Research conducted at the US Dairy Forage Research Center in Madison (WI) has shown the potential for improving milk nitrogen (N) efficiency by more than 20 % through balancing amino acids and reducing crude protein levels fed. Milk nitrogen efficiency is a measure of how much of the protein (hence nitrogen) fed appears in milk versus being wasted and posing environmental challenges.
The forage base consisted of alfalfa silage (21 % of DM) and corn silage (28 % of DM). Rolled high moisture (HM) corn was the sole grain and protein supplementation was from soybean meal (SBM) and roasted whole soybeans. Soy hulls (5.8 % of DM) were fed in all treatments to avoid excessive starch intake and potential acidosis problems. Treatments with 24 cows were 18.6, 17.3, 16.1 and 14.8 % crude protein (CP). The highest CP diet had no supplemental methionine, but as CP was reduced in the other diets by replacing SBM with HM corn, Mepron® was fed to maintain methionine supply.
Effects on Nitrogen Use
As shown in the Table below, nitrogen and therefore crude protein was used more efficiently as diet protein declined. This occurred as a consequence of lower N intake, higher milk yield and higher milk protein yield for the 17.3 and 16.1 % CP diets. The lowest CP did not support adequate production and therefore cannot be recommended for commercial use.
The 17.3 and 16.1 % CP diets not only supported higher production but substantially reduced N excreted as urinary urea. Nitrogen in this form is the most likely to volatize and to contribute to air pollution.
Moving from an 18.6 % CP ration to 17.3 or 16.1 % CP improved the efficiency of capture of dietary N in milk by 14 and 21 %, respectively. These changes also reduced the N lost as urea in urine by 28 and 44 %, respectively. This represents a substantial opportunity for dairies to be more environmentally responsible without paying a production penalty.
Results from this experiment indicate that Mepron® plus HM corn can be used to replace part of the CP that is normally fed as soybean meal. By supplementation with Mepron®, it was possible to reduce dietary CP from 18.6 to as little as 16.1 % CP without losing production of milk and milk components. Reducing dietary CP to 14.8 % depressed milk production and resulted in mobilization of body protein, which could not be compensated for by Mepron® supplementation.
Amino acid balancing of dairy rations represents a viable approach to meet the challenges of environmental sustainability and production economics. The highest N efficiency, with no loss in performance, was observed in the Mepron®-supplemented diet containing 16.1 % CP.