Decreased weight gain and feed intake are common intoxication responses by most species (e.g., cattle, rats, rabbits, and horses) to consuming E+ tall fescue (Schmidt et al., 1982; Hoveland et al., 1983; Daniels et al., 1984; Neal and Schmidt, 1985; Bond and Bolt, 1986; Redmond et al., 1991; Aiken et al., 1993). Patterson et al. (1995) concluded from a review of grazing experiments with E+ tall fescue that steer average daily gain (ADG) ranged from 0.20 to 0.62 kg/d, substantially less than for endophyte free (E-) or novel nontoxic endophyte tall fescues (Parish et al., 2003). Aiken et al. (1993) reported that ADG of yearling geldings was less (P < 0.01) on high E+ tall fescue than on forage with low levels of infestation. However, gelding growth in wither heights was similar (P > 0.10) on both fescue regimes. Feed intake and growth in sheep did not appear to be as affected as for other species (Porter and Thompson, 1992). These data indicate potential species differences in susceptibility, but the exact cause of these differences remains to be determined and may provide clues to a potential solution.

General growth effects probably are due to the reduced feed intake and/or reported digestibility effects of E+ tall fescue in sheep, cattle, and horses (Hannah et al., 1990; Fiorito et al., 1991; Redmond et al., 1991; McCann et al., 1992), both of which would decrease available nutrients for growth and maintenance. Causes of reduced feed intake still remain elusive; potential effects of endophyte-produced tall fescue toxicants on feeding and satiety centers, gastrointestinal tract motility, blood flow patterns, heat stress, and/or a sensation of sickness are all possibilities. Recent food preference studies with rabbits (Panaccione et al., 2006) using genetically modified endophytes (Neotyphodium spp. related to wild-type in tall fescue) in perennial ryegrass demonstrated that rabbits preferred novel over E- perennial ryegrass, neither of which contained ergot alkaloids. The preference of rabbits for E- perennial ryegrass with clavine alkaloids (precursors to ergovaline) was equivalent to that for E- perennial ryegrass. However, E+ ryegrass with ergovaline (an ergopeptine) had a negative effect on intake levels in subsequent meals. This provides the first evidence that ergopeptines, not other potential toxicants in E+ tall fescue, may be linked to restricted feed intakes in grazing animals.

Controlled-intake digestibility studies have provided clues to nutrients most affected by the consumption of E+ tall fescue without the confounding influence of feed intake differences reported above. Hannah et al. (1990) reported reduced ruminal and total tract organic matter, neutral detergent fiber (NDF), and cellulose digestibility in sheep consuming diets containing E+ tall fescue seed with 3 mg/kg of ergovaline. In a similar study, Fiorito et al. (1991) reported a study in rumen-cannulated sheep in which dry matter consumption was forced via the cannula to balance intakes between E+ and E- tall fescue. They reported that total tract dry matter, NDF, and acid detergent fiber (ADF) digestibilities were depressed by the presence of E+ tall fescue hay in the diet. Likewise, Westendorf et al. (1993) reported similar findings with total tract dry matter, ADF, NDF, and crude protein (CP) digestibilities, which were suppressed in sheep by inclusion of E+ tall fescue in the diet. In an attempt to define the mechanism of these reported digestibility effects in ungulates, Harmon et al. (1991) conducted portal and hepatic nutrient flux experiments in limit-fed steers consuming E+ or E- tall fescue hay. They reported no differences in dry matter or N digestibilities between tall fescue treatments and ruminal total volatile fatty acid concentrations, and molar proportions were not affected. Net portal flux of acetate was increased, but no other effects on nutrient flux were noted. The one minor change in acetate taken by itself did not support major effects of E+ tall fescue on ruminal metabolism of nutrients.

Results of Harmon et al. (1991) are in contrast to those reported by Hannah et al. (1990), Fiorito et al. (1991), and Westendorf et al. (1993) for sheep fed either seed or hay diets. There are several reasons why the differences may have occurred, such as differing levels of ergot alkaloids, quality of the diet, species differences, and ruminal microbial populations. In support of diet quality as a potential factor, digestibility results with horses have been reported to change in relation to the type of diet fed. Redmond et al. (1991) and McCann et al. (1992) reported lower digestibilities for E+ tall fescue hay fed to mature geldings and yearling horses, respectively. In contrast, McCann et al. (1993), Pendergraft and Arns (1993), and Schultz et al. (2006) reported no effect of E+ tall fescue, either as hay or seed, but these studies included grain in the total diet. All these metabolism studies, when considered together, indicate that nutritional plane may affect the efficiency of digestion.


<--Previous       Back to Top      Next-->