Sixty-seven Holstein replacement heifers (19 mo) were fed a standard (59.7% TDN) or a high energy (69.3% TDN) diet until parturition. After parturition, primiparous cows were fed either 0 or 2.8% supplemental tallow for 150 d in a 2 x 2 factorial arrangement with prepartum treatments. High energy prepartum increased BW (693.5 vs. 663.7 kg) and body condition scores (3.72 vs. 3.55) at calving. Increased energy density of the diet prepartum did not affect milk yield or composition. Supplemental dietary fat postpartum increased milk yield approximately 1.5 kg/d, but the response was not observed until 7 wk postpartum. Heifers fed the standard diet prepartum and no supplemental fat postpartum had higher DMI than other treatments. Heifers fed high energy prepartum and supplemental fat postpartum lost the greatest BW and body condition from 1 to 5 wk postpartum. Heifers fed high energy diets prepartum had higher concentrations of blood NEFA, BHBA, and liver triglycerides. Increases in BW and body condition scores at calving above approximately 660 kg and 3.5, respectively, do not enhance lactation performance. When 2.8% supplemental fat was fed to primiparous cows, milk yield improved approximately 1.5 kg/d after wk 7 of lactation.

Sixteen multiparous Holstein cows were in a replicated 4 x 4 Latin square design with 21-d periods to examine the effects of incremental tallow addition to diets containing whole roasted soybeans on rumen fermentation and lactational performance. Total mixed rations were fed for ad libitum intake and contained, on a DM basis, 33% alfalfa silage, 12% corn silage, 14% roasted soybeans, and 41% concentrate based on ground corn and soybean meal. Treatments were 0, 1, 2, or 3% supplemental tallow. Diets contained 20% CP and ranged from 1.68 to 1.82 Mcal NEL/kg of DM. The DMI, milk yield, milk protein and fat yields, milk fat percentage, rumen acetate: propionate ratio, and in situ forage DM disappearance did not differ among treatments. A small linear decrease occurred in milk protein percentage as tallow feeding was increased (2.89 to 2.86%). Tallow supplementation increased total VFA concentration in rumen fluid and resulted in a linear decrease in rumen pH (6.17 to 5.99). Supplementation of 1 to 3% tallow to diets containing 2.8% supplemental fat from whole roasted soybeans had minimal negative effects on rumen fermentation and did not influence lactational performance.

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