Improved dietary nitrogen use efficiency (NUE = milk N ÷ N intake) and milk protein yield are needed to meet global demands for human edible protein and preserve environmental sustainability of the dairy industry. The objectives of this dissertation were to investigate methods to improve NUE while maintaining milk protein yield or vice versa. Another objective was to bridge knowledge gaps between endogenous protein stores and their mobilization for milk production.
In experiment 1, oscillating the day-to-day concentrations of crude protein (CP) was evaluated as a feeding method to improve NUE without jeopardizing milk protein yield or causing protein mobilization. In a randomized block design, 30 Holstein cows [119 ± 21 d in milk (DIM); 667 ± 69 kg of body weight (BW)] were fed 1 of 3 treatments: 1) a diet with 16.2% CP [109% of metabolizable protein (MP) requirements] fed continuously (109MP), 2) a diet with 14.1% CP (95% of MP requirements) fed continuously (95MP), or 3) diets oscillating every 24 h from 109MP and a diet with 11.9% CP (~ 78% of MP requirements) so that average CP was similar to 95MP (OSC). Dry matter intake (DMI) was lower for OSC compared to 95MP (22.2 vs. 23.2 kg/d) but milk protein yield was similar (avg. 1.06 kg/d). Milk urea-N increased for OSC versus 95MP (10.9 vs. 10.2 mg/dL) despite lower N intake (474 vs. 512 g/d of N) and similar milk N outputs (avg. 171 g/d). This may be from greater CP digestion for OSC compared to 95MP (65.3 vs. 61.7%); however, oscillation did not improve NUE (averaged 0.35 g milk N/g N intake) and increased urinary N output (0.32 vs. 0.24 g urine N/g N intake). Empty body (EB) CP was unchanged over the 50 d treatment period. Overall, greater CP digestion and MUN at similar milk N outputs suggested that more urea was available for recycling to the rumen for OSC; however, increased urinary N excretion indicated that the urea was not well utilized for microbial CP synthesis. Lesser energy intake may have limited improvements in NUE.
In experiment 2, we evaluated whether high MP diets using one or blends of rumen undegradable protein (RUP) and AA sources, and whether substituting forage NDF (fNDF) or non-forage NDF with RUP improves milk and milk protein yield, and attenuates protein mobilization in fresh cows. We also evaluated parity effects and the mRNA expression in the longissimus dorsi of multiparous cows. In a randomized block design, 40 primigravid (664 ± 44 kg of BW) and 40 multigravid (797 ± 81 kg of BW; entering lactation number 2.6 ± 0.95) Holsteins were blocked by calving date and fed a common diet (11.5% CP). After calving to 25 DIM, cows were fed 1 of 4 diets: 1) deficient MP (DMP, 17% CP, 24% fNDF), 2) adequate MP met with high inclusion of soy protein (AMP, 20% CP, 24% fNDF), 3) adequate MP met with a blend proteins high in RUP and rumen protected (RP) AA sources (Blend, 20% CP, 24% fNDF), or 4) Blend but replacing fNDF rather than non-forage NDF with RUP (Blend-fNDF, 20% CP, 19% fNDF). The blend of protein and AA contained a treated soy and canola meal product, corn gluten meal, and RP His, Lys and Met. The blend of protein and AA was designed to mimic the AA profile of casein on a RUP basis. A common diet (17% CP) was fed from 26 to 92 DIM to evaluate carryover effects of treatment.
During treatment in experiment 2, AMP and Blend increased milk protein yields compared to DMP (1.13 vs. 1.06 kg/d) and reduced plasma 3-methyl-His concentrations (4.1 vs. 5.3 µmol/L), a biomarker of muscle degradation. Compared to AMP, Blend increased DMI (16.1 vs. 17.4 kg/d) and reduced empty body (EB) CP losses (-60 vs. -10 g/d), but milk protein yield and NUE were similar (avg. 1.13 kg/d at 0.31 g milk N/g N intake). Blend-fNDF did not affect milk protein yield compared to Blend (avg. 1.12 kg/d), but decreased DMI and energy-corrected milk (ECM) yields in multiparous cows (19.2 vs. 20.1 kg of DMI/d, 45.3 vs. 51.1 kg of ECM/d) whereas primiparous cows were the opposite (15.3 vs. 14.6 kg of DMI/d, 32.9 vs. 31.4 kg of ECM/d). Greater DMI for multiparous cows fed Blend carried over and was greater compared to AMP (23.1 vs. 21.2 kg/d) and Blend (21.3 kg/d) from 26 to 50 DIM. Blend also increased ECM yield in multiparous cows compared to AMP (49.2 vs. 43.5 kg /d) and Blend (45.4 kg/d) from 26 to 92 DIM. Little carryover effects of treatments on milk production were found in primiparous cows. Overall, feeding blends of RUP and AA to fresh cows improved the balance of AA supplied, increased DMI, and attenuated mobilization of EB CP. A better AA supply also improved longer term ECM yields in multiparous cows whereas in primiparous cows, it reduced mobilization of EB lipid during the fresh period. Our results also suggest multiparous fresh cows require more fNDF than primiparous cows.
Based on mRNA from the longissimus dorsi of multiparous cows, expression of genes related to the ubiquitin proteasome system (FOXO1, TRIM63, FBXO32) were increased at 7 DIM versus prepartum (-5±3.4 d relative to calving) and 25 DIM whereas genes related to fatty acid (FA) oxidation (LPL, PPARGC1A, UCP1) and oxidative muscle fiber types (MYH2, MYH7) were decreased. Genes associated with attenuating apoptosis (BCL2), oxidative stress and inflammation (SESN1, SESN2, SOCS2, NOS2), and fast twitch glycolytic muscle fiber (MYH4) were increased at 7 and 25 DIM compared to prepartum. Greater MP concentrations reduced expression of genes related to protein synthesis (MTOR, RPS6KB1) and degradation (FOXO1), inflammation (IFNG, TLR4), and endoplasmic reticulum (ER) stress (HSPA5 , DDIT) and increased genes associated with FA storage (PPARG) and glucose oxidation (LDH, MB). A better AA supply from Blend reduced expression of genes related to apoptosis (CASP8) and inflammation (TNFA) and increased genes associated with progression of the cell cycle (E2F1) and fast twitch glycolytic muscle fiber (MYH4). Less fNDF and energy intake (Blend-fNDF vs. Blend) decreased expression of genes associated with lipogenesis (PPARG, ACACA) and ER stress (BCL2, DDIT3, EIF2AK3, PPP1R15A, XBP1u) and increased expression of genes associated with inflammation (TNF), inhibition of myogenesis (MSTN) and autophagy (PEBP1). In conclusion, the mRNA expression data suggested the metabolism of skeletal muscle is altered at the onset of lactation. Multiparous cows also have a negative association between protein turnover, oxidative and ER stress, and apoptosis in skeletal muscle to milk production later into lactation.