Solid feeds (SF), comprising roughages and concentrates, represent an increasingly important source of nutrients for veal calves. From a welfare and economic perspective, there is a strong incentive to replace a considerable portion of the milk replacer (MR) by SF in the diet. However, interactions between MR and SF complicate the prediction of the nutritional value of these ration components, and adverse effects on health may occur when combining MR and SF. To investigate these interactions, various combinations of MR, concentrates, and roughages were tested in a series of large-scale studies.
When provided with unrestricted access to MR, concentrates, maize silage, hay, and straw over a 6-month period, calves markedly changed their preferences over time, and individual differences appeared very large. However, the ratio between digestible crude protein and digestible energy in the diet of choice appeared remarkably constant between calves. Another set of studies aimed at defining age-related changes in utilization efficiency of SF. It was demonstrated that stimulating early rumen development (before 12 wk of age) improves the nutritional value of each kg of SF in later life. In another study, it was shown that the nutritional value of SF increases with age. This effect is likely related to improved fermentation of fibrous SF. Increasing SF intake lead to an increase in the passage rates of concentrates and straw through the rumen.
Compared to the feeding of MR alone, nitrogen (N) economy of veal calves can be improved by feeding a low-protein SF, creating a N shortage in the rumen. Urea-N, likely originating from the MR, was demonstrated to recycle back into the rumen for microbial protein production. In a subsequent study, it appeared that the feeding of a high-protein SF improved ruminal degradation of fibrous SF relative to a low-protein SF at equal protein intake, balanced via the MR. Urea recycling was demonstrated to be unable to completely compensate a N shortage in the rumen. An important interaction between MR and SF can be the influence of SF on the proportion of MR flowing in the rumen, where it is fermented and potentially causes health problems. The current standard to measure this so-called ‘ruminal drinking’ is the Co recovery method, which requires sacrificing the calves. Several non-terminal methods to quantify ruminal drinking were evaluated in three consecutive experiments. From a meta-analysis of Co recovery data, it was shown that on average 17% of the MR fed flows into the rumen instead of the abomasum. No associations with SF or MR intake related variables were found. Potential adverse effects of replacing MR by SF include abomasal damage, particularly in the pyloric area. This generally increases with the intake of SF, particularly in the presence of sharp, abrasive particles, and more so with a 20:80 than with a 50:50 mixture of roughage:concentrate. Results indicated that early rumen development can offer some protection in later life.In conclusion, when taking interactions between MR and SF into account, it appeared possible to replace a considerable portion of MR by SF without compromising calf performance and health.