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FULL TEXT PAPER
Protein and Rumen-Protected Methionine and Lysine for Dairy Cows
by Michael S. Piepenbrink, Thomas R. Overton, and Jimmy H. Clark


TAKE HOME MESSAGES

  • Supplemental rumen-protected amino acids (RPAA) increased the concentrations of methionine (Met) and lysine (Lys) in plasma, suggesting an increased availability of absorbable Met and Lys for synthesis of milk and milk protein.
  • Supplying these RPAA to the cows did not increase the yields of either milk or milk protein.
  • Increasing the crude protein (CP) content of the diet from 14 to 18% increased the yields of milk and milk protein which suggest that some other nutrient was co-limiting or more limiting than Met and Lys for synthesis of milk and milk protein by cows fed the 14% CP diet.

INTRODUCTION

The most limiting amino acids for synthesis of milk and milk protein have been reported to be Met and Lys. Feeding these amino acids in an unprotected form to dairy cows results in their degradation by microbes in the rumen before they pass to the absorption sites in the small intestine. To supply additional Met and Lys for production of milk and milk protein, methods have been developed to protect these amino acids from microbial degradation resulting in the RPAA passing to the abomasum and small intestine where they are released and absorbed. Because the amounts and proportions of amino acids in duodenal digesta vary when different diets are fed, it is difficult to determine which amino acids are limiting. The amino acid submodel of the Cornell Net Carbohydrate and Protein System has been developed to predict dietary deficiency or excess for growing or lactating cattle. The objectives of this experiment were to investigate the effects on dry matter intake (DMI), milk yield and composition, and amino acids in plasma of dairy cows fed diets formulated to be deficient in Met and Lys or formulated to supply adequate Met and Lys.

MATERIALS AND METHODS

Ten multiparous Holstein cows averaging 128 days in milk were utilized in a replicated 5x5 Latin square with 14-day periods. Diets were formulated using the amino acid submodel of the Cornell Net Carbohydrate and Protein System either to provide adequate Met and Lys (18% CP) or to cause a deficiency of Met and Lys (14% CP). The 18% CP diet contained 26.0% corn silage, 24.0% alfalfa silage, 27.1% ground shelled corn, 13.4% soybean meal, 3.2% corn gluten meal, 2.8% soy hulls, and 3.5% minerals and vitamins. The 14% CP diet contained 20.0% corn silage, 30.0% alfalfa silage, 35.8% ground shelled corn, 6.5% soybean meal, 4.3% soy hulls, and 3.5% minerals and vitamins.

Increasing amounts of rumen-protected Met and Lys were fed to cows consuming the 14% CP diet to supply 0, 50, 100, or 150% of the predicted deficiency of Met and Lys. The availability of the amino acids in this prototype product at the small intestine was estimated to be 35% (Ajinomoto Company, Tokyo, Japan; 1993; personal communication). Treatments were: 1) 18% CP, 0 g/d Met, and 0 g/d Lys; 2) 14% CP, 0 g/d Met, and 0 g/d Lys; 3) 14% CP, 11 g/d Met, and 35 g/d Lys; 4) 14% CP, 22 g/d Met, and 70 g/d Lys; and 5) 14% CP, 33 g/d Met, and 106 g/d Lys. Milk production, milk composition, and DMI were measured during the last week of each period. Blood was drawn from the tail vein of each cow at 1000 hour on the last day of each period to determine concentrations of amino acids in blood plasma.

RESULTS AND DISCUSSION

Cows fed the 18% CP diet consumed 2.4 lb/d more dry matter than cows fed the 14% CP diet (Table 1). Yield of milk by cows fed the 18% CP diet was greater than that by cows fed the 14% CP diet without RPAA. This increase was probably the result of greater DMI by cows fed the 18% CP diet, which presumably improved the availability of amino acids and energy. Amount of CP in the diet did not affect percentage or yield of milk fat, but yield of milk fat by cows fed the 18% CP diet was slightly greater because milk yield was increased by 1.7 lb/d. This increase resulted in greater 3.5% FCM yield by cows fed the 18% CP diet. Feeding the 18% CP diet tended to increase the percentage and increased the yield of milk CP compared with the 14% CP diet without RPAA.

In contrast to the effects of amount of CP in the diet, RPAA did not affect DMI of cows. Milk yield also was not affected when RPAA were fed to the cows. Feeding RPAA affected milk fat yield quadratically and tended to affect milk fat percentage quadratically such that those yields and percentages were greater when 0 g/d Met plus 0 g/d Lys or 33 g/d Met plus 106 g/d Lys were fed and decreased when 11 g/d Met plus 35 g/d Lys or 22 g/d Met plus 70 g/d Lys were fed to the cows. This quadratic effect on milk fat yield resulted in a trend for a quadratic effect of RPAA on 3.5 % FCM yield such that yield was greater when 0 g/d Met plus 0 g/d Lys or 33 g/d Met plus 106 g/d Lys were fed. Percentage of CP in milk increased linearly as amounts of RPAA fed to cows increased; however, yields of milk CP were not affected by RPAA supplementation because milk yield decreased slightly.

The concentration of Lys in plasma tended to be greater for cows fed the 18% CP diet than for cows fed the 14% CP diet without RPAA but the concentration of Met in plasma was not affected by the amount of CP in the diet. Supplemental RPAA linearly increased the concentration of Met in plasma and quadratically increased the concentration of Lys in plasma, suggesting that Met and Lys were delivered postruminally for absorption from the small intestine.

Table 1. Effects of feeding supplemental protein and rumen-protected amino acids to dairy cows.

Variable 18% CP
No Lys
No Met
14% CP
No Lys
No Met
14% CP
35 g/d Lys
11 g/d Met
14% CP
70 g/d Lys
22 g/d Met
14% CP
106 g/d Lys
33 g/d Met
DMI lb/d 52.0 49.6 50.0 50.0 49.6
Milk, lb/d 60.1 58.4 57.0 57.3 56.6
Fat, % 3.81 3.80 3.77 3.73 3.85
Fat, lb/d 2.27 2.20 2.14 2.14 2.18
3.5% FCM, lb/d 62.8 61.0 59.5 59.5 59.9
Milk protein, % 3.46 3.43 3.42 3.45 3.48
Milk CP, lb/d 2.07 1.98 1.96 1.98 1.98
Met, µmol/dl 2.43 2.16 2.65 3.01 3.18
Lys, µmol/dl 9.11 7.96 9.51 11.06 10.00






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