PROTEIN-FAT BYPASS SUPPLEMENT FOR LACTATING DAIRY-COWS

Holstein cows (n = 46) were fed free choice a silage mixture balanced weekly throughout lactation using 13 and 36% CP grains to individualiz e CP for each cow; grains contained 15 and 20% of distillers grains wi th solubles, respectively. Cows were blocked by parity (1 vs. > 1) and assigned at calving to receive a commercial bypass protein-fat supple ment at 0 (control) or 6% of weekly projected 4% FCM yield throughout lactation. In peak lactation, supplementation raised dietary fat from 4.3 to 6.0% of DM, NE(L) from 1.64 to 1.70 Mcal/kg of DM, and undegrad able protein from 42 to 47% of CP and contributed about 25 and 20% of total CP in early and late lactation. Supplement reduced forage and to tal DMI significantly, which negated the potential nutritional value o f the supplement. Reduction in protein content of milk from supplement ed cows was small but significant; BW and yields of milk, SCM, and 4% FCM were not significantly affected by treatment. Supplementation incr eased fat test in parity 1 cows and lowered it in older cows. In wk 5 to 8, 21 to 24, and 37 to 40 postpartum, cows consumed 100 to 116% of the NRC recommendations of undegradable protein but only 65 to 94% of degradable CP needs; NE(L) intake generally was adequate except for pr imiparous cows in early lactation. Supplementation lowered Lys intake in early lactation. Addition of rumen-protected fat and undegradable, high quality protein mixture to the diet of lactating cows cannot be e ffective if its use reduces DMI or if degradable protein intake is ina dequate.