Effects of Stocking Density on Milk Fatty Acids Composition and Oxidative Stability of Mid- and Late-Lactating Dairy Cows

SIMPLE SUMMARY: The ratio of unsaturated fatty acids (UFAs)/saturated fatty acids (SFAs) of milk fat is known to play an important role in regulating milk oxidation capacity in lactating dairy cows. Recently, it was found that dietary antioxidant addition plays an important role in changing UFAs/SFAs ratio of milk fat and milk oxidation capacity in dairy cows, indicating the role of oxidative stress status in regulating milk oxidation capacity. Although stocking density can affect stress level in lactating dairy cows, whether stocking density may affect milk oxidation capacity remains to be answered. In the current study, we stocked the cows with different physiological stages (mid- and late-lactating) under different stocking density (100% vs. 75%), respectively. The plasma variables, milk fatty acids compositions, and oxidative index in the milk were further evaluated to clarify if stocking density can regulate the oxidation capacity of the milk. The results showed that, in mid-lactating cows, milk produced by low stocking density animals had lower oxidation stability due to the higher UFAs levels, compared with those of high stocking density cows. Milk oxidation capacity of late-lactating dairy cows did not differ when they were blocked under different stocking density. Our results suggested that stocking density did affect milk oxidation capacity but in a physiological-stage dependent manner. Nutritional strategies should be developed to improve the milk oxidation capacity of cows managed under a low stocking density. ABSTRACT: The objective of the current study was to investigate the effects of stocking density (SD) on the milk fatty acid profile and oxidation stability in mid- and late-lactating cows. Twenty-four multiparous mid-lactating Holstein dairy cows (milk yield = 34.5 kg/d (standard deviation 0.68), days in milk = 106 (standard deviation 2.2)) and 24 multiparous late-lactating animals (milk yield = 31.8 kg/d (standard deviation 0.98), DIM = 201 (standard deviation 3.5)) were allocated into 12 blocks based on parity, milk yield, and DIM and were randomly assigned to one of four treatments: mid-lactating cows with high SD (HSD, 100%), mid-lactating cows with low SD (LSD, 75%), late-lactating cows with HSD, and late-lactating cows with LSD. The experiment lasted for eight weeks. Lactation performance (milk yield and composition including fat, protein and lactose) was measured weekly. The milk fatty acids (FAs) profiles and oxidation stability indexes in milk were measured in the fourth day of the eighth week. Plasma variables were measured in the fourth day of fourth and eighth experimental weeks. Yield and composition (protein, fat and lactose) were not affected by SD in neither mid- nor late-lactating dairy cows. Among mid-lactating cows, LSD cows had higher contents of unsaturated FAs (total, C18:1 cis-9 and C18:2 cis-9 cis-12) and lower concentrations of saturated FAs (total, C14:0 and C16:0) in milk fat than those of the HSD animals. Moreover, LSD lowered SOD, GSH-px, and T-AOC activities and reduced the malonaldehyde content in the milk of mid-lactating cows compared with those of HSD cows. Mid-lactating cows under LSD had reduced cortisol and greater MDA contents in plasma than those of HSD cows. Our results suggested that the effect of SD on the milk FA profile and stability varied depending on lactation stages. In mid-lactating cows, although cows with LSD were less stressed, the milk they produced had lower oxidation stability due to the higher unsaturated FAs levels compared with that of HSD cows.