The protein in alfalfa silage is subject to extensive degradation to non protein nitrogen (NPN) in the silo. Furthermore, the NPN in alfalfa silage is rapidly degraded to ammonia in the rumen and, if not captured as microbial protein, will end up largely as urea-N excreted in the urine. Milk production and milk nitrogen (N) efficiency have improving potential through amino acid balancing. Milk-N per N intake is an index for the environmental impact of the fed protein and the efficient usage of the protein for milk production. A trial was conducted by Broderick and Muck (2009) to study the effects of adding rumen-protected methionine as Mepron® on production and nutrient utilization.
Thirty-six lactating Holstein cows were fed a 3 x 2 arrangement of diets formulated from alfalfa ensiled in bag, bunker, or oxygen-limited silos, and supplemented with either 0 or 15 g of Mepron®. Experimental diets averaged 41 % alfalfa silage, 24 % corn silage, 24 % high-moisture corn, 3.7 % soybean meal, 4 % roasted soybeans, 2 % ground shelled corn, 1 % minerals and vitamins, 16.7 % crude protein and 31 % neutral detergent fiber (NDF) on a dry matter (DM) basis. Alfalfa from the oxygen-limited silo showed greater nutrient preservation.
The more favorable composition of alfalfa from the oxygen-limited silo resulted in greater intake and digestibility but was not reflected in improved milk production which was not different among alfalfa sources. Because intake was greater but production was unchanged, feed efficiency (milk/DM intake) was lower on diets containing alfalfa from oxygen-limited silo versus ensiled in the bag or bunker silos.
As shown in Table 1, supplementation with Mepron® tended to improve feed efficiency and true protein yield, and increased N efficiency by 7 % across all diets. The improved conversion of dietary crude protein to milk protein suggested that the 40 g/d greater yield of milk true protein was a real effect due to providing the limiting essential amino acid.
The requirements for lysine and methionine for maximal milk protein concentration and yield are 7.2 % Lys and 2.4 % Met in metabolizable protein (MP), giving an optimal Lys/Met ratio of 3.0 (NRC 2001). Based on the NRC (2001) model, Lys supply was on average 6.7 % of MP and Met supply was 1.9 % of MP. Mepron® was used to correct the imbalance which increased Met supply in this study to 2.3 % of MP and reduced the Lys/Met ratio from an average of 3.5 to 2.9 (Table 2). This suggests that methionine limited production before Mepron® addition, and lysine was probably next limiting after Mepron® addition to the diets fed in this experiment.
Supplementation with Mepron® improved the amino acid balance by reducing the Lys/Met ratio to the optimal level. This resulted in a numerically greater protein yield and in a significantly improved N efficiency.