Nutrition
Although the neonatal calf stomach has four parts, the rumen is not functional at birth. At birth the abomasum is the only truly functional part and has a capacity twice that of the other compartments.
Liquid is shunted past the reticulorumen into the abomasum via the esophageal groove. The groove is closed by a reflex that is stimulated by the intake of liquids (milk is best, electrolyte and glucose water is usually effective, but straight water becomes ineffective in closing the groove fairly early in life). The first enzyme to start the process of milk digestion is salivary lipase. When whole milk enters the abomasum, casein is denatured by the acidic (pH 2.0) conditions. Rennin cleaves a specific peptide bond in K-casein, which in the presence of calcium ions causes coagulation of the casein proteins. Fat is entrapped within the coagulum, but the whey proteins, lactose, and soluble minerals and vitamins are excluded into the liquid portion (whey) as the coagulum contracts. The soluble components enter the small intestine within 2 to 3 hours after a meal, whereas the casein coagulum is digested more slowly.265 Casein is partially digested by the abomasal protease pepsin. Polypeptides released from caseins enter the small intestine for further digestion. Milk fat trapped in the abomasum undergoes some digestion by the enzyme pregastric lipase, which is secreted in the mouth. The products of its action are diacylglycerols and free fatty acids, which enter the small intestine for further digestion and absorption. Lactose is hydrolyzed to its component sugars, glucose and galactose, by the intestinal brush border enzyme lactase, and the monosaccharides are absorbed into the epithelial cells by specific active transport proteins.265 The age at which ruminal digestion begins depends on the diet of the calf. Transition to ruminal digestion begins at 3 to 4 weeks of age; the earlier dry feed is provided, the earlier ruminal development starts.The energy requirements of the calf may be divided into maintenance and growth requirements, with the magnitude of maintenance requirements influenced by environmental
■ TABLE 19.4
Maintenance Requirements for Metabolizable Energy as Affected by Body Weight and Environmental Temperature in Calves Less Than 21 Days Old
BW
(kg)
Environmental Temperature (° C)
20 10 0 -10 -20
(Maintenance ME, Mcal/day)
| 30 | bgcolor=white>1.281.63 | 1.97 | 2.38 | 2.67 | |
| 40 | 1.59 | 2.02 | 2.45 | 2.96 | 3.31 |
| 50 | 1.88 | 2.39 | 2.90 | 3.50 | 3.91 |
| 60 | 2.16 | 2.74 | 3.32 | 4.01 | 4.48 |
Data from National Research Council: Nutrient Requirements of Dairy Cattle, ed 7, Washington, DC, 2001, National Academy Press.
BW Body weight; ME, metabolizable energy.
conditions. The basal maintenance requirement for neonatal calves is between 44.7 and 52.4 kcal/kg/day.266,267 Energy requirements for growth are between 268 and 307 kcal/100 g of body weight gain.267,268 Since whole milk contains about 70 kcal/100 mL, a 40-kg neonatal calf needs about 3 L of whole milk for maintenance and an additional 4 to 5 L to gain 1 kg of body weight. Cold stress significantly increases energy requirements. A 40-kg calf at -20o C requires 3.31 Mcal/day to meet maintenance requirements and maintain body temperature compared to only 1.59 Mcal/day at 20o C.265 Similarly heat stress increases maintenance energy requirements by 20% to 30%.265 The energy requirements of calves managed at different temperatures are presented in Table 19.4.
The requirements for a sick, stressed calf are not well established.Conventional milk replacers contain 20% to 22% crude protein (CP),269 but calves benefit most from the increased feeding when milk replacers contain higher protein and lower fat (up to 30% CP with 15% to 20% fat).269 Digestible protein requirement for maintenance is 0.5 g/kg, and the amount required for growth is about 22 g/100 g gain in body weight. Digestibility of the protein is an important concern in evaluating milk replacers. Some milk replacers substitute legume proteins for milk-derived proteins. Legume-derived proteins contain antinutritional factors that include antigenic proteins, lectins, and trypsin inhibitors.270 Trypsin provides a protective mechanism against clostridial disease by cleaving the clostridial beta toxin.271 Soybean flour has been used experimentally to induce Clostridium perfringens type C enterotoxemia in lambs.272
Many calves are raised according to the “calf starter method,” which became popular in the early twentieth century.273 According to this method, calves are fed limited amounts of milk (approximately 10% body weight). Milk intake is limited to encourage intake of calf starter, facilitating weaning at about 7 to 8 weeks of age. Forage feeding is discouraged during the milk feeding phase, as it lowers starter intake. Rumen papillae growth is stimulated by production of volatile fatty acids (VFAs), in particular butyrate, which is highest in calves that are consuming larger volumes of starter.274 During the last decade several studies have challenged the “calf starter method.” Limiting how much calves are fed has been shown to lead to poor weight gains275 and a higher risk of disease and abnormal behaviors, indicating reduced calf welfare.276 Providing greater quantities of milk or milk replacer improves growth and feed efficiency.276 To achieve biologically appropriate growth, milk feeding rates are approximately twice those of the calf starter method.
This calf feeding approach is often referred to as the “accelerated feeding” method. A rule of thumb is to provide 1.5% of body weight as milk solids during the first week of life, then 2% of body weight from the second week of life until the week before weaning, when one feeding is dropped.277 Intake of starter lags behind that of calves fed on conventional systems, but increases at approximately the same rate once the amount of liquid is cut back.277 To avoid or minimize growth slumps around weaning, calves should not be weaned until they are consistently eating 1 kg of starter daily.265 With accelerated feeding programs it is not uncommon to see a postweaning growth slump. This may reflect poor rumen development due to delayed starter intake. Subclinical acidosis may also be a problem when well- grown calves with poor rumen development eat larger volumes of starter. Variable manure quality in a pen of calves is commonly observed in this scenario. Calves may also have a preoccupation with chewing on inanimate objects in the pen and an increased incidence of respiratory disease. The link between subclinical ruminal acidosis and risk of pneumonia is recognized in beef feedlots, but the mechanism is not yet known.278 Starter intake, nutrient composition, physical form, and palatability along with the age of the calf affect the risk of rumen acidosis. Experimental trials that feed forage to preweaned dairy calves have produced inconsistent results. The fermentation rate of a starter formulation influences the risk of rumen acidosis and the need to include forage in the diet. Grain type, particle size, and processing affect the fermentation rate of starch in the rumen. Trials in which provision of forage increased starter intake tended to be those feeding a pelleted starter. Conversely, trials in which forage had a negative or no effect on starter intake have often involved the feeding of a textured starter where starch fermentation is slow.279A rule of thumb for preweaning growth is for calves to double their birth weight by time of weaning at 7 to 8 weeks of age. Higher rates of growth observed early in life are associated with reduced breeding age280 and higher milk yield when the calf matures.281 One study reported that approximately 25% of the variation in first-lactation milk production could be explained by preweaning average daily body weight gain.281 Subsequent studies have reported variable milk production responses to preweaning growth. A more recent meta-analysis reported a positive yet less dramatic relationship between first lactation performance and preweaning average daily weight gain; however, the magnitude of the relationship was less than the relationship between the postweaning and puberty periods.282,283