Summary. Nineteen

Variation in response to fescue toxicosis was examined in inbred and linecross mice. In Exp. 1, exposure to a 50% endophyte-infected tall fescue diet (E+) reduced ADG of males from six inbred lines, but ADG of males from one line was modestly higher on E+. Lines differed (P < .01) for reproductive organ weight, but the diet x line interaction was not significant. In Exp. 2, an apparently susceptible (C57) and an apparently resistant line (FVB) were mated to produce inbred and linecross offspring. The reduction in weight gain caused by the E+ diet did not differ significantly among the genetic groups. In Exp. 3, C57 and C57 backcrosses had smaller reductions in ADG during E+ vs control feeding periods than FVB and FVB backcrosses (P < .10). In Exp. 4, the E+ diet reduced litter size of mates of C57 males by one pup, whereas litter size of mates of FVB males was four pups larger (interaction P = .07). Neither diet, line, nor their interaction affected male reproductive organ weights or tissue proportions in testis cross-sections. In Exp. 5, the E+ diet did not affect weight gain of C57 or FVB males, but effects of the E+ diet on litter size of mates were similar to those in Exp. 4. Percentage of abnormal sperm was increased in C57 males on the E+ diet but decreased in FVB males (Exp. 5). Differences among inbred lines in susceptibility to fescue toxicosis may depend on severity of the challenge and life cycle stage when the challenge is imposed.

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