BIOAVAILABILITY OF COBALT SOURCES FOR RUMINANTS .1. EFFECTS OF TIME AND DIETARY COBALT CONCENTRATION ON TISSUE COBALT CONCENTRATION

An experiment was conducted with 27 mature wethers initially averaging 59.8 +/- 5.6 kg in body weight to study the effect of time and dietar y Co concentration on tissue Co concentrations as a potential bioassay for Co sources. The 3 x 3 factorial arrangement of treatments include d 0, 20, or 40 ppm added Co as reagent grade CoSO4 . H2O added to a ba sal diet containing 0.17 ppm Co and fed for 20, 40, or 60 d. Sheep wer e offered diets and tap water ad libitum. At the end of each period sh eep were weighed and killed for tissue collection. At 20 d only livers were taken; thereafter kidney, spleen, skeletal muscle, and heart wer e also collected. There was no effect of time (P = 0.07) or dietary Co (P = 0.84) on average daily feed intake. Cobalt increased in liver (P < 0.001) and kidney (P < 0.05) with increasing time, but other tissue s were not affected. Supplemental Co increased Co concentrations in al l tissues collected, but the best fit to a linear model occurred with liver, kidney, and heart, respectively. Based on the lack of improveme nt in fit to a linear model, 20 d was selected as the most efficient t ime for use in future bioassays. (C) 1997 Elsevier Science Inc.