Graduate Student Competition: ADSA Production Division Oral Competition, PhD Division

Greater feed N efficiency for milk N could enhance dairy farm profits and reduce manure N excretion and environmental risks. The objective was to evaluate relationships between feed efficiency for milk (FEmk, kg milk/kg DMI) and measures of feed N efficiency in dairy cows. Eight pairs of mid-lactating cows (primiparous or multiparous <16 DIM apart within each pair) with high and low FEmk of 1.78 and 0.98 kg/kg, respectively, were fed a TMR (47% DM) consisting of 28% corn silage, 27% alfalfa silage and 45% concentrate, containing 16% CP and 28% NDF (DM basis) for at least 4 weeks. During a 3-d period TMR, feed refusals, milk, total feces and total urine were measured, sampled daily and analyzed for N concentrations. Data were analyzed as a randomized complete block with cow pairs as blocks and FEmk as treatment. High FEmk cows had 15% greater N intake (NI), 32% greater fecal N (FN) excretion and 5% lower apparent N digestibility than low FEmk cows (see Table 1). Despite similar digested N high FEmk cows secreted 1.9 times more N in milk than low FEmk cows, yet urine N (UN) and manure N (FN + UN) excretion were similar. High FEmk cows had greater feed N efficiencies (30% of NI secreted as milk N) than low FEmk cows (19%). Also, high FEmk cows excreted 45% less manure N/milk N and manure UN to FN ratio was 30% lower than in low FEmk cows. High FEmk cows were more feed N efficient due to metabolic rather than digestive processes. Selection for high FE cows may increase feed energy utilization as well as feed N utilization and decrease risk of manure ammonia and nitrous oxide emissions due to reduced manure UN to FN ratio.