g/day; ~450 g of this is supplied from liquid feed, which equates to ~4 L of milk or reconstituted milk replacer/day for calves weighing 40–50 kg at birth. This amount should be divided between at least two feedings/day. The remaining dry matter should come from a high-quality calf starter, which is a concentrate mixture specially prepared for young calves. As calves grow, the amount of liquid feed/day remains constant, and increases in growth rate are accounted for by increases in calf starter consumption.
Liquid feeds for young calves include milk, waste milk, excess colostrum, and milk replacers. Milk and excess colostrum can be high-quality feeds for suckling calves, but adequate biosecurity precautions, such as pasteurization and screening of cows for chronic infectious diseases such as bovine leukosis and Johne’s disease, need to be in place.
Milk replacers are designed to mimic bovine milk and thus contain a source of protein, fat, and carbohydrate. Protein concentrations in milk replacers vary from 18% to as much as 30% on a dry-matter basis but typically are 20%–25%. Protein sources in milk replacers vary and may substantially affect the quality of the replacer. Proteins derived from milk sources, such as whey protein isolate, delactosed whey, dried skim milk, and casein, are generally excellent sources of protein, but the quality of even these protein sources may be affected by processing methods. Other animal proteins such as plasma proteins also may be of good quality. Plant protein sources vary in their acceptability, particularly for calves <3 wk old. Appropriately processed plant proteins may be acceptable but are generally less desirable than animal proteins for calves <3 wk old. Plant proteins acceptable for use in milk replacers include soy protein isolate and soy protein concentrate; these proteins may be processed to reduce antigenicity and to remove antinutritional factors such as trypsin inhibitor. The degree of processing varies by manufacturer, and not all milk replacers containing these protein sources are of equal quality. Unprocessed soy flour is an unacceptable protein source for milk replacers.
Fat concentrations in milk replacers vary from 10% to as high as 30%, with most in the range of 15%–20%. Fat sources usually contain some coconut oil and major contributions from tallow, choice white grease, or lard. Lecithin and/or monoglycerides are usually added as emulsifying agents. Fat concentration substantially influences the energy concentration of milk replacer. In cold climates in which high energy consumption is critical for young calves, fat concentration should be ≥15%. The drawback of higher fat concentrations is that the rate of starter consumption is reduced as replacer fat concentrations increase.
Early introduction of solid feed is important for replacement calf rearing. Solid feed stimulates rumen development. Calves are born with small, nonfunctional rumens, and rapid rumen development is critical for early weaning. Rumen maturation is stimulated by the presence of fermentation products, particularly butyric acid. Thus, introduction of highly fermentable substrate into the diet is important to rumen development. High-quality calf starters are composed of highly fermentable carbohydrates in a mixture that is coarse in texture, contains few fine or powdery particles, and has a relatively high fiber concentration (~12%–15% NDF). The crude protein concentration should be ~18%–24% on a dry-matter basis. Calves should not be fed hay before weaning. Hay consumption may actually impede rumen development, because hay is less fermentable than concentrate.
A critical factor in stimulating starter consumption is the availability of fresh water. Calves should have readily available fresh water. Water consumption will vary greatly by calf but may be >4 L/day in addition to milk or milk replacer.
The lower limit of the thermoneutral zone for calves between birth and 3 wk of age is 20º C and for calves >3 wk old, 10º C. Maintenance energy requirements increase as temperatures fall below these values. To compensate for these increased energy requirements, milk replacers with fat concentrations ≥15% should be fed in cold weather. Furthermore, the amount of dry powder should be increased by 50 g/day for each incremental decline of 5º C below the thermoneutral zone. For example, if calves <3 wk old are receiving milk replacer powder at 450 g/day at ≥20º C, the amount should be increased to 650 g/day at 0º C and to 900 g/day at –25º C. The dry powder should be reconstituted with proportionate increases in the amount of water. Feeding this amount of liquid frequently requires more than two feedings per day. In addition to milk or milk replacer, fresh water should be made available at least twice per day.
Diarrhea is a common condition in young calves and frequently results in life-threatening dehydration. Electrolyte solutions administered orally can be beneficial in supporting hydration and successfully treating calves with diarrhea. To help control and correct dehydration, it may be necessary to reduce milk or milk replacer feeding for a brief period after the onset of diarrhea. Nutritionally, however, the objective is for calves being treated for diarrhea to be back receiving milk or milk replacer as soon as possible. With appropriate use of oral electrolytes, milk or milk replacer feeding should be reinstituted within 12–24 hr after the onset of diarrhea. Electrolyte solutions can be fed along with milk or milk replacer. Supporting the calf with adequate nutrition speeds restoration of the gut epithelium and generally improves calf health and immunity.