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• Increasing the amount of CP to 22% CP in milk replacers fed to calves at 12% of body weight resulted in increased average daily gains of heart girth, body length, and body weight.
• Calves utilized N more efficiently as the amount of CP increased from 16 or 18% to 22 or 26% CP in milk replacer.
• The final body composition of calves contained greater amounts of lean tissue (protein and moisture) as CP content increased in milk replacers.
• The composition of body weight gain in calves fed 22 or 26% CP had decreased percentages of fat and ash and increased the percentage of moisture compared with calves fed lower levels of CP.

INTRODUCTION

Optimal contents of protein in milk replacers remain controversial. Because protein is the most expensive component of milk replacer, it may be desirable to feed an amount of protein that supports satisfactory but not maximal growth at a given feeding rate. Previous research with calves housed in an indoor calf barn has shown that body weight gain was not greatly different for calves fed milk replacers ranging from 16 to 24% crude protein (CP). However, increases in body weight gain do not necessarily reflect an increase in the amount of lean tissue deposition. Therefore, effects of protein content in milk replacer are most accurately determined by measuring effects on body composition. At equal metabolizable energy intake, insufficient protein is expected to result in greater deposition of body fat and less deposition of body protein.

Some evidence indicates that modern dairy animals are leaner than calves of 20 to 30 years ago. Little research on the body composition of calves has been conducted over the last 25 years. Furthermore, all-milk protein milk replacers fed today contain no dried skim milk but instead are based almost entirely on whey proteins. Consequently, our objective was to assess the effects of protein content in whey-protein-based milk replacers on nutrient utilization and body composition during growth of pre-ruminant calves. Our goal was to define the concentration of protein that would provide optimal growth of lean body tissue and most efficient use of dietary nitrogen at a feeding rate slightly greater than current recommendations.

MATERIALS AND METHODS

Forty male Holstein calves < 1 wk of age were purchased from a sale barn. Upon arrival, calves were supplemented with an electrolyte solution twice during d 1 at 1200 and 2200 h and were vaccinated according to standard operating procedures at the University of Illinois Dairy Research Unit. Calves were fed a standard all-milk milk replacer containing 20% crude protein and 20% fat during the first 2 wk. Following the adaptation period, calves were blocked by weight into four blocks of five calves and randomly assigned to an initial slaughter group or to one of four milk replacer treatments (8 calves per treatment). Milk replacers were formulated to be isocaloric and contained 16%, 18%, 22%, or 26% CP. Milk replacers were reconstituted to 12.5% solids and fed at 12% of body weight, adjusted weekly. Balances of nitrogen and energy were measured in a metabolism study during d 31-41. Calves were slaughtered on d 56; visceral tissues were removed and combined. Viscera and the empty body (visceral-free fraction which includes the head, hide, hooves, and tail) were ground and analyzed for chemical composition. Calves had free access to fresh water but no access to hay or starter. All calves were housed in individual calf hutches throughout the trial, except during the metabolism portion of the experiment. Calves were weighed, stature measurements were taken, and blood was sampled once weekly.

RESULTS AND DISCUSSION

As calves were fed increasing concentrations of CP, their N intake increased. Calves fed 18, 22, and 26% CP excreted increasing amounts of N in feces when compared to calves fed 16% CP, and the increase in N intake resulted in increased urinary N. There were no differences in the amount of N absorbed as a percentage of intake. Calves increased N retention efficiency, expressed as N retained as a percentage of intake, when fed the 22 or 26% CP milk replacers compared with 16 or 18% CP diets. Therefore, the efficiency of N utilization efficiency was greater for calves fed 22% or 26% CP.

Average daily gains of weight, body length and hearth girth increased when calves were fed increasing amounts of CP in their milk replacers up to 22%. There was no added benefit to feeding the highest level of CP. Calves that were fed 26% CP had increased amounts of protein and moisture in their final body compositions compared with calves fed the 16% CP milk replacer. The amounts of fat and ash in the whole body were not affected by treatment. The composition of the gain was calculated as each component’s proportion of the total gain. Calves gained a decreasing proportion of fat and an increasing proportion of moisture as milk replacer CP increased from 16 to 22% CP. The proportion of protein remained unchanged. Therefore, calves fed 22% CP utilized N more efficiently, grew at a more rapid rate, and gained a smaller proportion of fat and larger proportion of lean tissue when compared to calves fed lower amounts of CP. Further improvements at 26% CP were modest.

These results were obtained with calves fed increasing amounts of milk replacer as they grew. The feeding rate (12% if BW; 1.5 pounds of powder for a 100 lb. calf) was greater than standard recommendations for calves offered starter at an early age. Because growth rates primarily determine protein requirements, optimal content of protein in milk replacer fed at lower or higher rates in this study may be lower or higher respectively.