The interest in using rapeseed and other locally produced protein feeds is increasing as the use of soy is questioned. The compound of amino acids underlies the beneficial effects of rapeseed meal in feed for dairy cows.
Many feeding trials with rapeseed have been performed. In a meta-analysis made by Pekka Huhtanen, SLU, results from many previous feeding trials were compiled. It showed that rapeseed meal gives as good milk production as soybean meal. Recently, Canadian researchers showed that one reason for the positive effects of rapeseed meal is due to the composition of amino acids.
Better utilization of protein
Rapeseed has a lower content of crude protein than soy, but in the experiment the diets were balanced for crude protein by feeding more of rapeseed meal. Then the rapeseed meal ration gave as good a milk yield as with soybean meal. In addition, cows fed rapeseed meal had lower urea content in milk, indicating a better utilization of nitrogen from oilseed rape than from soy. When the amino acids were analysed in cows' blood, a higher content of methionine was found when the cows got rape, and slightly higher levels of lysine and histidine, than when they were fed with soybean meal.
Expellers gave higher yields
Hexane-extracted soybean meal and rapeseed meal is not allowed in organic production, but feeding of rapeseed and rapeseed cake/expeller has given good results in experiments. In a Finnish trial, where the cows were fed additionally with higher forage proportion than in the Canadian study, it was found that expeller gave a higher yield and a tendency to higher levels of methionine in cows' blood than when they were fed soya expeller. The researchers concluded that cows with rape had a better nitrogen utilization.
Rapeseed also feeds the microbes in the rumen
Also in a Swedish experiment where rapeseed cake compared with a conventional concentrate containing larger proportions of soy, a positive effect on milk yield was found when rapeseed cake was used, despite that the protein in rapeseed cake is not very stable in the rumen. In order to still get a good utilization of the protein, it is important that the microbes in the rumen thrive and get energy for their growth and decomposition of feed proteins. Rapeseed and rapeseed cake contains fat, which provides energy to the microbes and thus microbe-protein is contributing with important amino acids which can be absorbed in the intestine. In another experiment, feeding of rapeseed cake was compared with feeding of rapeseed in an organic diet. Rapeseed is high in fat so it cannot be fed in large quantities without disturbing the balance of microorganisms in the rumen. The experiment shows that it can be economically viable to press the rapes to cakes.
Overall, rapeseed has a better amino acid composition than soy, which has positive effects on milk yield and nitrogen efficiency. Rapeseed in forms that can be used in organic diets often contains fat, which in moderate amounts contributes to the growth of microorganisms. In this way a double positive effect is achieved from feeding with rapeseed products.