Effect of production level and source of fat supplement on production performance, nutrient digestibility and blood parameters of heatstressed primiparous Holstein cows

Heat-stressed dairy cows are bioenergetically similar to early-lactation cows in that dietary energy may be insufficient to support maximum milk and production of milk components. Milk yield of heat-stressed dairy cows is usually decreased during the summer; therefore, increasing performance of cows is of particular interest. Fat supplementation is a common practice for increasing energy density in diets fed to high-producing dairy cows without sacrificing fiber content. In a very limited research studies, the use of Casalts of unsaturated fatty acids vs. saturated palm fatty acids has been investigated in the diet of heat-stressed dairy cows. Also, it has been shown that response to fat supplement can be related to production level. Hence, the object of this experiment was to investigate the effect of production level, source of fat supplement, and their interaction on milk yield and milk composition, nutrient digestibility, and blood biochemical parameters of heat-stressed primiparous Holstein cows.
Fifty-six primiparous Holstein cows were used in two separated group pens. Animals were blocked into two groups of high- (13474.7 kg) and medium-yielding (10438.1 kg) cows according to milk production and received one of two experimental diets containing either high palmitic acid palm fat or Ca-salts of unsaturated fatty acids both at 2.8 percentage of dietary dry matter. Temperature-humidity index and milk yield were recorded daily, and milk composition, nutrient digestibility, blood parameters, and complete blood count were measured every two weeks.
The average maximum temperature-humidity index was 75.50 over the experimental period that indicates cows experienced a mild to medium degree of heat stress. Main treatment effects interacted for raw milk yield (P = 0.03), 3.5% fat-corrected milk yield (P = 0.02), and milk fat yield (P = 0.02). Production performance and milk fat yield was greater for high-yielding cows fed palm fat relative to high-yielding cows fed Ca-salts of fatty acids, whereas source of fat supplement did not affect milk yield and milk fat yield in medium-yielding cows. Source of fat supplement did not affect nutrient digestibility. Irrespective of production level, concentration of blood cholesterol was greater for cows fed palm fat relative to cows fed Ca-salts of fatty acids. Also, concentrations of tri-glyceride, high density lipoproteins, very low density lipoproteins, and blood urea nitrogen tended to increase in cows fed palm fat. Source of fat supplement did not affect blood cells count (white blood cells, red blood cells, and platelets) and differential white cell count.
Source of fat supplement did not affect production performance of heat-stressed medium-yielding cows but palm fat supplement increased milk and milk fat production in high-yielding cows.