Crude protein oscillation in diets adequate and deficient in metabolizable protein: effects on nutrient digestibility, nitrogen balance, plasma amino acids, and greenhouse gas emissions

Authors: ERICKSON, MARY GRACE - University Of Wisconsin; Reinhardt, Laurie; Svaren, Levi; Sullivan, Michael; Zanton, Geoffrey; WATTIAUX, M.A. - University Of Wisconsin

Submitted to: Journal of Dairy Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/1/2023
Publication Date: 5/21/2024
Citation: Erickson, M., Reinhardt, L.A., Svaren, L.M., Sullivan, M.L., Zanton, G.I., Wattiaux, M. 2024. Crude protein oscillation in diets adequate and deficient in metabolizable protein: effects on nutrient digestibility, nitrogen balance, plasma amino acids, and greenhouse gas emissions. Journal of Dairy Science. https://doi.org/10.3168/jds.2023-24150.
DOI: https://doi.org/10.3168/jds.2023-24150

Interpretive Summary: This trial examined environmental output effects of two levels of dietary crude protein (CP; 13.8 and 15.5% of dry matter) fed in two patterns (CP oscillating ±1.8% at 48-h intervals, static CP) in mid- to late-lactation Holsteins. Greater CP reduced nitrogen use efficiency and increased manure N. Digestibility and gas production were similar across conditions. Contrary to our hypothesis, CP feeding pattern had no effects on measured variables, regardless of the CP level.These results are valuable to dairy farmers and nutritionists by demonstrating that dairy cows were not negatively affected by short-term reductions in dietary CP, although improvements in environmental outcomes were also not realized.

Technical Abstract: Reducing dietary crude protein (CP) is a well-established means to improve N use efficiency. Yet, few studies have considered if transient restrictions in dietary CP could reduce the environmental footprint of late lactation cows. We hypothesized that the effects of CP feeding pattern would be amplified at lower dietary CP. We tested CP levels below and near predicted requirements (LP, 13.8%; HP, 15.5%) fed in two patterns: where diets alternated ±1.8% CP every 2 days (oscillating; OF) or remained static (SF). Our study used a 2x2 factorial design with 16 mid- to late-lactation Holsteins (Mean = 128, standard deviation = 12 days in milk), divided into rumen-cannulated (n = 8) and non-cannulated subsets (n = 8). For each 28-day experimental period, we recorded feed intake and milk production and took samples of orts (once daily) and milk (twice daily) for four days. For the cannulated subset, we measured and sampled from the total mass of feces and urine production and collected plasma twice daily across four days. For the non-cannulated subset, we sampled gas emissions thrice daily for four days. For each subset, we fit linear mixed models with fixed effects for CP level, CP feeding pattern, and period and a random effect for cow. For selected body urea-N pools, we conducted time series analysis. Contrary to our hypothesis, we found no evidence that dietary CP level and CP feeding pattern interacted to influence N balance, nutrient digestibility, or gas emissions. Results showed HP maintained milk N but increased manure N, reducing N use efficiency relative to LP. For OF, urea-N in urine and plasma peaked 46-52 hours after the first higher-CP phase feeding. Nutrient digestibility and gas emissions were similar across treatments, except CO2 production was greater for OF-HP. In summary, measured variables were minimally affected by dietary CP alternating ±1.8% every 48 hours, even when average dietary CP was fed below predicted requirements (LP). Although our findings suggest that mid- to late-lactation cows are resilient to oscillation in dietary CP, oscillating CP neither reduced the environmental footprint by improving nutrient use efficiencies nor reduced the potential for direct and indirect greenhouse gas emissions.