Take Home Messages

• We compared a moderate fat conventional milk replacer (22% crude protein, 19% fat) with whole milk and a milk replacer formulated with similar composition as whole milk (25% crude protein, 28% fat). Diets were fed in amounts to equalize initial intakes of crude protein and metabolizable energy.
• Calves fed the moderate fat conventional milk replacer had greater average daily gains and greater gains of water and protein (lean tissue) than calves fed either high-fat diet.

Introduction

The optimal content of fat in milk replacers is poorly defined and controversial. Evidence in the early scientific literature suggests that fat may not be the best energy source to fuel lean tissue deposition. This finding could relate to the fact that ruminant tissues do not oxidize long-chain fatty acids as readily as do tissues from most nonruminant species. For example, we recently demonstrated that muscle from cows oxidized palmitate (a representative long-chain fatty acid) at much lower rates than the same muscle from rats, while cow muscle oxidized acetate at much greater rates than did rat muscle. As fat content is increased or decreased in milk replacer at the same crude protein (CP) content, the amount of lactose varies inversely. We speculated that lactose would be a better fuel for lean tissue growth in young calves than would fat. This experiment tested that hypothesis by comparing calves fed a typical moderate fat, high lactose milk replacer with a high-fat, moderate lactose milk replacer or whole milk.

Materials and Methods

Eighteen 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 three treatments: 1) a conventional milk replacer containing 22% CP and 19% fat; 2) a high-fat milk replacer containing 25% CP and 28% fat; 3) whole milk. Milk replacers contained only milk proteins, were reconstituted to 12.5% solids, and were fed twice daily. Feeding rates were determined to equalize initial intakes of metabolizable energy (ME) and CP, so that the only differences would be in the proportion of ME supplied by fat or lactose. Thus, the conventional milk replacer was fed at 14% of body weight, whereas the high-fat milk replacer and whole milk were fed at 11.65% of body weight (Table 1).

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. Weights of carcass, viscera, and the head-hide-hooves-tail fractions were recorded. These fractions were ground and analyzed for contents of water, protein, fat, ash, and energy. 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. Treatments were compared with orthogonal contrasts of 1) medium fat milk replacer versus high fat diets, and 2) high-fat milk replacer versus whole milk.

Results

Our experimental design was successful in obtaining statistically similar initial intakes of CP and ME (Table 1). Intake of lactose was greater for calves fed the medium-fat conventional milk replacer, as designed. Initial intakes of fat did not differ significantly among groups, but tended to be greater for the high fat diets. The ADG was greater for calves fed the medium-fat diet than for calves fed the high-fat diets, but efficiency of gain did not differ significantly. Although the content of protein in the final whole-body composition was greater for the high-fat groups, overall gains of water and protein (lean tissue) were greater for the calves fed the medium-fat conventional milk replacer (Table 1).

Table 1. Growth of male Holstein calves fed high-fat diets (milk or milk replacer) or high-lactose diet (milk replacer) at equal protein and energy intakes.

Diet

A recently published experiment from Cornell University using a somewhat similar design showed that calves fed high fat milk replacers deposited more fat, but lean tissue deposition and ADG were not affected. Taken together with our data, we conclude that high fat contents do not promote lean tissue deposition, and that modest fat contents actually may result in greater growth of lean tissue in young calves. Furthermore, literature data shows that higher fat contents in milk replacer decrease consumption of calf starter.