Response of Early Lactation Cows to Fat Supplementation in Diets with Different Nonstructural Carbohydrate Concentrations

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Abstract
Twenty-four high producing (36.7 kg/d, initially) Holstein cows in early lactation were used in a 63-d lactation trial to study the effects of low (28%), medium (31%), and high (37%) nonstructural carbohydrate concentrations in diets with added fat. Diets with isonitrogenous (17% CP) and fed as TMR with 41% sorghum silage. Nonstructural carbohydrate concentrations were altered by substitution of corn gluten feed and soybean hulls (50:50) for wheat and corn in the concentrate portion of the diet. Fat was added at 3% of DM in the form of yellow grease. Production of milk (38.5, 38.9, and 40.4 kg/d) and 3.5% FCM (38.2, 37.0, and 37.6 kg/d) was not significantly different for the cows fed low, medium, and high nonstructural carbohydrate diets, respectively. Milk fat percentages (3.47, 3.22, and 3.10%) decreased, and milk output per unit of DMI (1.5, 1.6, and 1.7) increased, as nonstructural carbohydrate concentration increased. Increasing nonstructural carbohydrate concentrations of the diet increased digestibilities of DM (58.4, 60.6, and 61.5%) and nonstructural carbohydrates (76.5, 79.6, and 80.4%) and decreased digestibilities of NDF (44.3, 43.7, and 40.6%) and ADF (46.3, 45.2, and 38.8%). Altering nonstructural carbohydrate content of the diet had no effect on AA supply to or uptake by the mammary gland or efficiency of utilization of AA for milk protein synthesis.