Abstract
Rumen protected amino acids (RPAA) are increasingly being used in dairy cattle diet formulation to obtain the required concentrations of lysine and methionine in metabolisable protein for optimal milk and milk protein production. The objective of this study was to evaluate the relative bioavailability of a liquid rumen protected lysine prototype using the milk composition 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) Methionine deficient (Met-) diet, (2) Met- diet supplemented with Smartamine M (SMM), (3) Met- diet supplemented with liquid rumen protected prototype (LRPMet) and (4) Met- diet supplemented with DL Met (DLMet), a hydroxyl analogue of methionine. After an adaptation phase all four groups received the Met- diet and thereafter switched over to the four treatments. Cows supplemented with SMM had higher milk protein and milk fat % compared to the other treatments and increased milk casein % significantly when compared to the Met-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 Met-, LRPMet, DL Met and SMM treatments, respectively. SMM again proved to be the RPAA with a 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.Keywords: Milk composition, rumen protected methionine, amino acids, Holstein cows
Highlights
Lysine (Lys) and methionine (Met) have been identified as the two most limiting amino acids (AA) for the synthesis of milk and milk protein (NRC, 2001; Weekes et al, 2006)
To supply additional Met and Lys, methods and technologies had been developed to protect these AA from microbial degradation resulting in the rumen protected AA (RPAA) passing to the abomasum and small intestine where they are released and absorbed
Milk yield was not affected by treatment and did not differ within any of the periods (P >0.05). This was expected since milk yield responses to Lys and Met supplementation are more common in early-lactation than in mid- or latelactation cows (Schwab & Ordway, 2001), as was the case in this study
Summary
Lysine (Lys) and methionine (Met) have been identified as the two most limiting amino acids (AA) for the synthesis of milk and milk protein (NRC, 2001; Weekes et al, 2006). To supply additional Met and Lys, methods and technologies had been developed to protect these AA from microbial degradation resulting in the rumen protected AA (RPAA) passing to the abomasum and small intestine where they are released and absorbed. Before commercialisation of such products or prototypes, information is necessary on the relative or apparent bioavailability of the specific AA (Ordway et al, 2009)
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