1855
Feeding Diets Inducing Milk Fat Depression to Heat-Stressed Dairy Cows on Performance, Energy Partitioning, and Antioxidant Status
Effects of grain source and dietary oil supplement on production performance, energy balance, metabolic heat production, and markers of liver function of heat-stressed lactating dairy cows were evaluated using eight multiparous Holstein cows (77.0 days in milk) in a duplicated 4 × 4 Latin square design with a 2 × 2 factorial arrangement of treatments. Experimental diets contained either ground barley or ground corn supplemented with either fish oil or soybean oil at 2% of dietary dry matter. Rectal temperature showed no change (averaging 38.9°C) regardless of diet but respiration rate tended (P =0.08) to be decrease in cows fed fish oil as compared to cows fed soybean oil (58.4 vs. 62.5 breath/min). Dry matter intake tended (P = 0.09) to be greater for barley- vs. corn-based diets (23.2 vs. 22.3 kg/d), but was reduced for the fish oil compared to soybean oil supplemented diets (21.1 vs. 24.3 kg/d; P < 0.001) which was negatively correlated with plasma concentrations of alkaline phosphatase (r = –0.45; P ≤ 0.01) and malondialdehyde (r = –0.26; P < 0.15). Actual milk yield and energy-corrected milk yield were not affected by grain source whereas feeding fish oil decreased milk yield as compared to soybean oil. Due to lesser dry matter intake, metabolic heat production was decreased in cows fed fish oil relative to cows fed soybean oil. Although feeding fish oil vs. soybean oil reduced net energy for both maintenance and lactation, net energy balance remained unchanged across treatments. However, back fat thickness positively changed (+4.0 mm) in cows fed corn- but not barley-based diets that were supplemented with fish oil vs. soybean oil (P = 0.10). There was an interaction between dietary grain source and oil supplement on in vitro indicators of plasma lipoperoxidation including basal and maximum conjugated dienes and calculated area under the curve which were greater in corn-based diets supplemented with fish oil vs. soybean oil. In vivo plasma lipoperoxidation estimated by the plasma level of the major lipoperoxidation product (malondialdehyde) was greater in cows fed fish oil vs. soybean oil which substantiated increased susceptibility of plasma lipoperoxidation in respective cows. Overall, results from current experiment suggest that in cows fed diets supplemented with soybean- vs. fish-oil biosynthesis in the mammary gland was prioritized over anabolism and oxidation in peripheral adipose and muscle tissues regardless of type of grain used.
Keywords:
Milk fat depression, Heat stress, Dairy cow