ADSA 2022 Abstract: Effects of dietary crude protein level and feeding pattern on nitrogen balance and nutrient digestibility
MaryGrace Erickson1, Geoffrey I. Zanton2, and Michel A. Wattiaux1
1Department of Animal & Dairy Sciences, University of Wisconsin—Madison, Madison, WI, USA
2USDA-Agricultural Research Service, US Dairy Forage Research Center, Madison, WI, USA
Limited research has assessed nitrogen (N) partitioning in relation to dietary crude protein (CP) level and feeding pattern. To test a hypothesized interaction, we examined 8 mid- to late-lactation Holsteins (M = 142, SD = 23 DIM) in a Latin Rectangle with 2x2 factorial treatments CP level (LP, 13.9%; HP, 15.4%) and feeding pattern (O = oscillating, S = static). O alternated 2 diets (O-LP 12.2-15.5%, O-HP 13.9-16.9% CP) every 48 hr so mean diet composition equaled that of corresponding S treatments (S-LP, S-HP). Diets exchanged soy hulls and ground corn with solvent soybean meal to maintain constant ratios of NDF:starch (1.18), rumen-degradable protein:CP (0.61), and forage:concentrate (60:40). Each 28-d period on d-25-28, we weighed and sampled total collections (3x/d) of feces and urine and collected samples of milk (2x/d) and orts (1x/d). We analyzed feed and feces period composites and individual urine samples. We fit linear mixed models with fixed CP level, feeding pattern, and period effects, and a random intercept for cow; computing model-implied means and standard errors (SEM). Neither CP level, feeding pattern, nor the interaction affected production of milk (38.9±1.9 kg/d) or fat-protein-corrected milk (37.1±1.4 kg/d). Neither CP level, feeding pattern nor the interaction affected dry matter intake (25.0±0.8 kg/d) or apparent total tract digestibility of neutral detergent fiber (aNDFom; 56.6±1.3%) or organic matter (72.9±0.7%). Relative to LP, HP increased N intake (546 vs. 627 g/d, SEM = 23) and urine N (153 vs. 207 g/d, SEM = 6), and tended to increase fecal N (183 vs. 201 g/d, SEM = 8). Neither CP level, feeding pattern, nor the interaction affected milk N (177±7 g/d), N digestibility (69.6±0.6%), or apparent N balance (38±11 g/d). Compared to LP, HP reduced N use efficiency (32.4 vs. 28.9%; SEM = 1.1) and increased N intake allocated to urine (28.4 vs. 33.5%; SE = 0.8) both of which were unaffected by feeding pattern or the interaction. Results showed CP level affected N partitioning regardless of feeding pattern, which had minimal effects.
Keywords: total collection, protein oscillation, dairy cow
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