Effects of rumen-protected methionine on lactating F₁ Holstein × Gyr cows grazing tropical pasture

The effects of rumen-protected methionine (RPM) in Holstein cows fed TMR are well documented, but limited information is available on the responses to RPM in grazing dairy cows. This study aimed to evaluate the effects of RPM supplementation on milk and solids yields, DMI, feeding behavior, and plasma AA of grazing F₁ Holstein × Gyr cows. We hypothesized that AA balancing with RPM would increase milk protein yield and ECM. Twenty-four multiparous cows (100 ± 55 DIM, 31.0 ± 4.8 kg milk/d, 590 ± 47 kg BW) were assigned to a randomized complete block design, blocked by milk yield and DIM. Within blocks, cows were randomly allocated to 1 of 2 treatments for 9 weeks during the summer, with the temperature-humidity index averaging 68.5 ± 4.6. The control concentrate (CON) contained ground corn, soybean meal, whole cottonseed, citrus pulp, minerals and vitamins. The RPM treatment was CON plus Smartamine M (Adisseo, France) at 0.060% (on average, 10.6 g/d) of the consumed diet DM, defined to achieve a Lys:Met ratio of 2.8:1 in MP according to the 2001 NRC guidelines at the beginning of the experiment, as the 2021 National Academies of Sciences, Engineering, and Medicine (NASEM) guidelines were not yet available. Cows grazed tropical pasture (Panicum maximum, cv. Mombaça grass), managed under rotational stocking with a pregrazing target sward height of 90 cm, and a new paddock allotted every day. Concentrate mixtures were offered individually 3 times per day. Data and sample collection happened on wk 3, 6 and 9, during which milk samples were collected from 6 consecutive milkings, spot fecal samples on the last 3 d and blood samples on the last day. Estimated pasture DMI was assessed using internal and external markers. Data were analyzed by MIXED procedure of SAS, with block as random effect and treatment, sampling week, and treatment × week as fixed effects, and cow as the repeated subject. The CON diet was deficient in Met (total NASEM estimated metabolizable Met supply = 36.4 g/d, efficiency of utilization = 0.78). Supplementation with RPM increased total estimated metabolizable Met supply to 45.0 g/d and reduced the NASEM efficiency of Met utilization to 0.68, whereas only 0.034% of DM (6.0 g/d) would have been required to achieve the NASEM target efficiency of 0.73. Feeding RPM increased milk protein (894 vs. 835 g/d), casein (699 vs. 649 g/d), and TS yields (3,363 vs. 3,197 g/d) and tended to increase milk protein (3.27% vs. 3.12%) and casein concentrations (2.56% vs. 2.43%). It also tended to increase ECM yield (26.3 vs. 25.0 kg/d) and pasture intake (8.0 vs. 6.6 kg/d), resulting in greater total DMI (17.7 vs. 16.2 kg/d). Feeding RPM did not affect Met plasma concentrations (35.9 vs. 31.5 µM for CON and RPM, respectively), but it decreased Gly (308.0 vs. 344.5 µM), Gln (192.0 vs. 215.1 µM), and total NEAA (1,103.4 vs. 1,201.9 µM). Overall, RPM supplementation increased milk protein yield and tended to increase ECM yield, indicating that Met was deficient in the CON diet for grazing F₁ Holstein × Gyr cows.