THE IMPORTANCE OF SPECIFIC AMINO ACIDS IN DAIRY COW DIETS.

Canola meal is the second largest protein feed in the northern hemisphere and is also fed in South Africa. Its utilization is expected to increase. However, a recent study by the same authors showed that even though higher inclusion levels of canola meal had a positive effect on production when it directly substituted a high protein maize-based dried distillers grains, there was an optimum point at 12 to 13.5% of dietary dry matter (DM) after which animal performance seemed to decline. It was concluded that only the amino acids methionine, phenylalanine and leucine could have limited production; the conclusion was based upon plasma amino acid concentrations at the highest canola meal inclusion level.

In the present experiment (referenced below), the objective was to determine if either methionine or phenylalanine, or both, was limiting performance of early lactation dairy cows fed a ration containing 18% of dietary DM as canola meal. This was achieved by supplementing a calculated amount of 7.5 g of intestinally absorbable phenylalanine per cow per day and/or 8.0 g of intestinally absorbable methionine per cow per day in a ruminally protected format to four pens of about 320 early lactation cows per pen in a 4 × 4 Latin square with 28 days experimental periods.

Dry matter intake (averaging 27.6 kg per day) was not affected by feeding either of the ruminally protected amino acids, or the combination. Phenylalanine supplementation alone had no effect on milk production or composition and body condition score (BCS) change compared to the Control, which did not receive ruminally protected amino acids. Methionine supplementation alone modestly increased milk protein and fat content, while decreasing milk lactose content and yield, but had no impact on BCS change compared to the Control. The combination methionine and phenylalanine supplementation decreased milk and lactose yields, as well as lactose content, while increasing milk protein content and the BCS change. Plasma methionine levels increased with both the methione alone and the methionine plus phenylalanine treatments and plasma tryptophan levels decreased with both the phenylalanine alone and the phenylalanine plus methionine treatments. However, the plasma phenylalanine levels did not change with any treatment. The authors interpreted the results as suggesting that the delivery of methionine to the tissues for protein synthesis with ruminally protected methionine feeding was higher than the requirements of the cows and caused an oversupply of methionine. The addition of phenylalanine to the methionine supplementation changed the way energy was utilized by the cows, redirecting energy liberated by methionine from milk components toward BCS gain.

Comment: Methionine, being the first limiting amino acid for milk production, for some time has been supplemented in a format that escapes rumen fermentation, since if not protected it will be broken down during rumen fermentation. However, the success of the supplementation has been variable. The results of this South African-US study can be seen as a breakthrough as (1) it was shown that a major reason for the variability is because the recommended ruminally protected methionine may be in excess of requirements and (2) that the methionine utilization depends on the levels of other amino acids with which the methionine  is associated, as they may also in excess or limiting.  

Reference:

N. Swanepoel, P.H. Robinson & L.J. Erasmus, 2015. Effects of ruminally protected methionine and/or phenylalanine on performance of high producing Holstein cows fed rations with very high levels of canola meal. Anim. Feed Sci. Techn. 205, 10-22.