Relationship between content of crude protein in rations for dairy cows and milk yield, concentration of urea in milk and ammonia emissions

During recent decades, efforts have been made in several countries to diminish the negative environmental influence of dairy production. The main focus has been on nitrogen and phosphorus. Modern dairy production in Western Europe is often based on imported feedstuffs, mostly protein-rich feeds. In Sweden at least, it is wished that the use of imported feedstuffs in animal production will decrease due to the risk of contamination with Salmonella and the ban of using GMO crops in Swedish dairy production. An experiment was carried out to investigate whether a lower content of crude protein in the diet would decrease the ammonia release from cow manure and whether a well-balanced diet using only feedstuffs of Swedish origin would maintain milk production. Five treatments were arranged in a Latin square design. Two different protein supplements made of ingredients of Swedish origin were each fed at two protein levels, and a fifth imported commercial protein mix was fed at the higher level. The treatments with low protein levels (13.1 to 13.5%) had a significantly lower milk yield, kilograms of ECM, but, on the other hand the net profit, milk income minus feed cost was nearly the same in all treatments except diet C, which had lower feed cost but also lower net profit due to lower milk yield. The content of urea in milk was higher with diets high in crude protein (17%) content. A decreased protein level in the diets did not influence the content of casein or whey protein, but the commercial concentrate showed a tendency to give lower values than the Swedish mixtures. The low protein diets gave significantly lower ammonia release from manure compared with the high protein diets. There were no production differences between the diets of Swedish feeds compared with the imported control. The readily fermentable beet pulp should have helped cows use the higher N diet more efficiently and increased the response. This gives the rumen microbes a possibility to match the inflow of protein with carbohydrates. Income over feed costs shows that it is possible to compile diets using products of Swedish origin and still be competitive. On the other hand, this structure may change quickly due to altered world market prices.