Influence of dietary nonfiber carbohydrate concentration and supplementation of sucrose on lactation performance of cows fed fescue silage.

There is interest in knowing if the source of nonfibrous carbohydrates (NFC) influences milk production and composition. Our objective was to determine the effects of source (starch or sugar) and level of NFC in the diet on these parameters. A 4 x 4 Latin square replicated five times using early-lactation (56 +/- 9 DIM) Holstein cows was used; cows were offered one of two levels of NFC and either no added sucrose or sucrose substituting for 10% of the corn. Diets were balanced to meet National Research Council requirements for total protein, energy, and minerals. Tall fescue silage was included at one of two levels (0.95 or 1.25% of BW as forage NDF), resulting in diets with 40 and 30% NFC. The remaining ingredients consisted of high-moisture corn, soybean meal, SoyPlus, minerals, and vitamins. Megalac (0.45 kg) was used in the low NFC diets. High NFC diets were lower (P < 0.01) in neutral detergent fiber (NDF; 31.5%) and crude protein (CP; 19.6%) than the low NFC diet (35.8% NDF and 21.0% CP). Sucrose containing diets were somewhat lower (P < 0.01) in NDF (33.1%) than the no sucrose added diets (34.3%), but diets did not differ in CP%. Cows offered the high NFC level produced more milk (39.6 kg/d; P < 0.05) than those offered the low level (38.3 kg/d), primarily due to higher dry matter intake (P < 0.05). Cows consuming the high NFC diet also had lower (P < 0.05) milk fat (3.25%) and milk urea nitrogen (MUN; 13.7 mg/dl), and higher (P < 0.05) milk protein (2.58%) and milk lactose (4.81%) concentrations than cows offered the low NFC level (3.46% milk fat, 17.5 mg/dl MUN, 2.51% milk protein, and 4.74% milk lactose). Fat yield was higher (P < 0.05) for cows fed low NFC diets than cows fed high NFC diets, whereas protein and fat yield were lower (P < 0.05) for cows fed low NFC diets than those fed high NFC diets. The NFC source did not influence dry matter intake or milk production or milk component yield (P > 0.05). Milk lactose (4.79%) and MUN (16.0 mg/dl) concentrations were higher (P < 0.05) for cows offered sucrose as a portion of the NFC compared with those not offered sucrose (4.76% milk lactose and 15.2 mg/dl MUN). Results suggest that cows fed sucrose may utilize diet nitrogen less efficiently than those not fed sucrose, when sucrose is replacing a portion of the high-moisture corn in the diet.