Blood amino acids and milk composition from cows fed soybean meal, fish meal, or both

Twelve multiparous Holstein cows at 48 +/- 8 d in milk were used in a 4 x 4 Latin square with 21-d periods to determine the effect on feed intake, mil k yield, milk composition, and blood amino acids when soybean meal was repl aced with fish meal. Fish meal substituted for soybean meal on an isonitrog enous basis at 0, 25, 50, and 100% of supplemental protein. Total mixed die ts were (dry matter basis) 25% corn silage, 25% alfalfa hay, and 50% concen trate mix. Intake of dry matter (27.9, 27.8, 26.1, and 25.8 kg/d for diets 1 to 4, respectively) was similar for all diets. Milk yield (37.5, 37.8, 37 .2, and 37.7 kg/d) was not affected by diets. Milk protein percentages (3.2 3, 3.24, 3.31, and 3.35) increased with 100% fish meal supplementation and tended to be higher, with 50% fish meal supplementation compared with 100% soybean meal diet. Milk fat percentages (3.18, 2.99, 3.04, and 2.87) and yi elds were lower with the 100% fish meal than with the 100% soybean meal die t. Molar proportions of ruminal volatile fatty acids and ammonia were not g reatly affected by diet. Fish meal supplementation slightly improved Met st atus, as shown when blood amino acid data were evaluated. Both extraction e fficiency and transfer efficiency of amino acids from the blood by the mamm ary gland indicated that Met, Lys, and Phe were the most limiting amino aci ds in all diets. Replacing as much as 50 or 100% of dietary soybean meal wi th fish meal may improve the amino acid balance and increase the protein co ntent in milk; however, feeding 100% fish meal will likely decrease milk fa t percentages.