Abstract
The objective of this experiment was to evaluate the effects of supplementing a rumen-protected source of Met, N-acetyl-l-methionine (NALM), on lactational performance and nitrogen metabolism in early- to mid-lactation dairy cows. Sixty multiparous Holstein dairy cows in early lactation (27 ± 4.3 d in milk, SD) were assigned to 4 treatments in a randomized complete block design. Cows were blocked by actual milk yield. Treatments were as follows: (1) no NALM (control); (2) 15 g/d of NALM (NALM15); (3) 30 g/d of NALM (NALM30); and (4) 45 g/d of NALM (NALM45). Diets were formulated using a Cornell Net Carbohydrate and Protein System (CNCPS) v.6.5 model software to meet or exceed nutritional requirements of lactating dairy cows producing 42 kg/d of milk and to undersupply metabolizable Met (control) or supply incremental amounts of NALM. The digestible Met (dMet) supply for control, NALM15, NALM30, and NALM45 were 54.7, 59.8, 64.7, and 72.2 g/d, respectively. The supply of dMet was 88, 94, 104, and 115% of dMet requirement for control, NALM15, NALM30, and NALM45, respectively. Milk yield data were collected, dry matter intake (DMI) was measured daily, and milk samples were collected twice per week for 22 wk. Blood, ruminal fluid, urine, and fecal samples were collected during the covariate period and during wk 4, 8, and 16. Data were analyzed using the GLIMMIX procedure of SAS (SAS Institute) using covariates in the model for all variables except body weight. Linear, quadratic, and cubic contrasts were also tested. Treatments did not affect DMI, milk yield, and milk component concentration and yield; however, feed efficiency expressed as milk yield per DMI and 3.5% fat-corrected milk per DMI were quadratically affected, with greater response observed for NALM15 and NALM30 compared with control. Acetate proportion linearly increased, whereas propionate proportion linearly decreased with NALM supplementation. Blood urea nitrogen linearly decreased with NALM supplementation. Total plasma essential AA concentrations were quadratically affected, as greater values were observed for control and NALM45 than other treatments. Plasma Met concentration was quadratically affected as lower levels were observed with NALM15, whereas Met concentrations increased with NALM45 compared with control. Nitrogen utilization efficiency and apparent total-tract nutrient digestibility were not affected by treatment. Supplementation of NALM at 15 or 30 g/head per day resulted in the greatest improvements in feed efficiency without affecting N metabolism of early- to mid-lactation dairy cows.
Highlights
Diet formulation on a CP basis has been shown to have several shortcomings over the past decades (Schwab and Broderick, 2017)
Accepted November 1, 2021. *Corresponding author: diwakarvyas@ufl.edu yield; feed efficiency expressed as milk yield per dry matter intake (DMI) and 3.5% fat-corrected milk per DMI were quadratically affected, with greater response observed for NALM15 and NALM30 compared with control
Feed efficiency expressed as milk yield per unit of DMI showed quadratic tendency with greater values observed for NALM30 compared with the control (P = 0.06)
Summary
Diet formulation on a CP basis has been shown to have several shortcomings over the past decades (Schwab and Broderick, 2017). Diet formulations for dairy cows have moved away from balancing CP requirements toward formulations based on providing ruminal NH3 and AA for ruminal and post-ruminal metabolism (Schwab and Broderick, 2017). Milk protein synthesis is limited by the AA that is in shortest supply relative to the AA requirements (Schwab et al, 1976). The goal for optimizing N efficiency is to maximize microbial protein synthesis in the rumen and supplement additional rumen-protected AA to meet the requirement for milk protein synthesis (Schwab et al, 1976). Balancing AA profile of dairy cattle diets is imperative, as it enhances milk protein synthesis, improves N efficiency, and reduces N excretion
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