Rumen protected amino acids are increasingly being used in dairy cattle total mixed ration formulation to obtain the required concentrations of the amino acids lysine and methionine in metabolisable protein for optimal milk and milk protein production. Lysine and methionine normally are limiting in generally-used feedstuffs used for dairy cow diets. The objective of this study was to evaluate the relative bioavailability of a liquid rumen protected lysine prototype, using milk composition as the distinguishing technique.
Forty mid-lactation Holstein cows were used in a complete randomised block design experiment and allocated to one of four experimental treatments. The treatments were: (1) a methionine deficient diet as Control, (2) the Control diet supplemented with Smartamine M (SMM) which supplies rumen protected amino acids, (3) the Control diet supplemented with liquid rumen protected lysine prototype (LRPMet) and (4) the Control diet supplemented with DL-methionine (DLMet), which is a hydroxyl analogue of methionine. After an adaptation phase, all four groups received the Control diet and thereafter 10 cows per treatment were switched over to one of the four treatments allocated to them.
The cows supplemented with SMM had higher milk protein and milk fat % compared to the other treatments. They also had higher milk casein %, when compared to the Control treatment. The milk protein percentages were 3.06, 3.25, 2.95 and 3.46 and the milk fat percentages 3.84, 3.93, 3.75 and 4.27 for the Control, LRPMet, DLMet and SMM treatments, respectively. SMM, as in previous experiments, proved to contain rumen protected amino acids with high relative bioavailability, while the LRPMet failed to elicit any milk yield or milk composition response.
The milk composition technique proved to be a simple but effective technique to evaluate the bioavailability of rumen protected products or prototypes. In addition, the results showed that there are promising amino acid supplementary products on the market to improve milk yield and milk composition, but it is important that they are tested thoroughly and independently.
Reference Erasmus, L.J., Bester,Z. & Coertze,R.J., 2013. Milk composition as technique to evaluate the relative bioavailability of a liquid rumen protected methionine source. S.Afr. J. Anim. Sci. 43, S93-S97.