Effects of Trace Mineral Source and Feeding Method on the Productivity of Grazing Braford Cows

Abstract

Braford cows (n = 160) were used to evaluate the effects of trace mineral source and feeding method on cow herd productivity over 3yr. In yr 1, cows were stratified by age and randomly allocated to one of eight groups (n = 20 cows per group). Cows were allotted to 8 of 16 bahiagrass pastures (8.0ha each, one empty pasture between groups) and were rotated each week. Cow groups remained intact throughout the study with the exception of culled cows, which were replaced with 3-yr-old pregnant heifers. Cows were offered 2.27kg of liquid molasses daily from early November until mid April. One mature Braford bull was placed into each cow group starting in early January, where the bull remained for 90 d. Bulls were rotated through cow groups each week. Two main factors were randomly applied to groups in a 2 × 2 factorial arrangement (two groups per treatment): 1) trace mineral source (inorganic vs organic) and 2) mineral feeding method (free-choice vs control-fed in a molasses supplement). Cow BW, body condition score (BCS), pregnancy rate, calving interval, and calf BW at weaning were obtained. Mineral intake was measured each week in pastures where mineral source was provided for ad libitum intake. In yr 1 and 2, liver biopsy collections were conducted for the determination of mineral status at the start and end of winter supplementation on the same six randomly chosen cows. Cows assigned to free-choice treatments consumed 23% less (P<0.001) mineral compared with control-fed cows, resulting in a greater (P<0.05) decrease in liver Zn (yr 1) and liver Cu (yr 2) concentration compared with control-fed cows. Mineral source and feeding method had no effect on cow BW, cow BCS, and calf BW at weaning. Young cows (3 and 4yr of age) consuming organic minerals had a greater pregnancy rate during yr 2 (P<0.05) and 3 (P=0.15) and a lesser (P<0.05) calving interval in yr 1 and 3, compared with cows consuming inorganic minerals. The supplementation of organic minerals (Cu, Zn, Mn, and Co) appears to increase the reproductive performance of young, but not mature, grazing Braford beef cows consuming molasses-based supplements.