Transition performance and metabolic variables in periparturient Holstein cows fed increasing amounts of organic zinc

Multiparous (n = 79) and primiparous (n = 68) Holstein cows were blocked by 305 previous mature equivalent milk yield and parity and assigned to one of three dietary treatments. Treatments differed in Zn content as follows: (1) 75 mg/kg of DM supplemental Zn provided entirely as Zn sulfate (CON); (2) 33.3 mg and 15.5 mg of Zn sulfate/kg of DM in the control close up and lactation diets respectively were replaced by a Zn amino acid complex (Availa RTMZn; MED); and (3) 66.6 mg and 40 mg of Zn sulfate/kg of DM in the control close up and lactation diets were replaced by a Zn amino acid complex (AvailaRTMZn; HIGH). Cows were housed at the Iowa State University Dairy farm, and were individually offered a TMR containing dietary treatments beginning at 28 +/- 15 d prior to expected calving date and continued until 250 DIM. Compared to controls, multiparous cows (but not heifers) fed OZn had increased (20%) colostrum IgG. Feeding increasing levels of OZn did not affect prepartum DMI, but compared to CON-fed cows decreased (P < 0.01) DMI during lactation (5.1 and 3.2 %, respectively). Feeding increasing levels of OZn improved ( P < 0.01) services per conception (38 and 24 %, respectively), and tended (P = 0.08) to increase NEFA concentration (455 vs. 409 &mgr;Eq/L). Feed efficiency measured as MY/DMI, FCM/DMI, and SCM/DMI increased (P < 0.05) 6.2, 5.8, and 5.9 % in HIGH OZn vs. CON-fed cows. MED-fed cows had reduced overall milk fat content (3.0%) compared to CON and HIGH-fed cows. Cows fed HIGH OZn had increased (P < 0.01) SCC compared to the CON and MED-fed cows (35%), but less ( P < 0.05; 17%) LnSCC than CON-fed cows. Feeding increasing levels of OZn reduced feed intake without affecting overall milk production and therefore improved feed efficiency.