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 individualize CP for each cow; grains contained 15 and 20% of distillers grains with solubles, respectively. Cows were blocked by parity (1 vs. > 1) and assigned at calving to receive a commercial bypass protein-fat supplement 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 undegradable 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 total DMI significantly, which negated the potential nutritional value of the supplement. Reduction in protein content of milk from supplemented cows was small but significant; BW and yields of milk, SCM, and 4% FCM were not significantly affected by treatment. Supplementation increased 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 primiparous 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 effective if its use reduced DMI or if degradable protein intake is inadequate.