Heat stress affects the milk yield, milk composition, serum oxidative status, and metabolites of Holstein cows during mid-lactation

Seasonal heat stress and metabolic disorders during mid-lactation are the main factors limiting production in Holstein cows, and several proteins and molecules involved in metabolic pathways are altered in response to stress. We investigated the effects of heat stress on the milk yield, milk composition, serum oxidative status, and metabolites in Holstein cows during mid-lactation to identify biomarkers associated with heat stress in serum and milk. Holstein cows with similar body condition scores (3.0 +/- 0.25), parity (2.5 +/- 0.5), and lactation days (115 +/- 5 days) were selected in August (heat stress, HS, n = 20) 2017 and March 2018 (non-heat stress, NHS, n = 20). Milk yield was recorded daily and serum was collected on days 1, 31, and 61. Serum and milk metabolites were analysed by gas chromatography-mass spectrometry on day 1. The results showed a significantly lower average daily milk yield in the HS group than in the NHS group (P < 0.05). The milk compositions of fat (%), lactoprotein, lactose yield, and milk solid-not-fat in the HS group were significantly lower than in the NHS group on days 1, 31, and 61 (P < 0.05). The levels of malondialdehyde were higher, whereas those of superoxide dismutase and glutathione peroxidase were lower in the serum of the HS group (P < 0.05) than that of the NHS group. The serum concentrations of D-glucose, 9,12-octadecadienoic acid, and D-lactose were significantly higher in the NHS group than in the HS group (P < 0.05). The concentrations of lactic acid and milk urea nitrogen in the NHS group were lower than those in the HS group (P < 0.05). The present data suggest that metabolic biomarkers are closely associated with heat stress in the serum and milk, which provides a basis for evaluating indicators of heat stress occurrence in mid-lactation cows.