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1 Department of Agricultural, Food and Nutritional Science, University of Alberta, Edmonton T6G 2P5
2 Dairy and Swine Research and Development Centre, Agriculture and Agri-Food Canada, Lennoxville, QC, J1M 1Z3
3 Purina Mills Inc., St. Louis, MO 63166-6812
4 Instituto de Producción Animal, Universidad Austral de Chile, Valdivia, Chile
Corresponding author: H. Lapierre; e-mail: lapierreh{at}agr.gc.ca.
Most prediction schemes of milk protein secretion overestimate milk protein yield from dairy cows at high protein intakes, thereby overestimating milk protein yield response to protein supplementation. This study was conducted to determine factors contributing to such an overestimation. Using published studies, a database was constructed that was limited to amino acid (AA) infusion studies, as then only the digestible amino acid of dietary origin needed to be estimated, whereas the amount infused was known exactly, thereby reducing the dependence on estimated values.
Although milk protein yield was positively related with total energy supply, and both digestible duodenal supply and infused AA, in this database there was no relationship between milk protein yield response above control treatments and the nutrient status of the cows (energy or protein). Total milk protein yield was defined as a function of individual AA supply, using a segmented-linear and a logistic model to obtain estimates of the efficiency of conversion of AA into milk protein. Except for Lys and Met supply, the segmented-linear model yielded lower root mean square error and better correlation, but both models were similar in their reliability. For both models, the estimated efficiency of conversion of AA to milk differed among AA. Estimations of the ideal profile of AA for lactating dairy cows were similar between models, with requirements for Lys and Met in line with 2001 National Research Council recommendations. The major difference is that the segmented-linear model yields a constant efficiency of conversion of an AA until requirements are met, with zero efficiency beyond this point. The logistic model allows for an estimation of the decreasing marginal efficiency of conversion of AA as the supply approaches the requirements. The use of variable efficiency factors should improve our ability to predict protein yield in response to supplemental protein.
Key Words: amino acid • requirement • lactation • efficiency
Abbreviation key: AAT = total digestible AA from diet and infusion, CPT = total CP supply from diet and infusion, EAA = essential AA, MP = metabolizable protein, MPT = total MP supply from diet and infusion, NEAA = nonessential AA, NELT = total NEL supply from diet and infusion, PDI = protein truly digested in the small intestine, PY = milk true protein yield, PY
= milk protein response, RMSE = root mean square error
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