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• Increasing the content of crude protein in milk replacer increases average daily gain and efficiency of gain.
• Increasing the content of crude protein in milk replacer increases lean content (water and protein) of whole-body tissue gain and decreases fat content of gain, without changing total tissue energy gain.
• Increasing the feeding rate increases average daily gain and efficiency of gain, without major impact on composition of whole-body gain.

INTRODUCTION

The proportion of total energy intake provided by protein can impact growth rates and body composition in many species. Requirements for protein in calves are directly related to the growth rate, because maintenance requirements for protein are quite small. Early research in New Zealand demonstrated that body fat deposition at a given total energy intake decreased as dietary crude protein (CP) in a milk substitute increased from 15.7% to 31.5%. More recently, an experiment in our research group showed that increasing protein from 16 to 26% CP in a milk replacer fed at 12% of body weight linearly decreased fat deposition and increased growth rates. The objective of this experiment to determine the interrelationships between CP content in milk replacers and the feeding rate. Response variables were growth rate, efficiency of gain, and rates of deposition of tissue components.

MATERIALS AND METHODS

Forty-eight male Holstein calves, purchased during the first week of life, were placed in hutches at the University of Illinois dairy unit. Calves were bedded on crushed rock without additional bedding to prevent consumption of material that might confound results on growth. Calves were fed whole milk at 10% of body weight for a 2-week adjustment period. Calves then were allotted to one of four isocaloric milk replacers containing 14, 18, 22, or 26% CP and one of two feeding rates (10% or 14% of body weight as reconstituted liquid). Milk replacers contained only milk proteins, were reconstituted to 12.5% solids, and were fed twice daily. Calves remained on treatments for 5 weeks, and amounts fed were adjusted weekly as calves grew. All calves were weighed and measured weekly. At the end of the 5-week feeding period, calves were slaughtered at the University of Illinois Meat Science Laboratory. The visceral organs were weighed, combined into a single fraction, and ground for chemical analysis. The head, hide, hooves, and tail were combined, weighed, and ground for chemical analysis. The whole carcass was weighed and ground. Contents of water, protein, fat, ash, and energy were determined in the ground fractions. Composition of an additional 8 calves from the same lot was determined after the end of the 2-week adaptation period so that changes in body composition due to treatments could be calculated. Data were subjected to analysis of variance techniques appropriate for a completely randomized block design with treatments in a 2 (feeding rate) H 4 (CP content) factorial arrangement. The main factor of CP content was partitioned into linear, quadratic, and cubic effects of increasing CP content.

RESULTS

Male calves were used instead of female because of lower cost, since growth of heifers and bulls is similar until at least 200 lb body weight. Increasing the feeding rate of milk replacer from 10% to 14% of body weight increased ADG (Table 1). As observed in earlier experiments, increasing CP content at the same metabolizable energy (ME) intake stimulated ADG (Table 1). Responses of ADG to dietary CP could be adequately described by linear equations; the quadratic effect did not achieve significance (P < 0.21). Efficiency of gain increased in a quadratic fashion (Table 1).

Body composition of calves was measured at the start of the experiments and after feeding for 5 weeks (Table 1). As dietary CP increased, the content of fat in the final whole body (including viscera) decreased and lean tissue (protein plus water) increased. Whole-body gains of water and protein (Table 2) were greater at the higher feeding rate and increased linearly as dietary CP increased. However, tendencies (P < 0.10) for an interaction of feeding rate and the linear effect of increasing CP indicated that the response to CP at the lower feeding rate was less than at the higher feeding rate. The amount of fat gained (Table 2) was greater at the higher feeding rate and decreased linearly as CP increased. Body energy gain was almost doubled at the higher feeding rate, but was not affected by dietary CP. This indicates that the effect of increasing dietary CP was to "re-partition" energy from fat to lean tissue deposition.

Table 1.Effects of feeding rate and crude protein (CP) content of milk replacers on average daily gain (ADG) and final whole-body composition in male Holstein calves.

The content of fat in whole-body gain (Table 3) decreased in a quadratic manner as dietary CP increased, with an interaction of feeding rate by quadratic effect of dietary CP. The content of water in gain showed an inverse pattern, whereas the content of protein in whole-body gain increased linearly with increasing CP but was not affected by feeding rate. These data are similar to earlier experiments with calves fed milk replacers based on skim milk proteins. Together, these data indicate that dietary CP (or protein to energy ratio), but not feeding rate, exerts a pronounced effect on composition of whole-body gain in young calves.

Table 2. Effects of feeding rate and crude protein (CP) content of milk replacers on whole-body gains of body components in male Holstein calves.

Table 3.Effects of feeding rate and crude protein (CP) content of milk replacers on composition of whole-body gain in male Holstein calves.