Calf Nutrition from Birth to Breeding

Vet Clin Food Anim 24 (2008) 55–86

Calf Nutrition from Birth to Breeding James K. Drackley, PhD Division of Nutritional Sciences, Department of Animal Sciences, University of Illinois at Urbana-Champaign, 260 Animal Sciences Laboratory, 1207 West Gregory Drive, Urbana, IL 61801, USA

The most recent United States Department of Agriculture–National Animal Health Monitoring System national survey reported that preweaning mortality of heifers alive at 48 hours of age was 7.9% [1]. Although slightly lower than previous survey results, industry-average morbidity and mortality of preweaned dairy calves remains unacceptably high in North America. Disease agents and environmental stressors interact with nutrition to determine disease susceptibility [2]. Labor for care and individual feeding of calves before weaning is the major cost of calf production, but nutritional inputs are also more costly during this period. Nutrition of young calves therefore remains of paramount importance for calf health and profitability of dairy operations. Although various approaches can be used successfully, they all must deal with the unique physiology of the calf born as a preruminant and transitioning to a functioning ruminant. Key aspects common to all systems include the composition and amount of liquid feed, water availability, and the first starter feeds offered. This article focuses on nutrition of calves before weaning and to breeding age, with primary emphasis on the preweaning and transition phases.

Overview of digestive physiology of the calf At birth the calf is functionally a nonruminant. Postnatal development of the digestive system occurs over three general phases [2]. The first is the preruminant phase, which is the first 2 to 3 weeks of age when the calf consumes negligible amounts of dry feed and relies almost entirely on milk or milk replacer for its nutrients. As the calf begins to eat some starter feed, it moves into the second or transitional phase. During this time, which lasts until the calf is weaned, the initial fermentation of starter feed in the undeveloped

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reticulorumen leads to rapid expansion of volume and differentiation of the rumen epithelium so that the volatile fatty acids (VFA) produced from microbial fermentation can be absorbed and used. The third phase is the ruminant phase, which begins at weaning and lasts the rest of the animal’s life. The ruminant depends on fermentation of dietary carbohydrates for most of its energy in the form of VFA, and obtains a large share of its amino acid requirements from proteins in the microbial biomass. Escape or bypass of some dietary proteins and carbohydrates, and dietary fats (which are not fermented in the rumen), supply the remainder of the protein and energy required. During the preruminant phase the solids in milk or milk replacer are digested by the animal’s enzymes in the abomasum and small intestine. Reflex closure of the reticular (or esophageal) groove forms a passage between the esophagus and omasum to ensure the passage of milk solids directly to the abomasum without entry into the reticulorumen. The complement of digestive enzymes present at birth and during the preruminant phase allows highly efficient digestion of milk proteins, lactose, and dietary triacylglycerols, but is less able to digest non-milk proteins or polysaccharides such as starch, which places stringent limits on the types and amounts of ingredients that can be included in calf milk replacers without compromising growth or health. When whole milk enters the abomasum, the casein proteins are denatured to some extent by the acidic (pH w2.0) conditions that result from secretion of HCl by the parietal cells of the abomasal mucosa. The inactive enzyme prorennin is secreted in the preruminant abomasum and is converted to the active enzyme rennin by the acidic environment. Rennin then 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. Casein is partially digested by the abomasal protease pepsin, which is secreted as the inactive form pepsinogen and activated by acid. Polypeptides released from caseins by pepsin enter the small intestine for further digestion. In the small intestine, casein fragments and the whey proteins are digested by the pancreatic enzymes trypsin, chymotrypsin, carboxypeptidase, and elastase. Peptidases on the intestinal epithelial brush border complete peptide hydrolysis and a mixture of free amino acids, dipeptides, and tripeptides is absorbed by way of specific transport proteins that presumably are similar to those characterized in other species. Milk fat trapped in the abomasum undergoes some digestion by the enzyme pregastric lipase, which is secreted in the mouth but remains active in the acid conditions of the abomasum. The products of its action are diacylglycerols and free fatty acids, which enter the small intestine for further digestion and absorption. Pregastric lipase is active only on the third position of triacylglycerols, which in milk fat is enriched in butyrate and other

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short- and medium-chain fatty acids. The butyrate and other short-chain fatty acids released are absorbed in the small intestine and are mostly oxidized for energy by the liver before reaching the peripheral circulation. The medium-chain fatty acids from 8 to 12 carbons in chain length possess potent antimicrobial activity [3], so this preliminary release in the abomasum may work with the acidic conditions of the stomach to prevent entry of pathogenic bacteria into the upper small intestine. Pancreatic lipase, in the presence of colipase and bile salts, hydrolyzes diacylglycerols and the remaining triacylglycerols to 2-monoacylglycerols and free fatty acids. The 2-monoacylglycerols and bile salts are essential for emulsification of the lipid components into micelles, which enable the hydrophobic lipids to cross the microscopic unstirred water layer of the intestinal epithelium. The fatty acids and 2-monoacylglycerols are absorbed into the epithelial cells, where they are reconverted to triacylglycerols and packaged into lipoproteins called chylomicrons. The chylomicrons, consisting of specific apoproteins and phospholipids surrounding the core of triacylglycerol, are secreted from the cells into the extracellular space, where they are picked up by the lymphatic system and delivered to the vena cava. In this way, dietary fatty acids are delivered to skeletal muscle, heart, and adipose tissue for use. 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. Sucrase activity is essentially absent in ruminants and so sucrose cannot be used other than by lower gut fermentation. Pancreatic secretion of amylase and the intestinal activity of maltase are low at birth but increase markedly over the first few weeks of life [4]. As the calf begins to consume starter concentrates, the microbial population ferments carbohydrates to VFA. Butyric acid, and to a lesser extent propionic acid, stimulates differentiation of the ruminal absorptive epithelium into its characteristic papillae [5]. Volume and musculature develop in response to physical bulk in the rumen and not fermentation products. As the papillae become functional and are able to absorb VFA, the pH of the rumen stabilizes and begins to increase. Until the pH is stable at greater than 6.0 the ability of cellulolytic bacteria to thrive is limited [6]. The challenge from a nutritional standpoint is that the rumen and postruminal digestive tract must be sufficiently developed to use starches and other non-fiber carbohydrates and non-milk proteins to support nutrient needs for maintenance and growth after weaning.

Nutrient requirements Energy and protein Like other animals, calves require nutrients for maintenance and for growth. Maintenance functions are those basic functions needed to keep the

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animal alive, but also include maintenance of body temperature in cold (or hot) climates, immune responses to infectious challenges [7], and accommodating stress responses induced by transportation or uncomfortable surroundings. Growth is the accumulation of new body tissue. Growth in young calves before weaning mainly occurs in the skeleton and muscle systems. Tissue growth is largely a function of protein deposition in bone and muscle, with corresponding mineralization of the protein matrix in bone. Some fat (primarily phospholipids) is deposited as part of normal tissue growth, with additional surplus energy deposited within adipose tissues as triacylglycerol. Rates of growth expressed as the percentage increase of body size (either as weight or height) are highest at birth and decline steadily thereafter [8]. Early nutrition thus centers on provision of adequate energy and protein, while ensuring that all required minerals and vitamins are consumed in appropriate amounts and ratios to overall energy intake. The National Research Council (NRC) [9] established energy requirements for calves less than 100 kg body weight (BW) in units of metabolizable energy (ME), which is determined by subtracting losses of energy in feces, digestive gasses (methane), and urine from total feed (or intake) energy. In young calves, loss of energy in methane is negligible and is ignored [10]. The ME requirements for maintenance under thermoneutral conditions are approximately 1.75 Mcal/d for a 45-kg calf. Whole milk contains about 5.37 Mcal ME/kg of solids, which means that a 45-kg calf requires about 325 g of milk solids, or 2.6 kg of whole milk (about 2.5 L), just for maintenance. Because most milk replacers are lower in fat content than whole milk, they have less ME per unit of solids (4.6–4.7 Mcal/kg). Consequently, a 45-kg calf requires about 380 g of milk replacer (about 3.0 L as fed) for maintenance. Amounts of milk solids consumed above maintenance can be used for growth. When the ME content of milk or milk replacer is not known, it can be estimated from nutrient composition. For whole milk, NRC [9] defines ME as 93% of the gross energy (GE), whereas for milk replacers a more reasonable figure is 90% of gross energy (GE) [11]. When whole milk (or pasteurized waste milk) is fed, the GE (total potential energy) in milk solids can be estimated using one of the following equations [2,9]: GEðMcal=kg as fedÞ ¼ ð0:0923  fat%Þ þ ð0:0492  SNF%Þ  0:0564

ðEq:1Þ

GEðMcal=kg as fedÞ ¼ ð0:0911  fat%Þ þ ð0:0586  true protein%Þ þ ð0:0395  lactose%Þ

ðEq:2Þ

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Resulting values then can be converted to a dry solids basis by dividing by the total solids content of the milk (eg, 12.5%). When milk replacer is fed, GE is calculated as [2]: GEðMcal=KgÞ ¼ ð9:21  fat%Þ þ ð5:86  protein%Þ þ ð3:95  lactose%Þ

ðEq:3Þ

If lactose content is not known, it can be estimated as: 100  %fat  %protein  %ash  2

ðEq:4Þ

The ash content of high-quality all-milk protein milk replacers is typically 6.7% to 7.2%, so a value of 7.0% ash can be used as an estimate if actual ash content is unknown. Ash content is a valuable measurement to obtain on milk replacer in addition to protein and fat contents. Like energy, protein is required for maintenance and growth as a source of amino acids. Unlike energy, however, the protein requirements for maintenance are small (about 30 g/d for a 45-kg calf) and are not believed to be substantially altered by cold or heat stress. Protein requirements are mostly determined by the rate of growth. On average 188 g of protein are deposited for every kilogram of BW gain in calves, which would require 250 to 280 g of crude protein (CP) intake from milk replacer [9]. The practical outcome of these principles is that body deposition of protein in the growing calf is essentially a linear function of dietary protein intake over the range of protein intakes that would be encountered in practice [12,13]. This effect is unrelated to energy intake of the calf as long as sufficient energy is available to use the additional protein to deposit body protein [12–14]. The NRC system [9] establishes requirements for ME and protein as a function of BW and rate of gain (Table 1). In this approach, needs for maintenance are met first, with nutrients in excess of those needed for maintenance being available to support growth. The system is based on energy-allowable growth, with protein requirements calculated to provide the amino acids necessary to support the amount of growth allowed by available energy. Several important principles can be demonstrated in Table 1. First, for calves to grow faster, they need to be fed more milk or milk replacer, or, in older calves, they must consume more starter. Calves clearly respond to greater intake of milk or milk replacer with greater BW gains [15–18]. Second, CP required in the calf’s diet as a percentage of dry matter (DM) is low for maintenance but increases as rate of gain increases. Third, CP content of the diet appears to approach a plateau at about 27% of the DM, which is similar to the CP content of whole milk solids (about 26% on a DM basis). Finally, these relationships highlight the importance of matching dietary protein and energy intakes with the expected growth performance of the calf. For example, feeding twice as much of a conventional milk replacer with 20% CP does not provide enough

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Table 1 Requirements for metabolizable energy and apparent digestible protein for a 50-kg calf at different rates of body weight gain under thermoneutral conditions Rate of gain (kg/d)

ME (Mcal/d)

0 0.20 0.40 0.60 0.80 1.00

1.88 2.37 3.00 3.70 4.46 5.25

ADP (g/d)

Required DM intakea (kg/d)

CP requiredb (% of DM)

31 78 125 173 220 267

0.40 0.45 0.63 0.78 0.94 1.10

8.3 18.7 21.4 23.7 25.1 26.1

Abbreviations: ADP, apparent digestible protein; CP, crude protein; DM, dry matter; ME, metabolizable energy; NRC, National Research Council. a Amount of milk replacer DM containing 4.75 Mcal ME/kg DM needed to meet ME requirements. b Amount of CP needed in DM of milk replacer to supply the amount of ADP that matches ME provided, with NRC assumptions and equations. Assumes only milk proteins fed. Data from National Research Council. Nutrient requirements of dairy cattle. 7th edition. Washington, DC: National Academy Press; 2001.

protein for lean tissue growth, and the surplus energy is converted to fat [11–14]. Conversely, feeding a high-protein milk replacer (eg, 28% CP) designed for ‘‘accelerated growth’’ at conventional feeding rates (454 to 568 g/d) provides excessive protein to the calves, which is not able to be used for additional growth because energy is limiting [12]. In this case the excess protein is degraded and the nitrogen excreted in urine. The current NRC system [9] is a vast improvement over previous systems. The model allows for differences in BW, rate of gain, and environmental temperatures to be used in determining nutrient requirements. The model was derived from older literature data, however, many of which were obtained in heavier animals (often veal calves) fed whole milk or skim milk–based diets. The system has been shown to overestimate energy accretion [17,19] and to underestimate protein requirements [19] of calves fed typically current diets. Recent growth experiments at the University of Illinois and Cornell University [11,12,17,20] have provided a large data set in which Holstein calves were fed from shortly after birth to ending BW of 63 to 100 kg, and were fed ‘‘modern’’ milk replacers containing whey proteins. These data have allowed development of modified NRC equations that better predict ME and CP requirements [19]. An example of the values determined by these equations is shown in Table 2. Comparison with the values in Table 1 shows that the modified equations result in slightly lower values for ME, and slightly higher values for CP, than NRC [9] estimates. Calves require protein in the diet as a source of the essential (or indispensable) amino acids. Although the amino acid requirements for calves have not been determined as accurately as those for young pigs and chicks, estimates from divergent sources are in general agreement and the predicted profiles for optimal growth are generally similar to those for pigs [2]. Values

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Table 2 Nutrient requirements and estimated gain/feed for a 50-kg calf under thermoneutral conditions, using the Cornell-Illinois equations Rate of gain, kg/d

Dry matter intake, % BW

ME, Mcal/d

CP, g/d

CP, % of diet DM

Estimated gain/feed

0.2 0.4 0.6 0.8 1.0

1.05 1.30 1.57 1.84 2.30

2.34 2.89 3.49 4.40 4.80

94 150 207 253 318

18.0 22.4 26.6 27.4 28.6

0.38 0.63 0.77 0.86 0.87

Data from Van Amburgh M, Drackley J. Current perspectives on the energy and protein requirements of the pre-weaned calf. In: Garnsworthy PC, editor. Calf and heifer rearing. Nottingham (UK): Nottingham University Press; 2005. p. 67–82.

for the most widely cited set of requirements [21] are in Table 3. As would be expected from a teleologic standpoint, the predicted requirement profile is similar to that provided by whole cows’ milk. Arginine appears to be deficient in cows’ milk but this likely reflects partial synthesis in the young calf as in other species [21]. Effects of environmental conditions on energy and protein requirements Requirements discussed to this point assume that calves are under thermoneutral conditions, which means that they do not need to expend energy to maintain body temperature. The thermoneutral zone for calves less than 21 days of age is 15 to 25 C [22,23]. Above or below this range, calves must Table 3 Essential amino acid requirements (g/d) for a 50-kg calf growing at 0.25 kg/d, and comparison with whole cows’ milk Essential amino acid (EAA)

Required (g/d)

Required (% of lysine)

Cows’ milk (g/100 g EAA)

Cows’ milk (% of lysine)

Methionine Cysteine Methionine þ cysteine Lysine Threonine Valine Isoleucine Leucine Tyrosine Phenylalanine Phenylalanine þ tyrosine Histidine Arginine Tryptophan

2.1 1.6 3.7 7.8 4.9 4.8 3.4 8.4 3.0 4.4 7.4 3.0 8.5 1.0

26.9 20.5 47.4 100 62.8 61.5 43.6 107.7 38.5 56.4 94.9 38.5 109.0 12.8

4.6 1.4 6.0 14.5 8.2 11.7 10.4 17.3 9.0 8.7 17.7 4.8 6.4 2.5

31.7 9.6 41.4 100 56.6 80.7 71.7 119.3 62.1 60.0 122.1 33.1 44.1 17.2

Data from Williams AP. Amino acid requirements of the veal calf and beef steer. In: D’Mello JPF, editor. Amino acids in farm animal nutrition. Wallingford, Oxon (UK): CAB International, 1994. p. 329–49.

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expend more energy to maintain body temperature; in hotter temperatures they pant and sweat, and in colder temperatures they shiver and use other means to increase heat production. This increase in energy expended becomes part of the maintenance energy requirement. For calves older than 21 days, the lower critical temperature falls to about 5 C, which means they are more able to withstand colder temperatures because of increases in body fat content and hair coat. The increased maintenance energy requirement in cold temperatures is built into the NRC model [9]. Table 4 shows the effects of BW and environmental temperature on maintenance ME requirements in calves less than 21 days of age. Note that maintenance requirements increase for larger calves as should be expected. As environmental temperature decreases, maintenance requirements for ME increase. If calves are fed the same amount of milk or milk replacer as in thermoneutral conditions, less energy is available to fuel growth. A 45-kg calf at 20 C requires about 725 g/d of milk replacer powder just to meet maintenance requirements and maintain body temperature, compared with about 382 g/d of powder under thermoneutral conditions. Since the NRC [9] publication became available, empiric observations during winters in the Northeast and Midwest United States indicate that these values provide reasonable estimates of the effects of cold stress for calves in cold housing. When calves are bedded deeply in straw, the effects of cold stress are less and thus the NRC equations may overestimate maintenance requirements [24]. Heat stress also increases the maintenance energy requirements of calves, although the exact amount needed for cooling has not been as well quantified as the effects of cold stress. Estimates based on data for older growing cattle [9] would indicate increased maintenance requirements of 20% to 30% during heat stress. Free choice water availability and shade are critical to maintain body temperature in young calves. Sand bedding also helps calves dissipate heat better than straw or wood shavings. Based on data from an Israeli study [23], an additional maintenance requirement may be needed by young calves undergoing transport. On Table 4 Maintenance requirements for metabolizable energy as affected by body weight and environmental temperature in calves less than 21 days old Environmental temperature ( C) 20 BW, kg 30 40 50 60

10

(Maintenance ME, Mcal/day) 1.28 1.63 1.59 2.02 1.88 2.39 2.16 2.74

0

10

20

1.97 2.45 2.90 3.32

2.38 2.96 3.50 4.01

2.67 3.31 3.91 4.48

Data from National Research Council. Nutrient requirements of dairy cattle. 7th edition. Washington, DC: National Academy Press; 2001.

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average, this amount is about 100 g of powder for calves weighing 43 to 50 kg. Calves should be fed this increased amount (in addition to any needed for temperature allowance) for 14 days following transport [19]. Minerals and vitamins Requirements for vitamins and minerals in young dairy calves are less well defined than for the young of animals such as pigs and chickens or even for older cattle. The current NRC recommendations [9] for vitamins and minerals therefore are expressed as percentages of the dietary DM rather than as actual amounts (Table 5). Detailed discussion of vitamin and mineral requirements in cattle can be found in the NRC [9]. Table 5 Concentrations of minerals and vitamins in whole milk solids and recommended for milk replacer, starter feed, and grower feed Nutrient Minerals Ca, % P, % Mg, % Na, % K, % Cl, % S, % Fe, mg/kg Mn, mg/kg Zn, mg/kg Cu, mg/kg I, mg/kg Co, mg/kg Se, mg/kg Vitamins A, IU/kg D, IU/kg E, IU/kg Thiamina, mg/kg Riboflavina, mg/kg Pyridoxinea, mg/kg Pantothenic acida, mg/kg Niacina, mg/kg Biotina, mg/kg Folic acida, mg/kg B12a, mg/kg Cholinea, mg/kg

Whole milk

Milk replacer

Starter

Grower

0.95 0.76 0.10 0.38 1.12 0.92 0.32 3.0 0.2–0.4 15–38 0.1–1.1 0.1–0.2 0.004–0.008 0.02–0.15

1.00 0.70 0.07 0.40 0.65 0.25 0.29 100b 40 40 10 0.50 0.11 0.30

0.70 0.45 0.10 0.15 0.65 0.20 0.20 50 40 40 10 0.25 0.10 0.30

0.60 0.40 0.10 0.14 0.65 0.20 0.20 50 40 40 10 0.25 0.10 0.30

11,500 307 8 3.3 12.2 4.4 25.9 9.5 0.3 0.6 0.05 1,080

9,000 600 50 6.5 6.5 6.5 13.0 10.0 0.10 0.5 0.07 1,000

4,000 600 25 d d d d d d d d d

4,000 600 25 d d d d d d d d d

a Values for whole milk are from Toullec [74]. No requirements are specified for starter or grower by NRC [9] because ruminal synthesis is assumed to be adequate. b Decrease to less than 50 mg/kg for veal calves [9]. Data from National Research Council. Nutrient requirements of dairy cattle. 7th edition. Washington, DC: National Academy Press; 2001.

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From a practical standpoint, whole milk is considered to be adequate in all nutrients except iron and perhaps manganese and selenium. Milk replacers are supplemented with minerals and vitamins to meet estimated requirements, most of which are close to those of whole milk, so true deficiencies or imbalances are rare in practice. Starters are supplemented with most minerals and the fat-soluble vitamins. Because of ruminal synthesis, no requirements are established for supplemental B vitamins in starter. The calf does not have a dietary requirement for vitamin C. Recommendations by the NRC [9] generally are followed closely by nutritionists in the field except for Vitamins E and A, which deserve special mention here. A series of experiments [25–29] at Kansas State University has built a convincing argument that vitamin E intake greater than the NRC recommendations is beneficial to calf health. The latest NRC committee modestly raised the requirement for vitamin E from 8 IU/kg to 10 IU/kg dietary dry matter [9]. The committee declined to raise the vitamin E requirement further in the absence of large clinical studies demonstrating health benefits. In light of the data suggesting health benefits most manufacturers of milk replacers routinely supplement with much higher amounts of vitamin E. Recommended Vitamin A intakes were increased in the last NRC document [9] largely on the basis of a recent study in which liver stores of vitamin A in neonatal calves were measured by repeated biopsy [30]. In reality, supplementation rates in most milk replacers are several times greater than even the new recommendation. This practice seems to have stemmed from limited evidence implicating effects of large doses of vitamin A in improving health of young calves [28]. The NRC subcommittee, however, cautioned that the evidence for benefits of the higher supplementation rates was not convincing. Milk replacers designed to be fed at accelerated rates should not contain massive oversupplementation of vitamin A to avoid toxicity problems. Water Despite its importance and recognition as the most critical nutrient, water nutrition is often the weakest link on farms. Calves must be provided with additional free water beyond what is consumed as part of the liquid diet. Empty body tissue in young calves ranges from 65% to 75% of total BW [12,17] and thus is the component deposited in greatest amounts during growth. Development of starter intake clearly depends on water intake [31]. Water intake does not cause scouring in calves; rather, calves that scour voluntarily increase their water consumption if it is available [31]. Water ideally should be available at all times to young calves, but as a minimum warm water should be offered after feeding and midday in cold climates. Separating water and dry feed containers physically or with dividers keeps calves from slopping water into the dry feed.

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Practical feeding programs General approaches Although nutrient requirements for calves are actually much more complex than the industry generally has recognized, practical feeding systems still can be made simple. Components include a source of liquid feed (milk or milk replacer) and a highly palatable starter feed. Differences in systems reflect composition of the feeds and the amounts offered. Traditionally, calves in North America have been fed limited amounts of milk or milk replacer (typically 8% to 10% of birth BW) with starter offered for ad libitum consumption from the first week of life. This amount of liquid feed is much lower than ad libitum intakes, which are in the range of 16% to 20% of BW or 2% to 2.5% of BW as dry solids [32]. The restricted liquid feeding approach arose in an attempt to stimulate early intake of starter and to minimize input costs of higher-value feed. In addition, early milk replacers were of poor quality and were not well used by calves at higher feeding rates [2]. Restricted feeding allows only for maintenance needs and about 200 to 300 g/d of growth under thermoneutral conditions (Table 6). As starter intake increases, typically doubling every week, enough nutrients are consumed to allow calves to begin to grow rapidly [33]. A contrasting approach is to allow calves much greater intakes of liquid feed during early life, which is closer to natural conditions in which calves would have ad libitum access to milk. These systems have been called by various names, including accelerated growth, enhanced nutrition, intensified nutrition, or biologically appropriate growth. Milk feeding rates are approximately twice those of conventional systems. An easy rule of thumb is to provide 1.5% of BW as solids during the first week of life, then 2% of BW from the second week of life until the week before weaning, when one feeding is dropped [34]. Intake of starter lags behind calves fed on conventional systems, but increases at approximately the same rate once the amount of liquid is cut back [34,35]. To avoid or minimize growth slumps around weaning, calves should not be weaned until they are consistently eating 1 kg of starter daily. Various systems have been used to deliver the Table 6 Expected average growth rates for calves of various ages under different nutritional programs Program and stage

Expected growth rate (kg/d)

Conventional milk replacer, ad libitum starter, d 0–42a Accelerated milk replacer, ad libitum starter, d 0–42b Moderate milk replacer, ad libitum starter, d 0–42c Weaned calves, ad libitum starter, !0.5 kg/d forage, d 56–84

0.5–0.6 0.6–0.8 0.55–0.65 0.85–0.95

a b c

Gains during d 0–21 would be 0.2–0.3 kg/d. Gains during d 0–21 would be 0.5–0.6 kg/d. Gains during d 0–21 would be 0.4–0.5 kg/d.

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increased milk volume, including larger volumes in buckets or nipple pails with hand feeding or various ad libitum feeding systems [2]. Benefits of improved nutritional status in the first 2 to 3 weeks may be improved ability to withstand infectious challenges, reaching breeding age (and thus calving age) sooner, and possibly increased milk production [36]. More recently, programs that are intermediate in nature have become established. These programs allow liquid intakes between those in conventional and accelerated programs [35]. These programs are reported to result in less slump in growth around weaning and fewer digestive upsets in calves than more aggressive liquid feeding programs [35], while still providing improved nutritional status during the critical first 2 to 3 weeks. A comparison of expected growth rates for calves under thermoneutral conditions when fed varying amounts of milk or milk replacer is shown in Table 6. Choice of liquid feed Liquid feeds used include whole salable milk, non-salable milk (unpasteurized or pasteurized), and milk replacers. All can provide excellent results and the decision to use one or another largely comes down to economics and convenience. Salable milk usually has higher value when sold than when fed to calves, and most commonly either non-salable milk or milk replacer is fed instead. Many producers successfully use a pool of all non-salable milk (colostrum, transition milk, milk withheld after drug treatment) to feed calves. Non-salable or ‘‘waste’’ milk often is viewed as free feed, but this ignores the opportunity costs of producing it and highlights the need for effective mastitis control on the dairy. The risks for ingestion of potentially pathogenic organisms in nonpasteurized waste milk have been well documented [37], and the use of nonpasteurized waste milk for calves should be strongly discouraged by the industry and the veterinary community. California studies [38,39] showed that pasteurization increased growth rates of calves and that calves fed pasteurized nonsalable milk were worth $8.13 more than calves fed nonpasteurized milk. Increased availability of on-farm pasteurizers at affordable prices has resulted in widespread adoption of the practice, which has been shown to result in acceptable growth and health of calves under field conditions [40]. Although proper pasteurization is effective in inactivating the causative bacteria for Johne’s disease [41] and mycoplasma mastitis [42], producers in strict biosecurity programs may want to should feed milk replacer. Care should be taken to maintain proper function and conditions of pasteurization, to ensure disease control and also to avoid overheating, which can damage the proteins in milk and cause decreased protein use. Day-today variability in composition of pooled waste milk also decreases gains and increases risks for digestive upsets, particularly on smaller farms where

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the variability in composition may be greater than on larger farms where a larger number of cows contribute milk to the pool. Milk replacers are fed on a majority of United States dairy farms [43]. High-quality milk replacers are excellent liquid feeds for young calves. Milk replacers usually are less expensive per unit of nutrient supplied than whole salable milk. Although more expensive than surplus colostrum, transition milk, or pasteurized waste milk, milk replacers have advantages in consistency of product from day to day, ease and flexibility of storage, and disease control. Maintaining as much consistency as possible in the diet for young calves minimizes chances for digestive upsets. This consistency may be particularly important when calves are raised under conditions of increased stress, such as cold or wet weather or during outbreaks of disease. Reports of poor calf performance on milk replacers most often are attributable to selection of an inappropriate or poor-quality milk replacer, to underfeeding the calf, or to an underlying disease or sanitation problem. As implied earlier, milk replacer nutrient content should be matched to desired calf growth rates. For calves fed on conventional restricted feeding programs, a CP content of 20% to 22% maximizes lean tissue growth [12]. For calves on more aggressive liquid feeding programs designed to increase early growth rates, CP must be in the range of 26% to 28% (see Tables 1 and 2). The major variable that results in differences in energy content of milk replacers is fat. Increasing fat content (and thus energy content of the milk replacer) increases daily gains [44] but may decrease starter intake [45]. In isocaloric feeds or feeding programs, dietary fat is preferentially and efficiently deposited as body fat [20]. Lactose is more readily used as an oxidative fuel to drive protein synthesis. Under thermoneutral conditions, a lower fat content of the milk replacer favors lean tissue growth and development of starter intake. Higher fat contents may be more desirable in cold feeding conditions. Cold weather feeding strategies Maintenance ME requirements, but not CP requirements, increase during cold weather. To maintain growth under these conditions requires increased nutrient intake. Several strategies can be used: (1) Increase the volume of milk or milk replacer fed at each feeding. Producers using conventional programs could easily increase the amount of liquid feed offered at each feeding, up to amounts typically used in accelerated schemes. (2) Provide a third feeding daily. For producers who are already feeding larger amounts of milk or replacer, adding a third (midday) feeding would be preferable to increasing volume at each feeding. This practice increases labor requirements and many producers are reluctant to adopt the strategy. (3) Switch to a higher energy (higher fat) milk replacer. Producers using lower fat milk replacers (eg, 15%) should switch to 20% fat during winter months. (4) Supplement milk replacer with added fat or additional milk solids. Research has

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demonstrated that additional fat added to milk replacer boosts daily gains during cold weather [46]. (5) Add additional milk replacer solids to each feeding to increase the concentration of liquid offered. Although this strategy increases nutrient intake [46] it is critical that free-choice water be available and not limited. Along with these strategies for managing nutrient intake by way of the liquid feeds, it is important that starter be managed properly to stimulate maximum intakes. Feed must be kept fresh and free from water or manure contamination, and supplemental water must be provided.

Ingredient composition of milk replacers Proteins Milk proteins The digestive enzymes in the young calf are especially adapted to digestion of milk proteins. Conversely, digestion of many non-milk proteins is limited early in life, and develops gradually over the first few weeks [2,4]. No nonmilk proteins have been found to equal the growth performance and health resulting from use of milk proteins, and so all-milk formulas remain the gold standard by which to compare other protein sources. Originally dried skim milk was the main milk protein source used in milk replacers [2]. Providing that the skim milk was not heat damaged during drying, the protein was used with high efficiency. Since the late 1980s, however, skim milk protein has been almost entirely replaced by whey proteins because of the marked increase in price of dried skim on the world market [2]. Currently, all-milk protein products in North America are formulated using dried whey (which provides the lactose needed), whey protein concentrates (both high and low protein), and delactosed whey. Whey proteins are highly digestible by calves, with properly processed whey protein concentrates having protein digestibilities of 94% to 97% that of skim milk [47]. In comparative trials in which milk replacers containing whey proteins or skim milk protein, or combinations of the two, have been fed, calf performance and health have been similar between the two milk protein sources [48,49]. Although comparative trials are not available, delactosed whey tends to produce inferior results to dried whey and whey protein concentrates. Lower performance may be attributable to the slightly greater non-protein nitrogen fraction (and thus fewer essential amino acids for a given CP content) but likely also to its high ash content. Tanan [47] reported that samples of delactosed whey contained substantially greater concentrations of ash (w20%), sodium, and sulfates than whey or whey protein concentrate. As a result, the average osmolality of the delactosed whey samples was 475 mosm/L, whereas samples of sweet whey and whey protein concentrate averaged 228 and 193 mosm/L, respectively [47]. High ash content of milk replacers (O10% of DM) coupled with high osmolality is more likely to cause digestive

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problems in calves [47]. Sulfate is also found in greater concentrations in acid whey powders than in the more prevalent sweet whey powders [47,50]. Sulfate concentrations greater than 0.06% in milk replacers (as fed) have been associated with increased water content of feces and greater risk for scouring, possibly because of low digestibility of sulfate in young calves [50]. Whey proteins by their nature do not clot (coagulate) in the abomasum as do the caseins in whole milk or skim milk powder. Although casein may be retained in the abomasum for up to 6 hours after ingestion, whey proteins leave the abomasum generally within 3 hours [47]. The essentiality of abomasal coagulation and proteolysis of casein cannot be demonstrated experimentally [2,51]. Regardless, that whey proteins do not coagulate in the abomasum is irrelevant to their use and cannot be used as a quality indicator for milk replacers based on whey proteins. On-farm clotting tests using rennin are therefore of no value in assessing milk replacer protein quality. Non-milk or alternative proteins Milk components are the most expensive in milk replacer formulas and as such it has long been desired to decrease their use in milk replacers. During 2007 the price of whey and whey protein concentrates skyrocketed because of increasing demand from the emerging Asian markets and increased use in human food or supplement applications. Consequently, interest has heightened again in potential uses of non-milk proteins for young calves. Most non-milk protein formulas have 50% or less of the milk protein replaced by alternate proteins, although some may be as high as 60% to 70% replacement [52]. This upper limit is set by the nutritional constraints of the alternate proteins and by the need to include a certain amount of whey to furnish lactose as a carbohydrate source. To be used successfully in milk replacers, alternate proteins must be well used by the calf and possess acceptable mixing and solubility properties. When taking these factors into consideration, the most popular alternate ingredients have been soy protein concentrates and modified wheat proteins. From a potential nutrition standpoint and cost basis, soy proteins have long been a desirable alternate protein source. Compared with requirements (see Table 3), soy proteins are slightly low in lysine but the amino acid profile is reasonably well balanced relative to lysine; methionine and threonine are modestly low [47,52]. Soy flour (finely ground soybeans or soybean meal) contains several antinutritional factors, however, including indigestible oligosaccharides, antigenic proteins, trypsin inhibitor, and several other detrimental compounds [53,54]. Full-fat soy flour is not readily available to the industry but should never be used in calf nutrition anyway. Defatted and heated soy flour can be used for calves older than 3 weeks of age but still results in inferior performance [55]. Soy protein concentrates are produced by extracting soy flour with hot aqueous ethanol to remove the indigestible carbohydrates and inactivate the antigenic proteins, such as glycinin and b-conglycinin [52]. The ethanol then is removed by solvent extraction and the product is toasted and dried.

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Soy protein also can be precipitated to form soy protein isolate [52]. Because this is a more extensive processing technique it results in a more expensive product that is targeted primarily into human food applications. Much of the soy protein isolate available to and affordable by the animal feed industry therefore is off-specification material that cannot go into the human food industry. As a result quality of soy isolates is extremely variable. In addition, solubility of soy isolates in milk replacers often is not good. The product is thus not as popular for milk replacer manufacture as soy protein concentrates [47]. Use of soy protein concentrate in milk replacers largely prevents digestive upsets but still results in decreased performance relative to whey proteins [56]. Most of the high-quality soy ingredients used by reputable manufacturers of milk replacers do not cause digestive problems under reasonable calf management, but growth performance may be decreased. This decrease might be acceptable in early weaning programs in which calf starter quality and management are satisfactory. Reasons that soy proteins do not produce performance similar to milk proteins have not been completely elucidated. Most antinutritional factors are inactivated by heat treatment, and antigenicity is greatly decreased by the hot aqueous ethanol treatment involved in production of SPC [53], yet adverse effects on growth and intestinal function still occur when SPC is fed to calves [57]. Inclusion of soy flour [58] or soy protein concentrate in replacement for skim milk or whey proteins decreases intestinal villus height when fed to young calves [55,56,59–61]. The cellulose and hemicellulose present in SPC may increase villus abrasion and cell desquamation and also increase mucus loss in the terminal small intestine [62]. In addition to alterations in villus size, various other intestinal abnormalities have been observed in calves fed low-antigenic soy protein products, including decreases in protein synthetic capacity [61], mucosal digestive enzyme activities [57,61], and absorptive capacity [59–61]; and increases in mucin secretion [63], immune activation [54], and specific endogenous protein loss [64]. Young calves have less well-developed enzyme systems to digest non-milk proteins compared with older cattle [2,4,53]. Nevertheless, plant-based proteins have high true digestibilities that are not greatly different from milk proteins. Apparent digestibilities are lower, however, because specific endogenous protein losses at the ileum are increased [63,64]. Montagne and colleagues [65] suggested that resistant dietary oligopeptides may interact with intestinal mucosa to stimulate endogenous protein secretion. Because endogenous protein loss has a different amino acid profile than body tissue, it is possible that the combined effects of dietary amino acid balance not being optimal and increased endogenous secretion might lead to lower performance. For example, even when lysine and methionine are equalized, another indispensable amino acid, such as threonine (which is rich in intestinal secretions and mucin), might limit calf performance relative to whey proteins [66].

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Wheat gluten is a by-product of wheat starch production. Native gluten is extremely viscous and is not readily soluble in aqueous solutions [67]. Modified wheat glutens are produced by mild acid or enzymatic hydrolysis. Modified wheat protein has good physical characteristics, possesses no antinutritional factors for young calves [52], and has high intestinal digestibility. Calf performance and health are acceptable but growth rates typically are 15% to 30% less than with all-milk proteins, particularly during the first 2 weeks of life [2,67]. Wheat proteins are markedly deficient in lysine and threonine, but are relatively high in the branched-chain amino acids (valine, isoleucine, and leucine). Among other possible protein sources for calf milk replacers, the most promising seem to be animal plasma proteins, which are still permitted to be fed to animals in the United States. Calves fed milk replacers containing plasma protein had similar [68] or improved performance [69,70] relative to all-milk controls. These proteins have good acceptability by calves, are highly digestible, and possess a reasonable amino acid profile, with the exception of very low isoleucine content. The upper limit for inclusion of plasma proteins has not been determined in calves because until recently they have been considered too expensive relative to whey proteins. Several other alternate proteins have been investigated [2]. Egg protein possesses an extremely well-balanced amino acid profile that has been used as a standard for protein quality determinations in laboratory animals. Calf performance when fed egg proteins has been disappointing [71,72], however, for reasons that have not been determined. Potato proteins have been investigated in Europe but are not widely used because of deficient amino acid profiles, physical constraints, and cost [2,64]. Fish protein preparations, such as fish protein hydrolysates and concentrates, have produced good results in calves depending on manufacturing process and starting material [2]. Their availability is limited, they are not easy to handle in milk replacers, and they may have objectionable odor (more a problem to human caretakers than to calves). Less highly processed fish flours, like soy flour, should not be used in calf milk replacers. Single-cell proteins, such as bacteria and yeast, have been researched and can be used in smaller amounts by calves, but are not widely available [2]. Meat solubles, whole blood proteins, pea flour, and lupin flour are not good alternatives for young calves and are not recommended [2]. Energy sources Carbohydrates Lactose is the main carbohydrate found in milk, averaging about 4.9% on an as-fed basis and about 39% on a dry solids basis. The digestive physiology of the young calf obviously is geared toward use of lactose as the major carbohydrate source. Typically, milk replacers have contained a greater amount of lactose than whole milk solids because of the lower fat content

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in milk replacers than in whole milk solids [52]. Lactose is almost completely digestible and the capacity for its use is high. When intakes of milk replacer solids are high (O0.9 kg/d) there may be some lactose that is not digested in the small intestine and is fermented in the large intestine, particularly if the calves are fed only twice daily [2,47]. This situation normally is not a problem for the calf. Adequate colostrum intake has been shown to be important for stimulating digestive and absorptive capacity for lactose in the intestinal tract [73], which may be part of the reason that colostrum-deprived calves do not grow as well as colostrum-adequate calves even in the absence of disease. As prices for whey and lactose have increased in recent years, interest has been renewed in alternate carbohydrate sources for young calves. Given the delayed development of digestive capacity for starch, replacement of large amounts of lactose by other low-cost carbohydrate sources is difficult. Small amounts of pregelatinized or hydrolyzed starch can be tolerated by calves less than 3 weeks old, with greater amounts usable by older calves [74]. Maltose also can be used in small amounts. Glucose (dextrose) and galactose are well used by calves. It has been suggested that the total amount of lactose replacement by glucose, maltose, and processed starch should not be greater than 8% to 10% of total dry matter [74]. Calves lack the enzyme activity sucrase and so cannot digest sucrose. In fact, sucrose results in severe osmotic diarrhea if fed to young calves [75]. Small amounts of fructose (a component of sucrose) can be tolerated by calves, as can sorbitol (a related sugar alcohol). Reports of use of other potential carbohydrate sources, such as dextrins and glycerol, do not seem to be available in the refereed scientific literature. These compounds are well used by young pigs [76,77]. Fats Because of the greater expense of milk fat, tallow, lard, and choice white grease are the most widely used fat sources in milk replacers in North America. Coconut oil and palm oil also are used. All of these fat sources are well digested by young calves and produce satisfactory results, although coconut as the only fat source causes fatty liver in calves [78]. Consequently, coconut oil usually is fed as a combination with other fat sources, such as in a 1:1 ratio with tallow [79]. In the European Union, animal fats no longer can be fed to calves or adult cattle. The most common replacement has been a mixture of 80% palm oil and 20% coconut oil [47]. Palm oil supplies sufficient amounts of the polyunsaturated essential fatty acids (linoleic and linolenic acids) to prevent deficiency. Feeding only vegetable oils rich in polyunsaturated fatty acids in milk replacers also was reported to cause fatty livers [80]. Early reports suggested that polyunsaturated vegetable oils or free fatty acids caused digestive upsets and poor growth [81], although it is now apparent that much of the problem in the early studies was poor emulsification of the fat in the milk replacer [82]. Free fatty acids are not absorbed well by the preruminant calf and

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cause decreased performance [83], possibly because the ruminant lysolecithin-based emulsification system for free saturated fatty acids has not yet developed. Bovine milk fat contains 4% to 5% by weight of the short-chain fatty acid butyrate (C4:0) and substantial amounts of the medium-chain fatty acids from C8:0 to C14:0 [84]. These fatty acids may have bacteriostatic effects in the upper small intestine [3], which may be at least a partial explanation for the anecdotal improvements in gastrointestinal health when scouring calves are switched from milk replacers to whole milk. Hill and colleagues [85] reported that supplementing milk replacers with sodium butyrate or with coconut oil (rich in medium-chain fatty acids) improved average daily gain (ADG) and decreased scour days in calves. They also reported improvements in ADG and health when milk replacers containing animal fats were supplemented with canola oil (to provide the essential fatty acids C18:2 and C18:3) [80]. Commercial milk replacer manufacturers usually incorporate fats into milk replacers as spray-dried blends of protein (usually whey protein concentrate) and fat, which greatly improves dispersion of the fat within the milk replacer and simplifies handling in the manufacturing plant. In addition, emulsifiers, such as lecithin and monoglycerides, may be used to further stabilize fat suspensions [52]. Care should be taken to mix the milk replacer at the water temperature specified by the manufacturer and to use a minimum of harsh mechanical mixing to avoid breaking down the fat suspensions [2]. A major challenge to producers who create milk replacers on-farm is to ensure dispersion of fat into droplets less than 3 mm in diameter [86] and keep this fat emulsion stable in solution. Scouring and decreased performance often result when fat is not properly incorporated into the milk replacer. Additives The feed industry is awash with various additives that are purported to improve health or growth of young calves. Unfortunately, the likelihood of finding a ‘‘magic bullet’’ supplement for improved health in calves is remote. Nevertheless, supplements may be helpful in improving health or growth of calves. One of the problems in evaluating the current literature on such additives is that most have been tested in calves fed at restricted intake, and thus have been more likely limited by energy and protein intakes than by the ingredients in the additives. The usefulness of additives may be enhanced if tested against the background of more adequate nutrition. Perhaps more importantly, for many of these additives biologically significant increments in performance or improved health may be too small to detect statistically in many experiments because of inadequate numbers of calves. It also is possible that several of the components discussed here may each provide small improvements that are additive, such that a product

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combining several different types of compounds might show substantial efficacy. One hope is that such combinations might be able to compensate for antibiotics if use in livestock feeding is eventually banned in North America. For example, a product containing probiotics, fructooligosaccharides, and allium supplemented to milk replacer without antibiotics provided similar calf performance and health as a medicated milk replacer [87]. The use of growth-promoting antibiotics in all animal production is coming under increasing criticism. Antibiotics typically added to ‘‘medicated’’ milk replacers (primarily oxytetracycline and neomycin) still are clearly effective in improving performance and health of calves [2,88–91]. For producers who have excellent sanitation and good nutrition programs, medicated milk replacers probably are not necessary. Under less than optimal management, however, or where producers are bringing calves from other farms or unknown sources on their farm, medicated milk replacers are almost always beneficial [2,91]. Coccidiosis is a common clinical or subclinical cause of poor performance, illness, and economic loss in dairy calves. Use of coccidiostats (decoquinate, lasalocid) in milk or milk replacer has been shown experimentally to lessen the effects of coccidiosis and improve gains [92–94]. Although use of coccidiostats in starters has been widely recommended and adopted, calves may not consume sufficient amounts of starter before 4 weeks to prevent infection during the first 2 to 3 weeks of life [94]. Probiotics are live bacterial cultures designed to increase colonization of the digestive tract with species that produce favorable effects and compete with pathogenic microorganisms for nutrients and attachment sites. Various probiotic bacterial cultures have been examined and promoted for use in dairy calves. Two probiotic products, a multispecies human-derived product and a calf-specific Lactobacillus product, were effective in increasing ADG and decreasing incidence and severity of disease in veal calves [95]. A combination of Bifidobacterium pseudolongum and Lactobacillus acidophilus in the diet of preweaned calves increased ADG and decreased scouring [96]. Calves fed a probiotic (composition not reported) without antibiotics had growth rates and health measures similar to those of calves fed a similar milk replacer with antibiotics [69]. Small but generally positive effects from bacterial probiotics have been noted in other studies [97–100]. Calves fed a live yeast product in milk replacer or in starter showed fewer days with diarrhea and improved ADG [101]. Prebiotics are compounds that are indigestible by the host animal’s digestive enzymes but are usable by microorganisms. These compounds provide substrate for growth of beneficial bacteria in the intestine or compete with pathogenic bacteria for intestinal attachment sites. Complex carbohydrates, such as oligofructose, mannanoligosaccharides, and others, have shown promise in young calves [93] and other species [102,103]. Calves fed mannan-oligosaccharides in milk replacer had improved health and greater starter intake than calves fed a control milk replacer or milk replacer plus

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chlortetracycline and neomycin [90]. Calves fed the trisaccharide galactosyllactose in milk replacer had greater ADG and fewer days scouring than control calves [89]. Interest has grown recently in the potential effects of various botanicals and essential oils in animal production. A series of experiments was reported in which a commercial mixture of plant-derived botanicals was fed in milk replacer for young calves [104]. Calves fed the botanicals had greater ADG and improved feed efficiency compared with the controls. There is considerable interest in other nutraceutical compounds for calves, such as garlic extracts or other herbal products. For example, allicin, a component of garlic, has antimicrobial properties against various microbial classes. A clinical trial demonstrated that allicin had little effect on the infection by Cryptosporidia in young calves [105], however. A recent study documented improvements in health and growth from addition of fresh or autoclaved rumen fluid to young calves’ diets [106]. The mode of action for the effect is uncertain but may be related to stimulation of the immune system by bacterial polysaccharides present in the rumen fluid extracts. Supplements prepared from serum or immunoglobulin fractions also may have promise for improving calf health [70,107].

Nutrition of calves consuming both milk and starter General aspects As calves begin to consume dry feed, the rumen develops in microbial population and absorptive function [5,108,109]. The source of nutrients begins to change from products absorbed directly from digestion of milk or milk replacer to a combination of dietary ingredients and end products of microbial fermentation (VFA, microbial protein). As discussed by Davis and Drackley [2] and NRC [9] the efficiency of use of nutrients during this transition is not greatly different between calves consuming only liquid diets and those consuming both starter and milk (or milk replacer). The only differences arise from the digestibility and metabolizability of dietary ingredients. Until calves are weaned the same principles of nutrient requirements can be applied to calves during this transition period. Fermentation of dry feeds to the VFA butyrate and propionate is necessary to drive growth and differentiation of the ruminal absorptive epithelium [5]. Increasing starter intake sustains a self-perpetuating feed-forward regulatory system that improves the ability of the calf to make use of the end products of the intake. It should be obvious that the key limiting factor here is getting calves to eat the dry feed at an early age, assuming that the starter is formulated with easily fermentable ingredients. Palatability and acceptability of the starter formulation to the calves therefore assume paramount importance. Several high-performance commercial calf starters are available in North American markets. Good calf starters allow high rates

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of fermentation and microbial protein synthesis, yet still provide some undegraded or bypass protein and starch to be digested in the lower tract. Whether or not to feed forage to young calves during the period they are receiving milk and starter has been controversial. Some of this arises from the notion that it is natural for calves to consume forage. Cattle developing under wild conditions, however, undoubtedly calved in the spring when grass was immature and abundant. The first solid feeds available for consumption by the young calf then would be fresh immature grasses that are rich in sugars and relatively low in fiber, and the fiber present would be nonlignified and highly digestible [9]. The high sugar content likely fermented to a relatively high proportion of butyric and propionic acids, which would be ideal to stimulate rumen development. In contrast, most forages offered to young calves in confinement are low in sugars and higher in more poorly fermented fiber. Use of typical forages by the underdeveloped rumen of calves today is clearly limited for several reasons. Although populations of cellulolytic bacteria become established within a few days after birth, they cannot increase to numbers necessary for significant forage fermentation until the rumen pH stabilizes at or greater than 6.0 [6]. The initial fermentations in the rumen result in pH being less than 6.0 for most of the first 10 weeks of life [109]. Use of cellulose is therefore severely limited during this time. Because the physical size of the rumen also is limited in the young calf, accumulation of undigested forage material in the rumen decreases voluntary intake of starter that could be fermented. Decreased availability of VFA from fermentation means a slower development of the absorptive papillae in the rumen, which in turn contributes to decreased ability to absorb fermentation acids to help raise rumen pH. Consequently, the amount of forage that can be digested to supply nutrients for the young calf is small. On the other hand, some fiber may be needed to maintain an abrasion factor to prevent abnormal development of rumen papillae [110]. If complete starters are fed that contain some ‘‘long’’ particles, such as rolled oats, alfalfa meal, beet pulp, or cottonseed hulls, then supplemental forage is not needed, especially if calves are bedded on straw. Calves fed only a pelleted starter ration and bedded on sand would likely do well to receive a small amount (!0.5 kg/d) of chopped or fine-quality grass or legume hay. Protein in starters As fermentation of the dry feed ingredients becomes established, so does the supply of microbial protein from the developing microorganism populations in the rumen. Relatively little is known about the dynamics of microbial protein production and dietary protein breakdown in the immature rumen, although clearly intestinal metabolizable protein supply is a combination of microbial and undegraded dietary protein [108]. Because of the paucity of data available to develop predictive models, prediction of metabolizable protein supply in young calves cannot be considered reliable at present.

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The NRC [9] lists the CP requirement for calf starters to be 18% of DM on an as-fed basis (about 20% of DM). Consistent with this recommendation, studies in which CP was titrated into calf starters showed responses up to 18% CP but no further increases in growth of calves when starter CP exceeded 18% [111,112]. Somewhat greater CP content may be beneficial for calves raised on accelerated programs, particularly in sustaining intake during the weaning transition and stimulating greater growth postweaning [34]. Efforts to increase metabolizable protein supply to the intestine by providing rumen-undegradable proteins in starter have largely been unsuccessful, as reviewed by Hill and colleagues [113], perhaps because of corresponding decreases in fermentable carbohydrates, decreased palatability, or imbalances between rates of carbohydrate fermentation and protein degradation. Because the rumen microbial activity is not fully developed, ruminal degradability of protein sources likely also is less than in adult cattle [113], which also might decrease the likelihood of response to additional rumenundegradable proteins. Imbalances in amino acid profile between the escape protein and microbial protein also may be a factor in lack of response. There is little evidence to indicate that protein sources other than soybean meal are necessary in starters for young calves. Energy sources in starters The main portion of energy in starters is derived from cereal grains, either corn or barley in most of North America. Studies that have examined various processing techniques for cereals have shown relatively small effects of extensive processing. For example, studies at Kansas State University showed minimal effects of processed corn compared with dry rolled corn [114]. Processing of sorghum grain had larger effects [115] as might be expected given the harder seed coat and more resistant starch in sorghum compared with other cereals. Lesmeister and Heinrichs [116] found increases in ruminal butyrate and stature growth when calves were fed roasted rolled corn compared with other treatments. Calves fed whole corn or dry rolled corn had better gains postweaning, suggesting that more fermentable grains during the preweaning period may be desirable but then less fermentable grains after weaning may produce slightly better performance. Fat as an energy-rich feedstock might seem to be logical for improving energy intake and thus growth in young calves. Studies in which fat has been added incrementally to starter consistently show decreased starter intake, and decreased or unchanged growth performance [45,117–120]. Addition of fat to starters beyond a small amount (eg, 1%) to aid in pellet formation or to decrease dustiness is therefore not recommended. Additives Several compounds are approved for use in starters as coccidiostats (decoquinate, monensin, lasalocid). The latter two compounds also have

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been demonstrated to improve feed efficiency in growing ruminants [113]. All starters should contain one of these compounds. Because effective dose as a coccidiostat depends on projected intake of starter DM, lower intakes than desired may make the compound ineffective in controlling coccidia infection and so coccidia control should begin in the liquid diet. Several studies have examined the use of probiotics, direct-fed microbial products, and prebiotics in calf starters. In general, results are variable and inconclusive; overall there is less evidence for efficacy of these products in starters than in milk replacers. Lesmeister and colleagues [121] found that yeast culture increased starter intake and ADG when it was included at 2% of starter DM. The same blend of plant botanicals that was effective in milk replacer also was effective when included in starter in increasing ADG [104]. Few other data have been published concerning the effects of botanicals or essential oils in calf starters. Trials have been conducted with various flavor compounds in an attempt to improve starter intake [122,123]. Results in general have not been positive, and calves seem to prefer the taste or smell of the diet they are familiar with regardless of what that is [113].

Nutrition from weaning to breeding General aspects Calves should be maintained on the starter ration until they are ready to move from the transition pens at approximately 10 to 14 weeks of age. At this time, they can be transitioned to a grower ration of simpler composition and lower protein content (approximately 16% CP on a dry basis). Forages also can be introduced at this time. Dry hay should be limited to ensure that calves consume the appropriate amount of grower diet, which is typically in the range of 2.5 to 5 kg daily as calves grow from 100 kg to breeding age. As more and more farms rely on corn silage as the major forage, careful attention must be paid to avoid excessive energy intake by growing heifers. Heifers must either be limit-fed [124,125] or some lower-energy feed (straw, cottonseed hulls, low-quality chopped hay or silage) must be blended with the corn silage. Producers should work closely with their nutritionists and veterinarians to ensure that diets deliver an appropriate amount of metabolizable protein to the lower digestive tract to support lean tissue growth without fattening. Use of various nutritional models, such as those provided by the NRC [9] or the Cornell Net Carbohydrate and Protein System [126], allows prediction of metabolizable protein supply, which can be quite different than dietary CP content. For example, diets based heavily on grass or legume silage may be high in CP (20%–24%), but because much of this is NPN the amount that can be used depends on supply of fermentable carbohydrates

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to capture the NPN as microbial protein. Situations can arise in which the supply of metabolizable protein may actually be deficient for optimal lean growth despite dietary CP concentrations that seem to be considerably in excess. A long-standing recommendation has been to avoid feeding silages to growing heifers until about 6 months of age. The origin of this recommendation is obscure. It is clear that heifers can thrive when fed corn silage or alfalfa silage from a relatively early age, particularly when fed total mixed rations (TMR). A common practice that works well is to begin to move heifers onto TMR in the early grower phase by mixing the starter grain with small amounts of the high-group lactation TMR in DM ratios ranging from as little as 9:1 to as much as 4:1. After several weeks on this regime, the heifers can easily move to a TMR containing substantial amounts of silages with no detrimental effects. Postweaning growth considerations Diets should be formulated to allow heifers to reach breeding age as quickly as possible without fattening. The target BW at first breeding is 55% of mature BW, with heifers calving for the first time at 82% of mature BW [9]. For Holstein heifers calving at 22.5 months at a target postpartum BW of 526 kg, the target BW gain prebreeding is 0.87 kg/d [9]. Although effects of prebreeding and postbreeding BW gain have been controversial, the preponderance of data indicate that growth rates between 0.8 and 0.9 kg/d are safe provided that metabolizable protein supply is adequate as predicted by NRC or Cornell Net Carbohydrate and Protein System models. Much of the concern about pre-breeding BW gain stems from early research that suggested impairment of mammary epithelial development (measured as decreased DNA accumulation in mammary parenchymal tissue) and decreased milk production during first lactation when heifers grew too rapidly (see review by Sjersen [127]). This negative effect was associated with greater deposition of fat in the developing mammary gland, and interpreted as a competition for tissue establishment with potential epithelial tissue. It is logical to assume, however, that if fat is being deposited in the mammary gland it also is likely being deposited elsewhere throughout the body. The negative consequences of overconditioning in cows of any age are clearly documented and well accepted. Decreased milk production is therefore likely attributable to general over-fatness [9]. Recent studies [128,129] at Cornell University have established that the rate of DNA accumulation in mammary parenchymal tissue is determined by chronologic age rather than rate of BW gain. Heifers were grown at rates of 650 g/d or 950 g/d and harvested at the same BW in 50-kg intervals. Heifers grown more rapidly reached the same BW at a younger chronologic age; for example, heifers grown more rapidly reached puberty

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approximately 108 days earlier than heifers grown more slowly. At the same BW, heifers grown at the higher ADG had less parenchymal DNA in the mammary glands, consistent with previous research. The rate of accumulation of DNA (mass per day) was similar between groups, however, and there was no impairment of parenchymal proliferation rates measured in vivo. The rates of parenchymal DNA accumulation in heifers are thus independent of plane of nutrition and are determined by chronologic age. These studies have effectively refuted the long-held dogma that higher ADGs before puberty are detrimental to future production by impairing mammary development. As long as diet formulation provides sufficient metabolizable protein to favor lean tissue gain and minimize overall body fattening, there is little basis for concern about target growth rates between 0.8 and 0.9 kg/ d to breeding age. Summary The general principles of growth and nutrients required are no different for young calves than for young pigs or any other species. Additional complexity is introduced, however, by the need to transition the young preruminant to functioning ruminant. The nutritional and digestive physiology of dairy calves as future ruminants needs to be the governing factor in designing practical feeding systems to meet nutrient requirements. Several strategies are available for raising young calves, ranging from restricted intakes of milk or milk replacer to more natural milk intakes that allow greater early growth but slightly delay development of solid feed intake. The principles behind these strategies should be understood to counsel producers on best management practices. Growth and health are intimately interrelated in young calves, and evaluation of existing or potential programs should be on an evidence-derived, results-oriented basis. Just as there are various nutritional strategies that can be successful (and systems to implement those strategies), there are many types of feeds available before and after weaning. It is important that appropriate feeds be selected for the given system to optimize performance. Likewise, differences in ingredients that affect price of feeds also may be reflected in differences in performance. Those working with farms to advise and troubleshoot must be able to relate these differences in cost–value propositions to the farm owners or managers. Appropriate nutrition postweaning until time of breeding centers on supplying sufficient metabolizable protein while avoiding excessive energy. Growth rates of 0.9 kg/d are entirely feasible with such systems and do not impair either mammary development or future productivity. As more is learned about the effects of early nutrition and growth on long-term health and productivity, it is likely that recommendations will continue to be modified to help ensure animal well-being and improve farm profitability.

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