Average daily gain of steers grazing E+ tall fescue has been used as one of the indicators of the severity of tall fescue toxicosis. Schmidt et al. (1982) fed E+ and E- tall fescue seed and hay to beef steers in a 53-d feeding trial. Steers fed E+ seed gained 0.76 kg/d less than those fed E- seed, while those fed E+ hay gained 0.38 kg/d less than those fed E- hay. Jackson et al. (1984) compared performance of steers consuming either E+ tall fescue or orchardgrass under controlled environmental conditions. Steer gain differences between E+ tall fescue and orchardgrass were a net of 14 kg, with E+ steers losing weight while orchardgrass steers gained weight. Reviewing the effects of fescue toxicosis on beef cattle productivity, Paterson et al. (1995) summarized 11 trials conducted in 10 states. Steer gains were 30 to 100% lower when steers consumed E+ rather than E- tall fescue.

In many of the earlier reports, before N. coenophialum-generated alkaloids had been recognized as a major influence on cattle performance, much effort was devoted to predicting animal ADG based on the endophyte infestation level in tall fescue. Garner et al. (1984), Crawford et al. (1989), and Stuedemann et al. (1985) suggested that for each 10% increase in endophyte infestation, steer gains were 45 g/d less.

Thompson et al. (1993) analyzed a pooled dataset from 12 independent grazing studies covering 13 yr and nine locations in the United States, using the mixed-models procedures implemented in the General Linear Mixed Model software (Blouin and Saxton, 1990). Forage treatment represented E+ levels from very low (≤5%) to very high (97%) in tall fescue stands with clover (Trifolium spp.) at 25 to 30% of the forage stand in spring and summer. An interaction was found between clover presence and E+ infestation level that helped explain animal performance differences noted in the dataset. Regression equations were developed to predict changes in ADG as a function of endophyte infestation level and clover presence. The regression equation for spring grazing with clover was: ADG (g/d) = 928 - 4.86 (% E+). The reduction in ADG associated with each 10% increase in E+ infestation was variable and influenced more by season of the year than by the presence of clover.


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