Feeding rates of dry matter, and more specifically protein and energy, are directly related to the amount and composition of gain for preweaning calves. A recent article in the Journal of Dairy Science (Bartlett et al., 2024; 107:7842-7850) detailed the growth and body composition of dairy calves fed at three milk replacer feeding levels. This study is very important in determining the energy requirements for growth and the impact on composition of body weight (BW) gain. Is there a limit to how much a calf can consume? Does it make them over-conditioned? How much energy and protein do they need?
All calves were fed a milk replacer containing 24.8% crude protein (DM), 18.9% fat, and reconstituted to 12.5% solids. The three feeding levels used were 1.25% of BW (DM basis); 1.75% of BW, and 2.25% of BW which were adjusted weekly as the calves grew. On an as-is basis the calves were fed 10, 14, and 18% of their BW when the milk replacer was reconstituted. The calves were only fed milk replacer during this period and were evaluated after 35 days on the feeding program to determine body composition.
As dry matter and nutrient intake went up the average daily gain increased linearly. The efficiency in which the calves used the milk replacer for BW gain also increased linearly. The gain-to-feed ratios ranged from 0.55 to 0.81. Other studies have also demonstrated an increase in feed efficiency when more intake is provided, which is partly explained by the dilution of maintenance but also because the first two months of a calf’s life is the time when they are most efficient at increases in BW and stature.
The fecal scores observed across these feeding rates (score of 1 to 4, with 1 being well-formed and 4 being like water) were higher with increased intakes. However, the growth and feed utilization were not diminished indicating that it was not likely due to pathogenic microorganisms, but rather the “loosening” of the feces due to higher intakes of the liquid diet. This agrees with other studies that have shown that higher intake doesn’t lead to scours.
The fat content of the whole body increased with increased feeding rates but overall was low across all treatments. Protein and water decreased with increasing feeding rates, which is the typical inverse relationship with fat. In general, this response is very typical of higher feeding rates and should not be a concern with over-conditioning. Sometimes if protein is inadequate for requirements there is a greater deposition of fat, but that was not the case in this trial. The efficiency of utilization for energy increases with increased intake because of the dilution of maintenance. Furthermore, protein efficiency was lowest on the lower intake, but was similar for calves fed 1.75 or 2.25%. Taken together this means that protein and energy intakes were reasonably balanced.
Overall, calves’ growth is determined by their intake, which determines energy and protein supply for maintenance and growth. Calves at 1.25% DM of BW did not provide adequate energy for maximal growth. By providing calves milk replacer at or above 1.75% DM of BW with adequate protein, there is a positive response to capturing the high efficiency of these animals for increased growth and body composition.
— Sarah Morrison