RESUMO
This study explored the metabolic adaptions to grazing conditions of two Holstein genetic strains (GS; North American, NAH; New Zealand, NZH) in two feeding strategies (FS; restricted, P30, vs. maximised, PMAX, grazing). Four groups (NAH-P30, NZH-P30, NAH-PMAX and NZH-PMAX; n = 10 cows each) were compared between -45 and 180 days in milk (DIM). NZH cows had lower (p = 0.02) fat and protein corrected milk (FPCM) yield and a tendency for lower (p = 0.09) body condition score concomitantly with a trend (p < 0.07) for higher average plasma insulin and lower (p = 0.01) 3-methylhistidine (3MH) at -45 DIM than NAH. Plasma glucose tended to be affected by the triple interaction GS × FS × DIM (p = 0.06) as it was similar between NAH-P30 and NZH-P30, but higher (p ≤ 0.02) for NZH-PMAX than NAH-PMAX except at 21 DIM. The physiological imbalance index was affected by the GS × FS interaction (p < 0.01) as it was lower (p < 0.01) only for NZH-PMAX versus NAH-PMAX. NZH cows had higher (p = 0.01) plasma thiobarbituric acid reactive substances at -45 DIM and tended to have higher protein carbonyls (p = 0.10) and superoxide dismutase (SOD) activity (p = 0.06) on average, and had higher (p < 0.01) α-tocopherol during mid-lactation than NAH Regarding the FS, FPCM was similar (p = 0.12) among them, but PMAX cows had higher (p < 0.01) plasma non-esterified fatty acids and 3MH, and lower insulin (p < 0.01) than P30 at 100 DIM. PMAX cows showed higher average SOD activity (p = 0.01) and plasma α-tocopherol at 100 and 180 DIM (p < 0.01). Under grazing, NZH cows can have a better energy status and lower muscle mobilisation but a higher redox reactivity. Maximising grazing can worsen energy status and muscle mobilisation while improving antioxidant response with no effect on FPCM.
