Submitted to: Journal of Dairy Science
Publication Type: Abstract Only
Publication Acceptance Date: 7/10/2006
Publication Date: 7/10/2006
Citation: Reynal, S.M., Broderick, G.A. 2006. Optimal nutrient intake and digestion for ruminal microbial protein and milk yields in lactating dairy cows [abstract]. Journal of Dairy Science. 89(suppl. 1):81. Interpretive Summary:
Technical Abstract: Individual-cow data from six in-vivo experiments (248 observations) conducted in our laboratory were used to study the response of microbial nonammonia nitrogen flow (MNAN) from the rumen and milk yield (MY) of lactating dairy cows to several dietary and digestion factors. Diets were based on high-moisture corn, corn silage, and legume silage (mainly alfalfa) and ranged from 13.5 to 20.3% CP (mean = 17.4%), from 43 to 55% NFC (mean = 49%), and from 22 to 30% NDF (mean = 25%). Urea, heated and unheated soybean meal, corn gluten meal, blood meal, canola meal, and cottonseed meal were used as protein supplements. Cows averaged 113 days in milk, consumed between 14 and 34 kg of DMI/d (mean = 23 kg/d), and produced between 14 and 55 kg milk/d (mean = 36 kg/d). Digestion variables were measured in vivo using the omasal sampling technique and total purines (1 study) and 15N (5 studies) as microbial markers. Data were analyzed using ProcMixed in SAS following the meta-analysis approach of St. Pierre et al. (J. Dairy Sci. 84:741) with study considered as random in the models. Response variables evaluated were OM truly digested in the rumen (OMTDR), rumen-degraded (RDP) and rumen-undegraded (RUP) protein, DM intake (DMI), N intake, dietary CP content, and ruminal fiber digestibility. MNAN was evaluated as a response variable for MY. The equation for MNAN was: MNAN, g/d = 36 (SE=82; P=0.68) + 32 RDP (% of DM; SE=12; P<0.01) - 0.8 RDP2 (% of DM; SE=0.43; P=0.06) + 0.57 OMTDR2 (kg/d; SE=0.06; P<0.01) The equation for MY was: MY, kg/d = -8.3 (SE=7.8; P=0.34) + 1.18 RUP (% of DM; SE=0.26; P<0.01) + 0.29 DMI (kg/d; SE=0.13; P<0.05) + 0.115 MNAN (g/d; SE=0.030; P<0.01) - 0.0001 MNAN 2 (g/d; SE=0.00004; P<0.01) Quadratic maxima for MY was at 573 g of MNAN. Milk yield responses to RUP supplementation (slope = 1.18) were substantially smaller than those predicted by the NRC (2001; slope = 1.85). For this data set, maximum MNAN was achieved at a dietary RDP level (20% of DM) that may result in inefficient N utilization and high N losses to the environment.