Nitrogen Efficiency in Late Lactation Dairy Cow

Our main objectives were to (Project A) determine the partitioning of N from alfalfa silage (AS), corn silage (CS), corn grain (CG), and soybean meal (SBM) in lactating cows; and (Project B) study the effects of incremental reduction in dietary crude protein (CP) concentration on N metabolism of late-lactation dairy cows.;In Project A, 12 multiparous cows were fed one of four dietary treatments constructed by replacing one feed ingredient of the unlabeled pretreatment TMR with its corresponding 15N-labeled ingredient (grown with 15N-labeled fertilizers). Cows were fed dietary treatments for four days and were fed the unlabeled TMR from day another four days. Results of different modeling approaches using a single exponential pool model suggested that N from concentrates contributed to a greater extent to milk N than N from silages. Corn grain N contributed the least to urinary N compared with other feed ingredients. These results provided evidence for distinct metabolic and digestive utilization of the N from AS, CS, CG, and SBM. Overall results suggested more excretion of urinary N and fecal N relative to milk N secretion for silages than for concentrates. In project B, 16 pens of eight late-lactation cows were fed for 12 weeks one of four diets that contained CP concentration of 16.2, 14.4, 13.1, and 11.8%. Reducing dietary CP reduced, in a linear fashion, the mass of dry matter, CP, and starch digested in the digestive tract. Ruminal concentrations of ammonia and branched-chain amino acids (AA) were linearly reduced with the reduction in dietary CP. Milk N secretion and excretion of urinary N and fecal N were linearly reduced with the reduction in dietary CP. Serum concentration of essential and non-essential amino acids were not affected by dietary treatments. Muscle N breakdown increased with the reduction of dietary CP concentration in week 1 and 4 of the experimental period, however this effect was not present in week 12. Muscle N mobilization was insufficient to maintain production level, but might have been instrumental in maintaining other physiological functions such as pregnancy.