Dynamic lactation responses to dietary crude protein oscillation in diets adequate and deficient in metabolizable protein in Holstein cows

Authors: ERICKSON, MARYGRACE - University Of Wisconsin; Zanton, Geoffrey; WATTIAUX, M. - University Of Wisconsin

Submitted to: Journal of Dairy Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/7/2023
Publication Date: N/A
Citation: N/A

Interpretive Summary: This trial compared feeding patterns where crude protein (CP) content oscillated at 48-h intervals to static CP feeding, at two median levels of dietary CP (13.8 and 15.5% of dry matter). Mid-lactation cows performed similarly across conditions. However, both CP feeding pattern and CP level affected urea-N metabolism, with milk urea-N concentration lesser for oscillating and lower CP. Cosinor mixed models (a method to evaluate regular, time-dependent changes) showed that milk urea-N concentration responded to dietary changes with a time delay of two days. Contrary to our hypothesis, we found no evidence that the effects of oscillating dietary CP on production differed depending on the median dietary CP level. These results are important for dairy farmers and nutritionists as they demonstrate that dairy cow performance is resilient to short-term changes in dietary CP. Additionally, milk urea-N concentration changes are time-sensitive indicators of dietary CP changes.

Technical Abstract: Limited research has examined the interaction between dietary crude protein (CP) level and CP feeding pattern. We tested CP level (LP, 13.8%; HP, 15.5%) and CP feeding pattern (OF = oscillating, SF = static) using a 2x2 factorial in sixteen mid- to late-lactation Holsteins (average days in milk = 128, standard deviation = 12). Cows ate total mixed rations formulated by exchanging soy hulls and ground corn with solvent soybean meal to keep constant ratios of neutral detergent fiber:starch (1.18), rumen-degradable protein:CP (0.61), and forage:concentrate (60:40). OF alternated diets (OF-LP 12.2-15.5% CP; OF-HP 13.8-17.3%) every 48 hr so mean diet composition equaled that of the corresponding SF treatment (SF-LP; SF-HP). In four 28-day periods, eight rumen-cannulated and eight non-cannulated cows formed two Latin Rectangles. We recorded each cow’s feed intake and milk production, took samples of orts (once daily) and milk (twice daily) on days 25-28 each period. We fit linear mixed models with fixed CP level, CP feeding pattern, and period effects, and a random intercept for cow; computing least squares means and standard errors (SEM). Neither CP level, CP feeding pattern, nor the interaction affected dry matter intake, feed efficiency, or production of milk, fat-protein-corrected milk (FPCM), fat, true protein, or lactose. Milk urea nitrogen (MUN) yield was lesser for LP, with no CP feeding pattern or interaction effects. LP and OF decreased MUN concentration with no interaction. CP level tended to interact with CP feeding pattern so that milk protein concentration was greatest for OF-HP, with neither main effect observed. Neither CP level, CP feeding pattern, nor the interaction affected concentrations of fat or lactose. OF and LP increased true protein:MUN yield with no interaction suggesting improved metabolic N utilization compared to SF or HP. Within OF, cosinor mixed models of selected variables showed that cows maintained production of FPCM across dietary changes but MUN followed a wave-pattern at a two day delay relative to dietary changes. Lesser MUN with OF contradicted prior research and suggested potential differences in urea-N metabolism between OF and SF. Results showed that cows maintained production of economically-relevant components regardless of CP feeding pattern and CP level. Contrary to our hypothesis, the effects of 48-hr oscillating CP were mostly consistent across CP levels suggesting the existence of adaptation mechanisms even when cows were on dietary CP level of 12.2% for 48 hours.