Effects of level and oscillation of dietary crude protein on ruminal conditions

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

Our trial examined dietary crude protein (CP) feeding pattern (O = oscillating, S = static) at two levels of dietary CP (LP, 13.8%; HP, 15.5%) to test for hypothesized effects on ruminal conditions. Using a 2x2 factorial arrangement, we assigned 8 mid- to late-lactation Holsteins (M = 135, SD = 9 DIM) to treatment sequences in a Latin Rectangle with four 28-d periods. O alternated 2 diets (O-LP 12.2-15.5%, O-HP 13.8-17.3% CP) every 48 hr so average dietary CP equaled that of the corresponding S treatments (S-LP, S-HP). Dietary differences were implemented by exchanging soybean hulls, ground corn, and expeller soybean meal with solvent soybean meal. On d 25-28 of each period, we collected fluid from the ventral rumen per cannula at -2, 0, 2, 4, and 11 hr relative to 1x daily feeding. We measured pH with a portable meter and used colorimetric methods to quantify NH3-N and total amino acids (TAA) on a flow injection analyzer. Data were analyzed using a linear mixed model with fixed effects for CP level, CP feeding pattern, their interaction, period, day, hour, and all treatment by time interactions, and a nested random effects structure. During d 16-27 of the fourth period, we monitored ruminal pH and redox potential (Eh) at 1-min. intervals with indwelling loggers and tested for differences in the daily minimum, mean, and maximum associated with diet changes in the oscillating condition (n = 4 cows). Ruminal pH (6.37 ± 0.03) and concentration of TAA (2.65 ± 0.11 mM) were unaffected by treatments, but NH3-N was greater for HP than LP (2.32 vs. 1.84 ± 0.12 mg/dL NH3-N, P<0.001). For all variables, non-significant feeding pattern by day interactions indicated that rumen conditions were unaffected by dietary changes in the O condition (P = 0.326 to 0.726). This finding was consistent with indwelling logger measurements, which showed similar daily pH and redox potential across higher- and lower-CP phases within the oscillating CP feeding pattern (P = 0.117 to 0.833). In conclusion, large variation in dietary N concentration had little effect on ruminal pH and NH3-N and TAA concentrations.

Keywords:  rumen, oscillating, protein

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