The interactive effect of dietary fat supplementation and milk yield level on
dairy cows performance under heat stress has not been thoroughly investigated.
The purpose of this study was to evaluate the effect of production level, the
source of fat supplements and their interaction on dairy cows performance under
heat stress. In this study, 64 Holstein multiparous cows were divided into 2
groups and received one of two rations having either calcium salts of fatty
acids (Ca-FA) or high-palmitic acid (PA) supplements (2.8% of DM; dry matter).
After completing the experiment and based on maturity-equivalent milk, cows were
divided into two groups of high-yielding (14,633 kg) and medium-yielding (11,616
kg). Average temperature humidity index (THI) was 71 during the trial period.
Apparent digestibility of dry matter (p = 0.04), organic matter
(p = 0.05), and neutral detergent fiber (NDF;
p = 0.04) for cows fed Ca-FA were greater than cows fed PA.
The milk fat content in high-producing cows was 0.3% greater than
medium-producing cows (p = 0.03). The milk protein content in
cows fed Ca-FA was greater than cows fed PA (p < 0.01).
High-producing cows had greater serum cholesterol (p = 0.02)
than medium-producing cows. The cows fed PA tended to have a greater BUN than
cows fed Ca-FA (p = 0.06). Alanine aminotransferase and
aspartate aminotransferase tended to be increased by PA, which indicates that
cows in PA treatment may have experienced more adverse effect on the liver
function than cows on Ca-FA. Therefore, under heat stress and in 90 d trial,
milk production level does not affect the cows' response to PA or Ca-FA.
Although cows fed Ca-FA received lower energy than those fed PA, they
compensated for this shortage likely with increasing the digestibility and
produced a similar amount of milk.
Background
It is not clearly known whether parity can affect the outcomes of starch reduction in the diet of lactating dairy cows.
Introduction
A 2 × 2 factorial study was conducted to evaluate the effects of reducing starch in the diets with similar protein and energy contents on lactation performance, ruminal fermentation, nutrient digestibility, behaviour and blood metabolites in primiparous (PP) and multiparous (MP) dairy cows.
Methods
Twenty PP cows (DIM = 37 ± 10; 40 ± 5 kg/day of milk; mean ± SD) and 20 MP cows (DIM = 37 ± 9; 48 ± 5 kg/day of milk) were used in present study. Treatments were a factorial arrangement of two levels of starch (high vs. low) and two parity categories (PP vs. MP): (1) high‐starch diet (29.2% ± 0.70) and PP cows (HS‐PP); (2) low‐starch diet (22.3% ± 0.52) and PP cows (LS‐PP); (3) high‐starch diet and MP cows (HS‐MP) and (4) low‐starch diet and MP cows (LS‐MP). All diets were formulated to be similar in crude protein (16.1 % of dry matter) and NEL (1.60 Mcal/kg of dry matter) contents. The amount of metabolise protein was 2688 g/day in high‐starch diet and 2728 g/day in low‐starch diet. The experiment was conducted over two consecutive periods and included 4 weeks for adaptation and 3 weeks for data collection.
Results
Dry matter intake and the yield of milk true protein and lactose increased but milk fat: protein ratio and nutrient digestibility decreased for cows fed the HS diets compared with the LS diets. The ruminal proportion of propionate was greater but acetate, the acetate to propionate ratio and sorting against long particles (19 and 8 mm) were lower for cows fed the HS diets than the LS diets. Multiparous cows had a greater nutrient intake and milk yield, longer rumination meal length, greater BW, but lower plasma total antioxidant capacity, non‐esterified fatty acids, faeces pH compared with PP cows. An interaction between parity and the dietary level of starch was detected on feed efficiency measured as FCM yield/DMI in the way that only within PP cows low‐starch diet was more efficient than HS diets. We found another interaction effect of parity × starch on back fat thickens (BFT) change in the way that only within PP cows BFT change was greater for HS compared with LS diet.
Conclusion
Overall, regardless of the benefit derived from feeding a reduced‐starch diet by partially replacing grains with sugar beet pulp in the diets on nutrient digestibility, a reduced‐starch diet may be used more efficiently in PP than in MP cows but at expense of body reserves (i.e. BFT) loses.
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