Milk Replacers for the Neonatal Calf

Symposium on Calf Diarrhea

Milk Replacers for the Neonatal Calf

Michael S. Hand, D.VM., Ph.D.,* Elaine Hunt, D.V.M.,t and Robert W Phillips, D. VM., Ph.D:t

History and politics have had as much to do with the advent and content of milk replacers as has science. Commercial milk replacers first became available in the early 1950s.94 Prior to that time, there was little economic pressure to encourage the development of milk substitutes. Milk was relatively inexpensive; therefore, farmers simply fed whole milk to calves. Although the fluid milk industry has long had an important relationship to the welfare of the general public, Widespread and detailed economic regulation of this industry by government is of comparatively recent origin. The dairy industry, like most others, was thrown into confusion by the abrupt decline in business activity and consumer purchasing power that began in 1929 as a result of the Depression. These chaotic conditions, coupled with the timehonored problems of inelastic demand and seasonal fluctuations of supply, finally prompted legislative intervention. The Agricultural Adjustment Act of 1933, the amended version of 1935, and the Market Agreement Act of 1937, were the first national attempts to stabilize the dairy industry by institutionalizing classified pricing of milk according to use. 9 These laws resulted in a classification scheme that was intended to make it attractive for dairy farmers to produce and sell grade-A fluid milk. However, pricing mechanisms were not specifically described, so dairymen had no guarantee that it would be profitable to upgrade their product. As a result, a significant price differential did not develop, and milk prices were not profoundly affected. Enter World War II and associated milk shortages. In an effort to increase production, wartime legislation in the form of the 1941 Steagall Amendment introduced specific pricing guidelines for the classes of fluid milk. 9 This experience allowed for postwar legislation in the form of the Agricultural Act of 1949. 9 ,109 This law did establish a mechanical procedure *Associate Professor, Department of Anatomy, Physiological Sciences and Radiology, North Carolina State University School of Veterinary Medicine, Raleigh, North Carolina tDiplomate, American College of Veterinary Internal Medicine; Assistant Professor, Department of Food Animal and Equine Medicine, North Carolina State University School of Veterinary Medicine, Raleigh, North Carolina tProfessor, Colorado State University College of Veterinary Medicine and Biomedical Sciences, Fort Collins, Colorado

Veterinary Clinics of North America: Food Animal Practice-Vol. 1, No.3, November 1985

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for determining the support price of milk, which resulted in higher, more stable prices. 110 Subsequently, whole milk became too expensive to feed to calves. Alternatives were sought, and the evolution of modern milk replacers was initiated. The first true milk replacers (1950s to 1960s) were made primarily from dried skim milk (carbohydrate and protein source) and animal fat.70 The early methods used to add fat limited its content to less than 10 per cent (versus approximately 30 per cent in dry whole milk). Calves fed these low-fat products grew poorly.94 Technology for adding fat improved so more could be added, and good-quality milk replacers became available. During the mid-1960s, skim milk prices climbed. Casein and whey were feasible alternatives and were substituted for skim milk. This milk replacer was still comprised primarily of milk constituents and worked well. Then, agricultural policies in the major casein-producing countries effected an increase in casein price·s and the milk replacer industry was forced to consider other protein sources. 94 Over the next several years, a variety of nonmilk proteins (primarily by-products) were evaluated for use in milk replacers. These included cottonseed, oat, pea, rapeseed, potato, soy and wheat Hours, distillers' by-products and brewer's yeast, corn, alfalfa, field bean, meat and fish by-products, bacterial sludge, blood, and others. 2,10,15,17,27,42,74,7~,114,115 When incorporated into milk substitutes in large amounts, many of these protein sources resulted in poor calf performance. However, continued efforts eventually resulted in soy protein concentrates and chemically treated soy Hours that permit acceptable performance. As a result, many modern milk replacers contain these products as a portion of the protein. 77 Currently, fish by-products (fish protein hydrolysates) are showing promise as a protein source. 19,44,69 Extensive trials with a commercial bacterial single-cell protein in a calf milk substitute indicate some success with this product as well. 85 Undoubtedly, milk replacers will continue to evolve as economic pressures prompt the industry to modify their products. Hopefully, some of these changes will represent advances. At the present time, a good-quality milk replacer is approximately as expensive to feed as whole milk. For this reason, many dairies carefully conserve colostrum from the cow, either through refrigeration or fermentation techniques. They raise calves on this diet through careful management of the colostrum. This is the least expensive way to feed a calf and probably the most nutritious. Many dairies do not hesitate to raise calves on whole milk after colostrum sources have been exhausted. The economic basis for this practice is that the reduced incidence of gastrointestinal disease, decreased mortality, and higher growth rates justify the loss of milk income. Current figures on cost effectiveness of feeding high-quality milk replacers versus whole milk support this rationale, for high-quality milk replacers are expensive. Despite the superiority of feeding calves whole milk or colostrum, it is estimated that 50 to 70 per cent of the dairy calves in this country are fed milk substitutes. 58 EVALUATION OF COMPONENTS Energy The fasting metabolism of the preruminant calf is greater per unit surface area or metabolic body weight (body wt. 0.75) than that of a mature cow.

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Furthermore, the metabolic rate of the calf is highest at 2 days of age and declines significantly by 4 weeks of age (129 kcal per kg metabolic body weight and 82 kcal per kg metabolic body weight, respectively).86 The apparently inefficient use of energy by very young calves is compounded by the fact that even whole milk digestibility is lower the first week of life. 65 Perhaps this is associated with gradual closure of the gut to immunoglobulins. The gross energy density of whole cow's milk is approximately 0.73 kcal per ml. 57 Because the digestibility of the nutrients in whole milk is approximately 95 per cent, the digestible energy content should be approximately 0.69 kcal per ml (but will vary with fat content and age of calf).86 This compares closely to a published value of 0.688 kcal per ml. 73 The energy nutrients include carbohydrates (both soluble and insoluble), fat, and protein. Little dietary protein is utilized for energy unless it is fed in excess, is of low biologic value, and/or the diet is deficient in other energy nutrients. Growing animals have a strong anabolic tendency and are less likely to catabolize protein reserves to support energy expenditure than are adults. 17 The soluble (nonfiber) carbohydrate content is not required to appear on milk replacer labels. It is referred to as the nitrogen-free extract (NFE) in the proximate analysis scheme and is determined by subtracting all the other fractions (per cent crude fiber, per cent crude protein, per cent crude fat, per cent moisture, and per cent ash) from 100 per cent. Lactose is the primary soluble carbohydrate used in milk substitutes. 7o Varying amounts of glucose have also been used successfully, but this is generally uneconom~ ical. 70,85 Gastrointestinal tolerance to lactose and glucose can be affected by the dietary fat content. 85 The lack of digestive enzymes in the young calf for sucrose, maltose, and various starches precludes the use of these carbohydrates in milk replacers. 25,66,75.111 These substances can cause diarrhea when fed to young calves. The fat content of dry whole milk is approximately 30 per cent. The fat content of dry milk replacers should be between 10 to 25 per cent. 68,75,81,95 Besides functioning as an energy nutrient, fat also supplies the calf with essential fatty acids. 85 The quantity of fat included in a milk replacer depends on the digestibility of the fat, the level of production desired, and the ambient temperature. There is considerable variation in digestibility of different fats by calves, the reasons for which are not entirely clear. Much of the variation can be accounted for by fatty acid chain length and degree of unsaturation. Digestibility decreases with increasing chain length and increasing saturation. 85 The position of the fatty acid in the triglyceride molecule is also important. Palmitic acid is more efficiently utilized when it occupies the 2 position of a triglyceride. 85 The apparent mean digestibility of several fats used in milk replacers are as follows: butterfat,. 97 per cent; coconut, 95 per cent; unhydrogenated palm, 93 per cent; lard, 92 per cent; lard tallow, 90 per cent; maize, 88 per cent; and tallow, 87 per cent. 85 Sunflower, soybean, and hydrogenated soybean and fish oils have also been used successfully. 70 Calf age can also affect fat digestibility. Very young calves (less than 2 weeks) do not digest nonmilk fats nearly as well as milk source fats. 65 Homogenization of fat also alters digestibility. Mean globule size of 3 to 4 f..Lm or less results in improved fat digestibilityBS and improved nitrogen retention. 75 Poor protein digestibility can also interfere with fat digestion. 69 Feeding fats that

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contain a high proportion of long chain unsaturated fatty acids (for example, groundnut oil or corn oil) have been associated with an increased incidence of diarrhea and other health problems, even when supplemented with vitamin E.85 Antioxidants should be included to prevent rancidity of fats, particularly those containing a high proportion of unsaturated fatty acids. Butylated hydroxyanisole (BHA) or butylated hydroxy toluene (BHT) are commonly used for this purpose in milk replacers. 85 Generally speaking, milk replacers high in milk fat lower the risk of diarrhea. This may be because fat itself has a constipative effect or because the presence of fat in a diet allows the soluble carbohydrate fraction to be lowered. Soluble sugars, fed in excess, can have detrimental effects. 85 One guideline is that 10 per cent fat is the minimum acceptable level in a milk replacer when the temperature-wind index (TWI) is greater than 5°C. Twenty per cent fat is suggested when the TWI is below 50C. 58 Veal rations usually contain 16 to 25 per cent fat. High dietary fat promotes deposition of carcass fat but will not affect muscle or bone fat content above a fat intake adequate for normal growth. 85 It should be obvious that simplistic statements regarding fat content of milk replacers (such as the one made at the beginning of this paragraph) should be interpreted with some caution. Crud-fiber content has been used as a rough index of the amount of milk replacer of plant origin, because more fiber would be added as the per cent of plant protein increases. It has been suggested that each 0.1 per cent of crude fiber in a milk substitute indicates that 10 per cent of the protein is of plant source and, therefore, the crude fiber content of milk replacers should be less than 0.5 per cent and preferably not in excess of 0.25 per cent. 58 However, considerable variability in the crude fiber content exists even within similar plant products. For instance, soybean Hour may contain 2.5 per cent crude fiber while soybean meals contain up to 7 per cent. 61 As a nutrient, crude fiber may be somewhat beneficial. Barr reported a reduction in diarrhea in calves fed milk replacers containing 10 per cent fat that had fiber added to the milk replacer. The fiber was added to the liquid and presumably bypassed the rumen. 7 Product information from one calf milk replacer manufacturer indicates that their research with all milk protein replacers demonstrated that adding up to 3 per cent fiber reduced calf scours by 10 to 15 per cent. 6 Because most calves that are hand-reared are housed, or at least partially protected from weather extremes, usually the most significant variables in determining energy needs are body weight (maintenance) and rate of gain. The rate of gain desired depends upon whether dairy herd replacement, beef production, or veal calves are being fed. The recommendation has been made that dairy replacement heifers should be fed to gain 0.875 per cent of their body weight per day for the first 3 months. 83 This would be 0.35 kg per day for a calf at 40 kg or an average of 0.54 kg per day over a 3-month period for a calf with a 40-kg birth weight. Calves intended for beef production should gain 1 per cent of their body weight per day for the first 3 months. 83 For the same 40-kg calf, this would represent a gain of 0.4 kg per day at 40-kg birth weight or a mean gain of 0.65 kg per day over the gO-day period. However, these two groups actually share the same short-term production goal: getting to weaning age in good health. These calves should be

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Table 1.

Theoretical Milk Replacer Guaranteed Analysis List

Crude protein Crude fat Crude fiber Ash

not not not not

less than *23.0% less than 12.0% more than 0.3% more than 10.7%

*"Not less than" can also be construed to mean "not significantly more than" because these ingredients are the more expensive components of the milk replacer.

fed in a manner that allows reasonable growth but, more importantly, avoids fattening and reduces the incidence of gastrointestinal disease. They should be fed high-quality feeds, particularly early in life. For veal production, a weight gain of 1. 5 per cent per day for the first 90 days is recommended. 83 This would be 0.6 kg per day for 40-kg birth weight or an average gain of 1.28 kg per day for the first 3 months. Many models have been devised for determining energy requirements for preruminant calves. 86 Most are mathematically cumbersome and difficult to use. Therefore, a «thumb rule" system will be discussed (keeping in mind its shortcomings). This shorthand method of determining a calf's digestible energy requirement was most recently published by Medina and colleagues. 58 It proposes an allowance of 50 kcal per kg body weight per day for maintenance, plus 3 kcal per gm body weight gain per day. These figures have been determined by averaging and rounding values published by Blaxter and Wood 14 (52.4 kcal per kg body weight per day for maintenance, 307 kcal per 100 gm body weight gain per day) and Brisson and colleagues 16 (44.7 kcal per kg body weight gain per day for maintenance, 268 kcal per 100 gm body weight gain per day). For example, using this system, a 40-kg calf gaining 0.35 kg per day would require 3050 kcal of digestible energy per day [(50 kcal per kg X 40 kg) + (3 kcal per gm X 350 gm)]. This is a valuable guideline and will provide a reasonable energy intake for healthy young calves. Whether or not a milk replacer contains adequate energy depends upon its energy density and digestibility. The approximate energy density can be easily determined from the information on the product label. A more accurate determination can be made by analysis by a commercial feed laboratory. Either way, it is necessary to determine the amount of soluble carbohydrate, fat, and protein in the diet and to affix energy and digestibility values to these nutrients so that total digestible energy content can be estimated. The crude protein, crude fat, crude fiber, and usually ash are required to be listed on the product label. If ash is not listed, assume 10 per cent. If moisture is not listed, also assume 10 per cent (air dry). To obtain the soluble carbohydrate fraction (nitrogen-free extract), sum the per cent crude protein, crude fat, crude fiber, ash, and moisture. Subtract this sum from 100 per cent. The difference represents per cent soluble carbohydrate. For example, a product label lists the ingredients shown in Table 1. The soluble carbohydrate fraction of this milk replacer would be 44 per cent [(100 per cent)-(23 per cent crude protein + 12 per cent crude fat + 0.3 per cent crude fiber + 10.7 per cent ash + assumed moisture 10 per cent)].

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The digestible energy content of the energy nutrients can be estimated from their average gross energy content. The average gross energy of soluble carbohydrates is 4 kcal per gm. 56 In a milk replacer, most should be lactose. If this carbohydrate component is considered to be 95 per cent digestible, the resulting digestible energy is 3.8 kcal per gm. Fats also vary in their energy density, but an average gross energy content is 9.4 kcal per gm. 56 Fats are generally quite digestible (92.5 per cent); therefore, they yield about 8.7 kcal of digestible energy per gm. The mean gross energy of proteins commonly used in milk replacers is 5.7 kcal per gm (for example, soybean, 5.5 kcal per gm, and casein, 5.9 kcal per gm).56 Protein digestibility varies considerably in calves. l,63,65,84,85 Assumptions regarding non milk protein digestibility in calves under 2 to 3 weeks of age would most likely be inaccurate and, in any event, should be conservative (see protein discussion) relative to older calves. l,63,90,104 Milk replacers containing raw or heated soy protein or all milk protein were compared in calves 35 to 38 days of age. 63 The average digestibility of the protein was 76.6, 89.8, and 95.5 per cent, respectively. In another trial, mean protein digestibility of milk replacers containing soy flour, soy protein concentrate, or skim milk was 68.7, 81.8, and 95.2 per cent, respectively.l Therefore, an average overall digestibility of 85 per cent could probably be assumed for protein in milk replacers fed to calves over 3 weeks of age. Using the gross energy value of 5. 7 kcal per gm, the digestible energy of protein would be approximately 4.8 kcal per gm. With these values, the digestible energy density of 100 gm of this product would be estimated as follows: 23% X 100 g = 23 gm 23 gm X 4.8 kcallgm = 110.4 kcal 12% X 100 gm = 12 gm 12 gm X 8.7 kcal/gm = 104.4 kcal

Crude protein Crude fat

Soluble carbohydrate

44% X 100 gm = 44 gm 44 gm X 3.8 = 167.2 kcal

Total digestible energy 382 kcal/100 gm or 3820 kcal/kg (1736 kcalllb)

The other feed constituents are not a source of energy for the preruminant calf and, therefore, are not included. Carrying the example further, the amount of this milk replacer powder necessary to feed a calf would be determined as follows. Assume a 40-kg calf is being fed to gain 0.35 kg per day. His maintenance energy requirement would be 2000 kcal of digestible energy (40 kg X 50 kcal per kg). To support 0.35 kg gain per day, he would need an additional 1050 kcal (350 gm X 3 kcal per gm) for a total of 3050 kcal. This level of production would require 0.80 kg (1. 76 lb) milk replacer powder per day (3050 kcal + 3820 kcal per kg). Dilution factors will be considered later, and this example continued within that discussion. The reader should be reminded that the thumb rules used in these calculations may not be relevant to calves under 3 weeks of age unless very high-quality milk replacers are being used. Protein It has been recommended that milk replacers should contain a minimum of 25 per cent crude protein. 58 However, even much higher protein levels

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may not assure adequate protein nutrition. Several important variables must be considered, including age. It is not nice to fool Mother Nature, especially when considering alternative dietary protein sources for calves less than 3 weeks of age. Older calves are better able to utilize nonmilk proteins. Young calves have lower gastric acid and protease activity than their older counterparts. 90,I04 Differences in ability to digest protein also occur between breeds of calves the same age, being more favorable in the larger breeds. 89,92,104 Besides breed variability, it has been shown that calves with higher birth weights are better able to utilize soy protein in milk replacers. 1 In any event, casein is certainly the protein of choice for milk substitutes. However, the high cost of skim milk powder continues to prompt research efforts to find a suitable casein alternative. A great variety of nonmilk protein sources have been tested (see introduction), but only two are used currently in substantial amounts by the milk replacer industry: soy Hour and soy protein concentrate. 77 These two products are far from ideal. Soy protein is not utilized as effectively by the calf as it is by other species. 77 Soy products do not form a coagulum in the abomasum, which affects transit time. Soy protein may be more resistant to enzymatic degradation than milk protein and contains antinutritional factors to which the calf is uncommonly sensitive. The most important of these factors are antigens and trypsin inhibitor. 77 Ingestion of unmodified soy protein antigens results in a gastrointestinal allergic response that causes abnormalities in digestive and absorptive processes. 77 This response usually appears after several feedings, 100 and the calves produce high titers of serum antibodies. 101 One study reported that soy Hour increased the weight of both small and large intestinal tissue of calves. 91 In another study, feeding calves soy protein resulted in broadening and shortening of villi, which reduced surface area. 8 These morphologic changes are similar to those reported for a human infant allergic to soy protein. 3 Treatment of soy protein with ethanol and/or heat has been shown to greatly reduce this allergic response. 51 Soybean trypsin inhibitor is also heat sensitive and may be largely inactivated by the heat that is applied during normal processing. The content of inhibitor in raw, defatted Hakes is approximately 60 units per mg, whereas soy Hour usually contains less than 10 units per mg. 77 Even this low level of inhibitor is of nutritional significance to calves, particularly the very young. Ingestion of trypsin inhibitor interferes with protein digestion by decreasing pancreatic exocrine secretory activity and by inactivating both trypsin and chymotrypsin. 37,38 Heat treatment and acid or alkali treatment of soy protein have been shown to inactivate trypsin inhibitor. 20,21 Although chemical and/or thermal treatment of soy protein decreases both its antigenicity and trypsin inhibitor activity, these, as well as other non milk proteins, are still less well utilized by calves under 3 weeks of age. An Israeli study found that 10-day-old calves fed fully toasted soy protein concentrate gained only 35 per cent of that achieved with milk protein. 63 This improved to 95 per cent during days 30 to 46 of the trial. Another study from Israel further demonstrates the effect of age on protein digestibility. 64 Seven 10-day-old calves fed soy protein concentrate experienced a 20 per cent reduction in growth. Calves started on the soy protein concentrate at

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30 days of age gained as well as calves fed milk protein. Even processed milk shows a lower protein digestibility than whole milk when fed to calves less than 2 weeks old. Calves from 10 to 14 days of age had 81 per cent protein digestibility when fed whole milk, and 57 per cent with reconstituted evaporated milk. 65 United States workers recently reported similar results. Holstein calves were fed milk protein, soy protein concentrate, or full fat soy flour milk replacers. 1 Protein digestibility from 10 to 15 days of age was 90.1, 56.6, and 61.3 per cent, respectively. The calves were maintained on these diets, and protein digestibility coefficients were again determined from 30 to 35 days of age and found to be 95.2, 81.8, and 68.7 per cent, respectively. Fish protein concentrates are also in use. These can be soluble or insoluble, depending upon whether or not they are produced by controlled hydrolysis or solvent extraction. 85 Fish protein concentrate reduces gastric enzyme secretion; acid secretion is not affected. 116 As with other nonmilk proteins, a clot is not formed in the abomasum, resulting in a shortened transit time. In general, fish protein concentrates have given better results than soybean products. 85 However, at high inclusion rates, some mortality has been noted associated with a relative vitamin E deficiency due to residual oil (polyunsaturated fats) in a solvent extracted fish-protein product. 33,55,59 Lower gains, relative to casein, have also been noted at these higher inclusion rates, particularly in young calves. 24 ,34,36,44,45,103 However, good weight gains are possible in older calves fed fish-protein concentrate at a more moderate inclusion rate. 24,34,36,45,69,91,103 Protein digestibility can be reduced by improper processing of milk replacer constituents. Excessive heating of skim milk during drying has been shown to reduce protein digestibility and to be associated with an increased incidence of diarrhea. 52 Preheating prior to spray drying improves digestibility and results in a lower incidence of diarrhea. 70 Information regarding the amino-acid requirements of preruminant calves is scanty. The subject is not even mentioned in the latest NRC publication on dairy cattle nutrient requirements. 62 Some manufacturers supplement their milk replacers with methionine and lysine as a precautionary measure, particularly if their products contain soy protein or are to be used for veal calves. However, at least one milk replacer manufacturer does not supplement its product with amino acids, because they have not been able to demonstrate a positive response. A recent study evaluated three levels of methionine supplementation of a milk replacer containing soy flour. 76 The two lower levels of methionine (0.26 and 0.52 per cent) improved growth, whereas the highest (0.78 per cent) depressed growth. The two lower levels also increased the voluntary consumption of starter and hay, whereas the highest level did not. This study points out the complexities associated with amino-acid metabolism in the calf as well as the fact that specific amino-acid supplementation may improve or depress calf performance on milk replacers containing nonmilk protein. Economic factors may dictate in each individual instance whether a less expensive milk replacer that results in less gain is still more satisfactory than higher early weight gains obtained with a more expensive milk replacer. 44 One should not overlook the importance of management. The best managed

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calves may do better on lower-quality milk replacers than will poorly managed calves on high-quality milk replacers. Minerals and Vitamins Efficiency of dietary calcium absorption decreases with age. Calcium availability also varies as to source. The calcium from milk is more digestible than that from other feeds. Young calves can absorb 92 to 98 per cent of the calcium in milk. This digestibility coefficient has been shown to fall to 83 per cent in 170-kg calves. 12,39,46,98 The requirement for calcium will vary with rate of growth. For 0.3- to O.4-kg gain per day, calves should receive 6 to 8 gm of calcium per day. 62 Like calcium, phosphorus digestibility varies with source. Milk phosphorus is most readily available (94 to 99 per cent). 14,26,53 Phosphorus digestion from other feeds is about 73 to 76 per cent. 106-108 Phosphorus requirements also vary with growth rate. For 0.3 to 0.4 kg per day, 4 to 5 gm per day are required. 62 Calves have been shown to grow well when calcium:phosphorus ratios have varied from 1.2 to 6:1. For optimal utilization, however, the ratio should be similar to that in bone (2.2:1).86 If dietary requirements for both minerals are properly met, consideration of the ratio should not be necessary. Milk replacers made from all-milk products are usually adequate in both calcium and phosphorus. Nonmilk ingredient milk substitutes require supplementation. 58 Milk contains approximately 0.013 per cent magnesium. 62 The suggested requirement for young calves is 0.07 per cent of the diet dry matter. 62 Magnesium deficiency (milk tetany) can occur in calves growing at a rapid rate on very large amounts of milk or milk substitutes and having no access to other supplementary feeds. 86 It is usually seen i{l calves fed exclusively milk for more than 8 weeks such as veal calves. 58 Magnesium absorption efficiency decreases with age and is lower on dry than liquid diets. 86 Calves at 4 weeks of age had magnesium digestibility coefficients ranging from 75 to 90 per cent. By 3 months of age, these values had fallen to approximately 40 per cent. 97 Diarrhea will reduce magnesium absorption 99 as will the inclusion of low digestibility fat, generating magnesium soaps.82 Dry diets naturally contain about five times the magnesium content of milk. 86 Soluble salts of iron, copper, cobalt, zinc, manganese, and iodine should be included in all milk replacers. 75 Alterations in mineral content of whole milk may interfere with abomasal clot formation and digestibility of the milk. A description of the rennet-clot test for whole milk is provided in another article in this symposium ("Therapeutic Agents Used in the Treatment of Calf Diarrhea"). This may also prove to be a beneficial test for evaluating the digestibility of milk replacers, but the authors of this article have had no experience with this technique. Under natural conditions, vitamin-A supplementation of calves is usually not necessary.87 Whole milk contains relatively low levels of vitamin A.62 Colostrum is much higher in vitamin-A content than whole milk. It contains 6 times as much vitamin A and 12 times as much carotene. 47 Adequate colostrum intake generally assures adequate vitamin A nutrition until the calf is ingesting hay or forage, which is usually a good source of vitamin A unless it is excessively bleached. Vitamin-A supplementation is necessary

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in the more artificial management systems with intensive rearing, high growth rates, and subsequent high intakes of concentrates or milk substitutes. 87 Because vitamin A generally functions to control cell differentiation and mitotic rate, rapidly growing animals require more vitamin A.29,40 Vitamin-A toxicity can occur from overzealous supplementation. In one study, 138 J.Lg per kg body weight reduced plasma ascorbic acid and tocopherol concentrations. Changes in protein distribution in cerebrospinal fluid occurred at intakes greater than 200 J.Lg per kg and bone mineralization was affected at greater than 1327 J.Lg per kg. 41 The recommended vitamin-A intake by calves is 30 J.Lg per kg body weight87 or 12,000 to 30,000 IV per kg dry matter. 85 Most milk replacers contain adequate levels of vitamin D. 58 High levels of this vitamin are toxic. Toxicities have been produced experimentally by giving 1 X 106 IV (25 mg) daily. 11 A recommended amount for milk replacers is between 1800 to 3500 IV per kg dry matter. 75,85 This would provide about 0.035 to 0.07 mg of vitamin D per 40-kg calf, which is considerably below the toxic level. When high-levels of unsaturated fatty acids are used in milk substitutes, vitamin E (alpha-tocopherol) nutrition becomes very important. If the ratio between dietary vitamin E and unsaturated fatty acids gets too low, a nutritional myopathy can develop. Milk substitutes should contain 15 to 35 mg vitamin E per kg dry matter. 85,93 Milk replacers are often supplemented with vitamin K and the B complex group.87 However, there is little evidence that young calves will be deficient in these vitamins under most conditions. Cyanocobalamin could be a possible exception. 87 This vitamin should be included in milk replacers at 20 to 40 J.Lg per kg dry matter. 58 Although a deficiency of vitamin C has not been considered to be a problem, the vitamin has frequently been added to milk replacers in small quantities. 87 However, recent work indicates that dietary supplementation with vitamin C in greater quantities than previously used may be of benefit. 23 This study evaluated the effect of the addition of ascorbic acid to the diet at the rate of 1.75 gm per calf per day for a 42-day period in both hypo- and normogammaglobulinemic calves. Significant elevations in IgG and IgG 1 levels were noted in the hypogammaglobulinemic calves; no effect was observed in the normogammaglobulinemic group. Decreased mortality and incidence of diarrhea were also seen in the hypogammaglobulinemic calves supplemented with vitamin C versus those that were not. The bovine neonate may be marginally deficient in ascorbate because significant endogenous production does not occur until 4 months of age. 113 From this work, it appears possible that dietary supplementation in therapeutic amounts may be beneficial to colostrum-deprived calves. Certainly more investigation is warranted. Antimicrobials Antibiotics are often added to milk replacers at about 10 per cent of the therapeutic dose. If the calves are well managed, many veterinarians feel antibiotics probably have no beneficial effect in disease prevention. 58 Milk producers and manufacturers of milk replacers do not share this opinion

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and it is difficult to obtain a milk replacer that does not contain antibiotics. Research conducted by a manufacturer of milk replacers showed no difference in average daily weight gain in calves fed milk replacers with and without antibiotics. Calves receiving antibiotics in the milk replacer had a lower incidence of diarrhea and respiratory disease. In-vitro bacterial resistance to the antibiotics used in these calves increased to 100 per cent. Further studies by the manufacturer indicated that the combination of neomycin-terramycin did a better job of controlling diarrhea and respiratory disease than did chlortetracycline. Calves receiving no antibiotics in the milk replacer experienced 16.7 per cent mortality, whereas those receiving neomycin-terramycin in the milk replacer experienced 6.5 per cent mortality. 5 Long-term in-vivo studies on the effect of antimicrobials in milk replacers are needed to ascertain whether pathogenicity of bacteria may be enhanced by prolonged low-level antimicrobial exposure, as has been indicated by invitro studies. Of additional concern, some of the antimicrobials utilized most commonly in milk replacers, such as tetracyclines and aminoglycosides, will bind to ionized calcium, and when suspended in liquids containing calcium, may be ineffective. 35 Dilution Rates of Milk Replacers Overfeeding liquid diets to calves has been thought to increase the incidence of diarrhea. 13,43,54,60,88 However, Mylrea60 showed that feeding whole milk free choice to calves (9 to 38 days of age), when compared with restricted feeding of whole milk, did not result in a significant change in fecal dry-matter content. At this time, dilution rates of milk replacers have not been extensively studied. Pettyjohn and colleagues fed calves ad-libitum milk replacers containing 5, 10, 15, 20, and 25 per cent dry matter. 71 The incidence of diarrhea from 29 to 56 days of age was observed. Calves fed 5 and 10 per cent dry matter diets had the lowest number of diarrhea days. Efficiency of feed conversion was similar for the 5, 10, and 15 per cent dry matter groups but much lower for the calves fed 20 and 25 per cent dry-matter rations. The 10 per cent dry matter concentration reconstitution rate resulted in a relatively low incidence of diarrhea but acceptable growth performance (0.95 kg per day) in this study. Stiles and co-workers studied milk replacers fed to young calves (3 to 20 days of age) reconstituted to three concentrations of dry matter (7, 9, and 11 per cent) and fed at fluid intake levels of 7.5, 10, and 12.5 per cent of body weight. lOi However, it should be noted that they reported that all calves in the study lost weight and experienced scours. The greatest weight loss occurred in the groups fed milk replacers containing 7 per cent dry matter and fed at the lowest rate of fluid intake (75 per cent). They also observed that at a fluid intake of 12.5 per cent of body weight, calves had more severe scours and were more dehydrated. In their study, the calves fed milk replacers reconstituted to 9 or 11 per cent dry matter and fed fluid at 10 per cent of body weight per day suffered less severe scours and the least amount of weight loss. Jenny and colleagues conducted a study to observe the effects of dry-matter concentration and fluid intake. 49 Calves were fed once daily and offered calf starter. Dry-matter concentrations were 10, 13, 16, and 19 per cent. Fluid intake rates were 6, 8, and 10 per

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cent of body weight per day. They noted that overall gain was not affected by varying fluid intake or dry matter concentration. Incidence of scours increased with increasing dry matter concentration. Duration of scours was extended with higher levels of dry-matter concentration and greater rates of fluid intake. Calves fed milk replacers reconstituted to 10 to 13 per cent dry matter and fed fluid at 8 per cent body weight per day had fewer intestinal disturbances and obtained satisfactory gains (0.51 to 0.57 kg per day). From these studies, it would appear that for overall performance in young calves, milk replacers should be reconstituted to 9 to 13 per cent dry matter and be fed at a fluid intake of 8 to 10 per cent of body weight per day. Most mixing directions result in a product that is 12 to 15 per cent dry matter. 31,72,112 Continuing the example from the energy discussion, a dilution rate for milk-replacer powders could be calculated. It was determined that a 40-kg calf gaining 0.35 kg per day requires 3050 kcal of digestible energy or 0.80 kg of a milk-replacer powder with an energy density of 3820 kcal of digestible energy per kg. Because the moisture content was 10 per cent, the dry matter content of the milk-replacer powder is 90 per cent. If 13 per cent dry matter is desired in the reconstituted product, Pearson's square can be used to determine the ratio of water to add to the milk replacer powder. 28 To use Pearson's square to formulate any mixture, place the desired percentage of the ingredient in question in the center of a square (see example below). Place the percentages of the ingredient in each of the parent constituents at each corner of the left side of the square. Subtract diagonally, taking the smaller number from the larger, regardless of whether its position is at the center or corner of the square (90 - 13 = 77; 13 - 0 = 13). The two differences are recorded at the diagonal corners on the right side of the square and summed (13 + 77 = 90). Two fractions are generated from each difference divided by the sum (13 -;- 90 = 14.4%; 77 -;- 90 = 85.6%). These fractions represent the relative amount of each parent constituent directly across the square that is required to produce a mixture containing the ingredient percentage in the center of the square. Milk-replacer powder dry matter is 90%

13

desire 13% dry matter

I Water dry matter is 0%

I 77 90

13 -:- 90 = 14.4% milk-replacer powder 77 -:- 90 = 85.6% water

To determine the ratio of water to powder, divide 85.6 per cent by 14.4 per cent, which equals 5.94 (approximately 6) parts water per one part milkreplacer powder. For the calf in this example, 0.80 kg of milk-replacer powder would be mixed with 4.8 kg of water (0.80 X 6 = 4.8). This would result in approximately 5.6 kg of fluid. However, the rate of fluid intake by this theoretical calf would be (5.6 kg -:- 40 kg X 100) 14 per cent. This exceeds

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the general recommendation of 8 to 10 per cent of body weight for fluid intake. At this point, several options exist. The calf could simply be fed less milk replacer and a lower weight gain should be accepted. The calf could be fed the milk replacer at this dry-matter concentration and rate of fluid intake. However, according to the aforementioned studies, there may be a greater likelihood of diarrhea. The milk-replacer powder could be reconstituted to a higher percentage of dry matter. Again, according to the previous discussion, this may result in an increase in the incidence of diarrhea. A milk-replacer powder of higher energy density could be selected. If one was chosen that contained 20 per cent crude fat, 26 per cent crude protein, 8 per cent ash, and 12 per cent moisture, the estimated digestible energy content would be 4280 kcal per kg as fed or (4280 -;- 0.90) 4756 kcal per kg dry matter. If this were fed at 13 per cent dry matter concentration and a fluid intake of 10 per cent body weight, only 0.52 kg of milk-replacer powder (dry-matter basis) could be fed, as shown below: 40 kg body weight

X

10% rate of fluid intake

= 4 kg fluid

4 kg fluid X 13% desired dry matter concentration = 0.52 kg milk replacer powder (dry-matter basis). To determine the as-fed amount, divide 0.52 kg by 0.90 (90 per cent dry matter) to get 0.58 kg. This much powder would contain approximately 2473 kcal of digestible energy (0.52 kg dry matter X 4756 kcal per kg dry matter). Assuming 50 kcal per kg for maintenance, the 40-kg calf would require 2000kcal, leaving 473 kcal for gain. Assuming 3 kcal per gm gain, 473 kcal of digestible energy would allow a gain in body weight of only 157 gm (0.35 lb) per day. Although several options exist, none appear ideal according to recommendations based on the previously discussed research. However, for the 2- to 3-week-old calf, moderate or even no gain in body weight may be acceptable. In 1970, Radostits and Bell stated that there was a widespread, but poorly documented, belief that calves fed at or below maintenance for the first 10 to 14 days of life suffered less from digestive upsets than those fed to gain weight. 75 More recent data indicate that calves can be maintained at near birth weight for up to 3 weeks with no detrimental effect on performance through 12 weeks of age. 48

PROSPECTUS Acidified milk replacers were developed in the late 1970s and tested extensively in Europe. 30,105 These products are now appearing in the United States. Acidified milk replacers utilize a blend of organic acids that are added as preservatives, so that the skim milk and whey constituents can be prepared and stay fresh for up to 3 days in ambient temperatures of 95°F or less. The pH of these replacers is adjusted to approximately 5.6. Additional emulsification of fat prevents separation of this constituent. Thus, an ad-lib supply of cold milk is available for the calf. 30 The milk is mixed and retained in a large sealed plastic barrel in a common calf pen and is delivered through a pipeline containing a nonreturn valve to one or both of two teats available to the calves. The calves nurse small quantities of the milk frequently during

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the day and night. 105 The manufacturer states the digestion of the product is improved and total milk consumption actually increases. 4 Manufacturer information about the product suggests that a calf can experience a total weight gain in 3 weeks that would typically require 5 weeks using traditional milk replacers.4 There are as yet no figures available concerning the costbenefit ratio of this feeding method, although certainly the increased consumption of milk replacer will increase costs. The manufacturer claims that reduced labor requirements, more flexible work routines, simple and inexpensive systems, reduced energy costs (not having to heat water), low overhead group penning, and nonspecialized buildings will more than offset increased costs due to increased consumption. 4 This method requires early supervision of the calf to assure adaptation to the teat system. These milk replacers should help diminish labor costs in group-housing situations, but American preference towards outdoor hutches and indoor calf crates may limit the labor-saving advantages of this feeding system. In addition, careful observation of individual calves is still necessary to ascertain that milk intake is adequate and to assure the health status of each calf. While evaluating acidified milk replacers, investigators reported an increase in live-weight gain, increased feed conversion efficiency, and a decreased incidence of alimentary tract disorders. 3o,105 These findings were especially significant in calves 3 weeks of age and less. 105 Two possible explanations exist for the decline in intestinal problems. The first defense the body has against intestinal infection is the gastric acid trap, which inhibits proliferation of coliforms introduced by mouth. In addition, cyclical re-establishment of a relatively acid pH in the proximal duodenum following ingestion and abomasal digestion of a small quantity of milk inhibits coliform proliferation in this critical section of bowel. Engorgement of milk results in prolonged alkalinization of the proximal duodenum, which would be more conducive to coliform proliferation in this region of the gut. Ad-lib ingestion of small.quantities of milk replacer would more closely simulate the natural feeding behavior of the neonate and may be useful in protecting the proximal small intestine from coliform proliferation. 96 Calves fed fermented waste milk with 0.6 per cent N aH C03 added experienced an increase in scouring above calves not receiving alkalinizing agents. 50 Perhaps oral alkalinizing electrolyte solutions may be detrimental to the normal neonate. Thus, according to the manufacturer, the advantages of acidification include milk preservation for up to 3 days, restriction of amount ingested at each feeding due to the tart flavor of the product, improved protein digestibility, and lowered gastric pH, which inhibits invasion and establishment of pathogenic organisms. 4 The acidified milk replacers presently available in this country are allmilk formulations with 20 per cent protein and 14 per cent fat. Soy protein milk replacers settle rapidly and, therefore, are not acceptable for this type of feeding program. Antibiotics are not added to these milk replacers. Enzymes are presently being utilized to predigest proteins from sources that are less expensive but otherwise poorly digested by the calf (for example, fish protein hydrolysate, which was discussed earlier).69 This process may allow less expensive protein sources to be used in milk substitutes. In addition, attempts are being made to develop milk replacers with a much lower percentage of dry matter, because a reduction of drying costs would also

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decrease the expense of the replacer processing. In order to achieve this, hydrolysis of milk protein and lactose is necessary to give a long shelf-life to the product. Hydrolyzed skim milk milk replacers may increase calf performance by increasing the digestibility of the constituent nutrients. 30 Denatured whey protein (called lactalbumin) has been successfully digested through the use of an alkaline protease called esperase (an inexpensive enzyme commonly used in the detergent industry). Utilization of this process resulted in production of a powder containing 32 per cent protein and 44 per cent lactose, which is similar in composition to skim milk powder. Because the end product is not bitter and is derived from the by-product of the cheese industry (whey) future milk substitutes may be composed of everincreasing amounts of solubilized, denatured whey protein. 67 Enzymatic predigestion of cereal starches may result in increased utilization of Hours in future milk substitutes. The complete replacement of lactose by partially hydrolyzed cereal starch has been reported to be successful in veal operations in France. 58 An interesting concept that is currently being investigated is the combination of preserved colostrum extended with reconstituted soybean Hour and whey powder. This would allow the dairy farmer to extend colostrum feeding until the calf reaches 4 weeks of age, thus decreasing the total rearing costs of raising dairy calves. Calves receiving the lowest levels of soybean Hour and whey powder displayed the best response to the diet. However, least-cost diet combinations could easily be devised for each dairy with a product similar to this. 22 Studies should be developed to compare growth rates, scour index, and so on before this program is widely accepted. Oncedaily whole-milk feedings interchanged with once-daily feedings with soy protein-based milk replacers in older calves has given satisfactory results. 32 Processing (pasteurization) of waste milk (mastitic or antibiotic-treated milk) may also be feasible on very large dairies. I8

SUMMARY A rather long list of broad generalizations exists regarding milk replacers for calves. At least two more general statements should be added to that list: (1) Most broad generalizations regarding milk replacers should be applied cautiously; and (2) if young calves (less than 2 to 3 weeks) are to be fed milk substitutes, the products should be of high quality. Our primary goal in feeding baby calves should be health oriented-that is, not to predispose to or cause diarrhea through diet. This means using milk -source ingredients in milk substitutes targeted for young calves. The preponderance of data presented in this article underscores that statement. Milk replacers containing non milk sources of major nutrients are better fed to older calves.

REFERENCES 1. Akinyele, 1.0., and Harshbarger, K.E.: Performance of young calves fed soybean protein replacers. J. Dairy Sci., 66:825-832, 1983.

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2. Alpan, S.O., Oldfield, }.E., Claypool, D.W., et al.: Digestibility of alfalfa protein concentrate (Pro-Xan) in milk replacer diets of calves. J. Dairy Sci., 62(Abstr.)(Suppl. 1):86, 1979. 3. Ament, M.E., and Rubin, C.E.: Soy protein: Another cause of the Hat intestinal lesion. Gastroenterology, 62:227-234, 1972. 4. Anonymous: A new formula for dairy calf rearing. Anim. Nutr. Health, March/April:26-28, 1984. 5. Anonymous: Antibiotics in milk replacers: Pro or con? In product Information Sheet, Land-a-Lakes, Arden Hills, Minnesota. 6. Anonymous: Land-a-Lakes Milk Replacer Guide. Edition 4, 1976. 7. Barr, G. W.: Developments in calf starters and milk replacers. In Proceedings of the Kansas Formula Feed Conference, 1978, p. B-1. 8. Barratt, M.E.J., Strachan, P.J., and Porter, P.: Antibody mechanisms implicated in digestive disturbances following ingestion of soya protein in calves and piglets. Clin. Exp. Immunol., 31 :305-312, 1978. 9. Benedict, M.A., and Stine, O.C.: Fluid milk. In The Agricultural Commodity Programs. Baltimore, The Lord Baltimore Press, Inc., 1956, pp. 441-489. 10. Bhatty, RS., and Christison, G.I.: Digestibility of pea proteins by preruminant calves. Can. J. Anim. Sci., 60:925-930, 1980. 11. Blackburn, P.S., Blaxter, K.L., and Castle, E.J.: Vitamin D3 toxicity in calves. Proc. Nutr. Soc., 16:xvi, 1957. 12. Blackwood, J.H., Morris, S., and Wright, N.C.: The nutritive value of raw and pasteurized milk for calves. The assimilation and retention of nitrogen, phosphorus, and calcium. J. Dairy Res., 7:228-237, 1936. 13. Blaxter, K. L., and Wood, W. A.: Some observations on the biochemical and physiological events associated with diarrhea in calves. Vet. Rec., 65:889-892, 1953. 14. Blaxter, K. L., and Wood, W.A.: The nutrition of the young Ayrshire calf. 5. The nutritive value of cow's whole milk. Br. J. Nutr., 6:1-12, 1952. 15. Bouchard, R, Brisson, G.}., and Julien, J.P.: Nutritive value of bacterial sludge and whey powders for protein in calf milk replacers and on chromic oxide as indicator of digestibility. J. Dairy Sci., 56:1445-1449, 1973. 16. Brisson, G.J., Cunningham, H.M., and Haskell, S.R: The protein and energy requirements of young dairy calves. Can. J. Anim. Sci., 37:157-167, 1957. 17. Bryant, J.M., Foreman, C.F., Jacobsen, N.L., et al.: Protein and energy requirements of the young calf. J. Dairy Sci., 50:1645-1653, 1967. . 18. Bushnell, R B.: Personal communication, 1983. 19. Campos, O.F., Huber, J.T., and Bergen, W.G.: Partial substitution of milk protein with spray-dried fish solubles or soy protein concentrates in calf milk replacers. J. Dairy Sci., 65:1240-1246, 1982. 20. Colvin, B.M., and Ramsey, H.A.: Growth of young calves and rats fed soy Hour treated with acid or alkali. J. Dairy Sci., 52:270-272, 1969. 21. Colvin, B.M., and Ramsey, H.A.: Soy Hour in milk replacers for young calves. J. Dairy Sci., 51 :898-904, 1968. 22. Cruywagen, C. W.: Optimizing soybean Hour, whey powder and colostrum ratios for rearing dairy calves. S. Afr. Anim. Sci., 12:103-112, 1982. 23. Cummins, K.A., and Brunner, C.J.: Dietary ascorbic acid and plasma immunoglobulins in dairy calves. J. Dairy Sci. (submitted for publication, 1985). 24. Dodsworth, T.L., Owen, J.B., Mackie, I.M., etal.: Fish protein hydrolysate as a substitute for milk protein in calf feeding. Anim. Prod., 25:19-26, 1977. 25. Dollar, A.M., and Porter, J.W.G.: Utilization of carbohydrates by the young calf. Nature, 179:1299-1300, 1957. 26. Duncan, C. W.: and Huffman, C. F.: The effect of daily massive doses of viosterol upon calcium and phosphorus metabolism and blood calcium and inorganic phosphorus in calves. J. Dairy Sci., 17:83-91, 1934. 27. Duthie, I.E., Edwards, D.C., and Rogers, B.: Preliminary studies on the suitability of field bean (Vicia faba L.) protein isolate for lambs and calves. Proc. Nutr. Soc., 33(Abstr.):40A, 1974. 28. Ensminger, M. E., and Olentine, C. C., Jr.: Feeding standards: Ration formulation. In

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Feeds and Nutrition: Complete. Edition 1. Clovis, California, Ensminger Publishing Co., 1978, pp. 557-582. Erwin, E.S., Grainger, R.B., Algeo, J., et al.: Effect of protein, energy, and antioxidant on hepatic vitamin A in steers. J. Anim. Sci., 20{Abst.):931, 1961. Fallon, RJ., and Harte, F.J.: Milk replacers for the eighties. Farm Food Res., 13:180-184, 1982. FCX Calf Maker Product Information Label. FCX Inc., Raleigh, North Carolina, 1985. Fuhrman, T.: Personal communication, 1983. Genskow, RD., and Harshbarger, K.E.: Fish protein concentrate in calf milk replacer formulas. J. Anim. Sci., 29{Abstr.): 158, 1969. Genskow, R.D., Harshbarger, K.E., and Wendlandt, RM.: Effect of feeding fish protein concentrate in milk replacers on plasma-free amino acid value. J. Diary Sci., 52:932, 1969. Giovanni, R, and Warren, RG.: Drug distribution and elimination. In Principles of Pharmacology. St. Louis, C. V. Mosby Co., 1983, pp. 22-45. Gorrill, A.D.L., Nicholson, J.W.G., and Power, H.E.: Effects of milk, fish, and soybean protein in milk replacers, and feeding frequency on performance of dairy calves. Can. J. Anim. Sci., 52:321-328, 1972. Gorrill, A. D. L., and Thomas, J. W.: Body weight changes, pancreas slice and enzyme activity and proteolytic enzyme activity and protein digestion in intestinal contents from calves fed soybean and milk protein diets. J. Nutr., 92:215-223, 1967. Gorill, A.D.L., Thomas, J.W., Stewart, W.E., et al.: Exocrine pancreatic secretion by calves fed soybean and milk protein diets. J. Nutr., 92:86-92, 1967. Hansard, S.L., Comar, C.L., and Plumlee, M.P.: The effects of age upon calcium utilization and maintenance requirements in the bovine. J. Anim. Sci., 13:25-36, 1954. Hazzard, D.G., Grifo, A.P., Jr., Rousseau, J.E., Jr., et al.: Effect oflevel of ration intake and duration of vitamin A deficiency upon some biochemical constituents in serum, cerebrospinal fluid, and aqueous humor of Holstein calves fed fixed carotene intakes. J. Dairy Sci., 45:91-104, 1962. Hazzard, D.G., Woelfel, C.G., and Calhoun, M.C., et al.: Chronic hypervitaminosis A in Holstein male calves. J. Dairy Sci., 47:391-401, 1964. Hinks, D.E., Peers, D.G., and Armishaw, M.: The replacement of skim milk by cooked potato flour in milk diets for calves. Anim. Prod., 19:351-358, 1974. Hodgson, J.: The development of solid food intake in calves. 5. The relationship between liquid and solid food intake. Anim. Prod., 13:593-597, 1971. Huber, J.T., and Campos, O.F.: Enzymatic hydrolysate offish, spray-dried fish solubles, and soybean protein concentration in milk replacers for calves. J. Dairy Sci., 65:2351-2356, 1982. Huber, J.T., and Slade, L.M.: Fish flour as a protein source in calf milk replacers. J. Dairy Sci., 50:1296-1300, 1967. Hughes, J. S., and Cave, H. W.: Coefficients of digestibility of the constituents of milk to the balance of calcium and phosphorus in calves on a milk diet. J. Nutr., 4:163-169, 1931. Jacobson, N. L., McGillard, A. D.: The mammary gland and lactation. In Duke's Physiology of Domestic Animals. Edition 10. Ithaca, Cornell University Press, 1984, pp. 863-880. Jenny, B.F., and O'Dell, D.G.: Subsequent performance of calves held at near birth weight for the first three weeks oflife. J. Dairy Sci., 64:1735-1737, 1981. Jenny, B.F., Van Dijk, H.J., and Grimes, L.W.: Performance of calves fed milk replacer once daily at various fluid intakes and dry matter concentrations. J. Dairy Sci., 65:2345-2350, 1982. Keith, E.A., Windle, L.M., Keith, N.K., et al.: Feeding waste milk to dairy calves. Nutr. Abst. Rev., 52:760, 1982. Kilshaw, P. J., and Sisson, J. W. : Gastrointestinal allergy to soyabean protein in preruminant calves. Allergenic constituents of soyabean products. Res. Vet. Sci., 27:366-371, 1979. Lister, E.E., and Emmons, D.B.: Quality of protein in milk replacers for young calves. II. Effects of heat treatment of skim milk powder and fat levels on calf growth, feed intake, and nitrogen balance. Can. J. Anim. Sci., 56:327-333, 1976. Lofgreen, G.P., Kleiber, M., and Luick, J.R: The metabolic fecal phosphorus excretion of the young calf. J. Nutr., 47:571-581,1952.

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54. Loosmore, RM., Anderson, K.P., and Edgson, F.A.: Symposium on calf diseases. Vet. Rec., 76:1335-1348, 1964. 55. Makdani, D.D., Huber, J.T., and Michel, RL.: Nutritional value of 1,2-dichloroethane extracted fish protein concentrate for young calves fed milk replacer diets. J. Dairy Sci., 54:886-892, 1971. 56. Maynard, L.A., Loosli, J.K., Hintz, H.F., et al.: Bioenergetics. In Animal Nutrition. Edition 7. New York, McGraw-Hill Book Company, 1979, pp. 60-73. 57. Maynard, L.A., Loosli, J.K., Hintz, H.F., et al.: Lactation. In Animal Nutrition. Edition 7. New York, McGraw-Hill Book Company, 1979, p. 504. 58. Medina, M., Johnson, L.W., Knight, A.P., et al.: Evaluation of milk replacers for dairy calves. Compend. Contino Ed., 5: S148-S 155, 1983. 59. Michel, RL., Makdani, D.D., Huber, J.T., et al.: Nutritional myopathy due to vitamin E deficiency in calves fed fish protein concentrate as the sole source of protein. J. Dairy Sci., 55:498-506, 1972. 60. Mylrea, P.J.: Digestion in young calves fed whole milk ad libitum and its relationship to calf scours. Res. Vet. Sci., 7:407-416, 1966. 61. National Research Council: Atlas of Nutritional Data on United States and Canadian Feeds. Washington, D. C., National Academy of Sciences, 197L 62. National Research Council: Nutrient Requirements of Dairy Cattle. Edition 5. Washington, D. C., National Academy of Sciences, 1978. 63. N itzan, Z., Volcani, R, Gordin, S., et al.: Growth and nutrient utilization by calves fed milk replacers containing milk or soybean protein concentrate heated to various degrees. J. Dairy Sci., 54:1294-1299, 1971. 64. Nitzan, Z., Volcani, R, Hasdai, A., et al.: Soybean protein substitute for milk protein in milk replacers for suckling calves. J. Dairy Sci., 55:811-821, 1972. 65. Noller, C.H., Ward, G.M., McGilliard, A.D., et al.: The effect of age of the calf on the availability of nutrients in vegetable milk replacer rations. J. Dairy Sci., 39:1288-1298, 1956. 66. Okamoto, M., Thomas, J.W., and Johnson, T.L.: Utilization of various carbohydrates by young calves. J. Dairy Sci., 42:920, 1959. 67. Okeefe, A. M., and Kelly, J.: Solubilization of denatured whey protein. Neth. Milk Dairy J., 35:292-297, 1981. 68. Olson, W. A., and Williams, J. B.: Effect of five levels of animal fat in calf milk replacers. J. Dairy Sci., 42:918-919, 1959. . 69. Orskov, E.R, Soliman, H.S., and Clark, C.F.S.: Use offish protein hydrolysate in milk replacers. Anim. Feed Sci. Technol., 7:135-140, 1982. 70. Otterby, D.E., and Linn, J.F.: Advances in nutrition and management of calves and heifers. J. Dairy Sci., 64:1365-1377, 1981. 71. Pettyjohn, J.D., Everett, J.P., and Mochrie, RD.: Responses of dairy calves to milk replacer fed at various concentrations. J. Dairy Sci., 46:710-714, 1963. 72. Purina Nurse Chow #200 Product Information Label. Ralston Purina Co., St. Louis, Missouri, 1985. 73. Radostits, O.M., and Acres, S.D.: Herd health and management. In Bovine Medicine and Surgery. Santa Barbara, American Veterinary Publications, Inc., 1980, pp. 21-98. 74. Radostits, O.M., and Bell, J.M.: Nutrient digestibility by newborn calves fed milk replacer. Can. J. Anim. Sci., 48:293-302, 1968. 75. Radostits, O.M., and Bell, J.M.: Nutrition of the pre-ruminant dairy calf with special reference to the digestion and absorption of nutrients: A review. Can. J. Anim. Sci., 50:405-452, 1970. 76. Ramsey, H.A.: Methionine supplementations of milk replacers containing soy Hour. J. Dairy Sci., 66:195-196, 1983. 77. Ramsey, H.A.: Non-milk protein in milk replacers with special emphasis on soy products. Review paper presented at the Annual Meeting of the American Dairy Science Association, 1982. 78. Ramsey, H.A., and Rutledge, K. S.: Glandless cottonseed Hour in milk replacers for young calves. J. Dairy Sci., 52(Abstr. ):933, 1969. 79. Ramsey, H.A., and Willard, T.R: Symposium: Recent advances in calf rearing. Soy protein for milk replacers. J. Dairy Sci., 58:436-441, 1975.

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Department of Food Animal and Equine Medicine School of Veterinary Medicine North Carolina State University Raleigh, North Carolina 27606