As stated above, stands of tall fescue should be tested for endophyte at periodic intervals based on age and history of likely contamination events. There are at least six potential sources of contamination by toxic endophyte in new tall fescue stands (Hume and Barker, 2005) because of the increased competitiveness of infected plants (Hill et al., 1991) and the likelihood of reinfestation by wild endophyte strains:
The significance of contaminated seed has been mentioned. To prevent any chance of seed being mixed accidentally with toxic endophyte seed, a seed test should be done on seed samples obtained immediately before seeding. Since endophyte viability in seed declines gradually, seed tests older than several months could overestimate the actual endophyte level.
Herbicide applications and fallow cultivation may not have killed all established infected plants, and therefore pasture establishment into a previously pastured area could have significant survival of infected plants. In Mansfield, Ohio, an old fescue pasture was planted to oat (Avena sativa L.) followed by orchardgrass (Dactylis glomerata L.), but was still found to have about a 10% tall fescue stand 2 yr later, with the surviving tall fescue being 85% E+ (D.J. Barker, unpublished data, 2003). In contrast, a Georgia study indicated that nontoxic E+ tall fescue can be established successfully by preventing viable spring/summer seed production and spraying twice with glyphosate in the fall before no-till planting (Andrae and Hill, 2007). Such a procedure has resulted in less than 2% of the tillers infected with wild-type endophyte in multiple on-farm demonstrations. It is possible that there are significant environment effects on endophyte viability and seedling recruitment from seedbanks. A 2-yr exclusion from tall fescue is recommended currently in Ohio to ensure that all prior tall fescue has been eliminated; shorter intervals may be appropriate in other locations.
When establishing E- tall fescue into a previously pastured area, there also is the potential for contamination from buried seed from the prior stand. Management to prevent seedhead formation in the summer before establishment can minimize seed loads. Pedersen et al. (1984) and Rampton and Ching (1966) reported that tall fescue seedlings were eliminated after 18 to 24 mo of burial. Therefore, as stated above, a 2-yr exclusion from tall fescue is recommended to ensure that any buried seed become nonviable.
Tall fescue seed can survive passage through the cattle digestive tract (Shelby and Schmidt, 1991). Contaminated tall fescue pasture, especially if allowed to reach the reproductive stage, and contaminated hay should not be available to cattle within 2 to 3 d of starting grazing E- or novel endophyte tall fescue stands. Seedlings emerging from cattle manure could be tested if the potential for endophyte contamination were possible.
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Fig. 15-6. Endophyte infected seedlings of tall fescue are germinating from seed carried in hay fed on this E- pasture. Careless management has accelerated endophyte contamination in the new pasture seeding (photo: D.J. Barker).
Most spring-cut tall fescue hay contains some viable seed, and hay harvested from old tall fescue pastures is a potential source of endophyte contamination (Fig. 15-6). Seed from hay should tested to determine the potential for endophyte contamination.
Feral wildlife, machinery, and wind at seed maturity are potential agents for the transfer of contaminated seed into pastures free of endophyte. Keeping adjacent E+ fields mowed or grazed, so that they do not produce seedheads, should minimize the transfer of infected seed to clean fields. There is little information on the dispersal range of tall fescue seed.
There are other factors that also might affect established tall fescue stands. These factors might accelerate contamination of noninfested stands. There is some evidence that endophyte levels increase with drought, especially in recently planted pastures, perhaps because of differential drought survival of infected plants (see Chapter 4). Checking endophyte levels the year after a severe drought is recommended.
Fertilization with both N and P may increase both endophyte presence and ergovaline concentration (Belesky et al., 1988; Malinowski and Belesky, 2000). Rottinghaus et al. (1991) found spring and fall applications of N increased ergovaline levels by about 2
μg /kg for each kilogram of N per hectare. Checking endophyte levels after fertilization usually is not necessary. Nonetheless, if there has been a large increase in fertilizer application over several years, endophyte levels should be checked after 5 yr because levels are likely to increase.
The extent to which livestock can discriminate between toxic E+ and E- tall fescue is debated. Producers have observed dramatic refusal by livestock to graze infested stands, but livestock selectivity between adjacent plants has never been tested in controlled experiments. In New Zealand, Edwards et al. (1993) found more complete grazing by sheep of E- ryegrass than of ryegrass infected with N. lolli. In a Knoxville, TN study, pastures drilled in 17-cm (7-in) drill rows, with four rows of E- tall fescue alternating with 4 rows of > 85% E+ tall fescue, were grazed with steers for 10 yr in a put-and-take system to maintain vegetation between 5- and 20-cm height (H.A. Fribourg and J.H. Waller, personal communication, 2008). There were no visible differences in the grazing of these rows, and plant density (i.e., numbers of tillers per linear foot of row) was the same for E- as E+ throughout the study. In Ohio, a study in turf tall fescue found that insect resistance of E+ grasses was affected by mowing height and frequency through the impact on alkaloid production (P. Grewal, personal communication, 2008). Testing endophyte after grazing practice changes is not necessary.
There are no significant effects of time of day, grazing intensity, or rainfall on endophyte infection. There is no evidence that companion plant species affect endophyte infection; however, since E+ plants are more vigorous and competitive than E- plants in associations with companion species such as white clover (Trifolium repens L.), denser clover stands are associated with lower levels of endophyte infestation (Fribourg et al., 1991).
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