Efeitos da sazonalidade e da suplementação com metionina na produção, composição do leite e metabolismo de vacas leiteiras

Abstract

Milk production and composition in tropical systems vary throughout the year and are affected by critical periods of heat stress. This thesis comprised two complementary studies. In the first, we characterized circannual rhythms of milk fat, protein, and somatic cell count (SCC) in Brazil and compared the relative contribution of seasonality and economic variables using monthly bulk-tank data (2013–2023) from six states (792 state-month observations) and Girolando records (10,987 observations; 718 cows; 41 herds), fitting 12-mo cosinor linear mixed models at market, herd, and cow levels, as well as models including milk, corn, and soybean prices. Significant annual rhythms (P < 0.001) were detected for fat, protein, and SCC in all states, and seasonality explained most of the variation, with minimal gains from adding economic variables (ΔR² ≤ 0.01). At the herd level, 65.9% were rhythmic for protein, 31.7% for fat, 34.1% for milk yield, and 22.0% for SCC; at the cow level, rhythms were observed in 46.2% (protein), 20.5% (fat), 51.6% (milk yield), and 22.2% (SCC), confirming patterns preserved across organizational levels. In the second study, we evaluated rumen-protected methionine (RPM) supplementation in mid-lactation primiparous Holstein cows during summer heat stress (THI > 68 for 68.3% of monitored hours; mean THI = 70.6) in a 9-wk trial with 80 cows (182 ± 64 DIM; 42.9 ± 4.7 kg/d) fed a control diet or RPM (0.75 g/kg diet DM; ~20 g/cow/d), with sampling in weeks 3, 6, and 9. RPM increased milk yield by 2.0 kg/d (44.9 vs. 42.9), protein yield by 50 g/d (1,464 vs. 1,414), lactose yield by 108 g/d (2,109 vs. 2,001), and total solids yield by 176 g/d (5,331 vs. 5,155), increased insulin (0.52 vs. 0.35 ng/mL), reduced plasma glucose (39.3 vs. 43.2 mg/dL), increased FRAP (32.9 vs. 28.5 μM), and reduced malondialdehyde (2.48 vs. 2.78 nmol/mL), without a consistent increase in mean vaginal temperature. In conclusion, circannual rhythms in milk composition and SCC are present and differ among Brazilian states, and RPM improves milk and solids yields while favoring endocrine and redox status in primiparous cows under heat stress, representing a promising strategy to mitigate summer losses in tropical dairy systems.