Grazing cow and calf responses to zinc supplementation.

This experiment evaluated weight gain responses to supplemental Zn by cows and calves grazing forage containing less than 20 ppm Zn. One hundred cow-calf pairs in each of 2 years and 120 paird in a third year were pastured together over a 63- to 77-day preliminary period during which cows were exposed to bulls. Each experimental period began in mid-June after bulls were removed and continued for 125 to 154 days. During this period, cattle grazed mature dry forage containing less than 20 ppm Zn and were fed protein supplements with or without added Zn. Estimated Zn intake by each cow-calf pair in the control group ranged from about 140 to 260 mg/pair/day. Daily Zn intake was supplied by forage (100 to 200 mg), basal supplement (32 to 57 mg), water (4 mg), iodized block salt (1 mg) an soil (1 mg). Each cow-calf pair fed the supplement with added Zn received an additional 860 to 900 mg Zn/day. Calves fed Zn gained 6% more (P less than .05) weight (.04 kg/day) between the two groups. Clinical signs of Zn deficiency were not observed in any animals. Conception which occurred before the experimental period, and subsequent calving rates were not affected by Zn supplementation.