Effects of an Estradiol Implant on Locoweed Consumption, Toxicity, and Recovery in Growing Beef Steers11This research was supported by funds from the New Mexico Agricultural Experiment Station and Grant #ARS 58-5428-6-122 from the USDA.

Abstract

<h2>Abstract</h2> Two experiments were conducted to determine effects of an estradiol implant (Compudose®) on locoweed consumption and toxicity in growing steers. In Exp. 1, 16 crossbred steers (185.3 ± 6.1 kg) were randomly assigned to two replicated treatments and received either an estradiol implant or no implant. Steers were assigned to one of four pastures and were rotated through all pastures, which differed in locoweed distribution, to allow equal access. Bite counts were recorded twice daily at 0600 and 1700h during a period when steers were likely to consume locoweed. For bite counts, steers were observed for 5min each, starting at 0600 and at 1700 h, and the number of bites taken of cool- and warm-season grasses, forbs, and locoweed were recorded. Blood was collected on d 0, 7, 28, 35, 63, and 119, and individual BW was recorded on d 0, 35, 63, and 119. Proportion of bites of locoweed consumed by implanted vs nonimplanted steers did not differ (P>0.10). Likewise, ADG, serum alkaline phosphatase activity, and thyroxine concentrations did not differ (P>0.10) between implanted and nonimplanted steers. In Exp. 2, 20 crossbred steers (212.3 ± 6.1 kg) were divided into four groups and individually fed in a 2 × 2 factorial arrangement of treatments. Treatments included: 1) estradiol implant + locoweed, 2) implant, no locoweed, 3) no implant + locoweed, and 4) no implant, no locoweed. Steers were implanted at d 0 and fed either a ground forage diet containing 80% sudangrass hay and 20% locoweed, or a diet of 100% sudangrass hay. Implanted steers had improved ADG vs nonimplanted steers (P<0.10) through 63 d on trial, but no differences were observed in steers fed locoweed vs sudangrass hay diets (P>0.90; locoweed x implant, P>0.10). Alkaline phosphatase activity was greater (P<0.05) for steers fed locoweed vs those not receiving locoweed on d 7, 14, and 21, whereas, thyroxine concentration was lower (P<0.06) in steers fed locoweed than those not fed locoweed on d 14 and 21. Estradiol concentrations were greater in implanted steers vs those not implanted (P<0.05). These results suggest no effect of an estrogen implant on locoweed consumption or on severity of locoweed toxicity by beef steers.