Fungal endophyte and alkaloid levels vary on an annual cycle. They are fairly constant when the same days are compared from year to year, but measurements can vary considerably among different months. This section considers the effects of timing and frequency of plant sampling on endophyte detection.
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Fig. 15-5. Average percentage of endophyte infection in pastures (Georgia) and seed fields (Oregon) of tall fescue sampled over 2 yr (adapted from Ju et al., 2006).
Endophyte infection within a tall fescue population is not static. Fields sampled over the course of the year in Georgia and Oregon showed similar trends of reduced endophyte infection during winter and spring months (Fig. 15-5). The reason for the depression in endophyte detection is unclear, but it appears to be related to cardinal minimum temperatures for endophyte growth vs. plant growth and the shift from vegetative to reproductive plant tissues following vernalization. Alternatively, the complex coordination between cell division in the hyphae and cell expansion in the leaf might become disrupted with the high rates of spring leaf expansion, with consequent lowering of infection in spring tall fescue stands. Whatever the mechanism, fields should not be sampled in spring, when endophyte levels are lower, than during the remainder of the year.
In addition to variation in the amount of endophyte, there is seasonal variation in the amount of ergovaline produced. High temperature and drought that occur in summer will result in a higher concentration of ergovaline. Rottinghaus et al. (1991) found that ergovaline concentrations in leaves and pseudostems in Missouri peaked in May (529 and 1083 μg/kg, respectively) and in seedheads peaked during July (5000 and 1700 >μg/kg in 1987 and 1988, respectively) (Fig. 15-4). After flowering, ergovaline levels remained high (up to 800 μg/kg) and did not decrease until November (Rottinghaus et al., 1991). In another Missouri study (Kallenbach et al., 2003) ergovaline concentration of stockpiled E+ ‘Kentucky-31' (KY-31) tall fescue decreased steadily during winter and reached the "safe" level of 200 μg/kg on 1 Feb. and 1 Jan. in 2000 and 2001, respectively.
Between 1995 and 2002, most recommendations for new tall fescue pastures were to establish E- cultivars; however, this status free of endophyte may be short-lived (Hill et al., 1998). Thompson et al. (1989) reported that six of the eight fields they tested had significantly higher endophyte levels 28 mo after planting (mean = 51%) than was present in the seed planted (mean = 28%). In contrast, there are some cases in Tennessee where E- stands have remained free of endophyte for more than 10 yr (H.A. Fribourg, personal communication, 2008).
There are at least six possible sources of contamination in new stands (see below). Depending on their severity, E- tall fescue stands generally become reinfested by toxic E+ tall fescue within 5 to 10 yr after planting. New E- pastures should be tested 1 to 2 yr after establishment and resampled each 5 yr after that.
Currently, there is almost no information regarding the potential for endophyte contamination in new nontoxic endophyte tall fescue stands. Current recommendations are to treat such novel endophyte tall fescue seedings in the same manner as new E- sowings. One hypothesis is that the presence of the nontoxic endophyte will reduce the rate of reinvasion by the wild toxic endophyte. In two spring-sown studies in Ohio, E- plots were 5 to 15% infected with toxic endophyte after 18 mo, but there was no evidence of toxic endophyte in tall fescue plots with nontoxic endophyte (MaxQ, trademark of Pennington Seed, Madison, GA) plots (Barker et al., 2005). Alkaloid analyses by ELISA are required to distinguish between toxic and nontoxic E+ tall fescue.
Old tall fescue pastures usually have stable endophyte levels. These may vary between 0 and 100% infestation, with an average of 58% throughout the United States (Shelby and Dalrymple, 1987). Lower levels in western states were related to a lesser use of the KY-31 cultivar (Shelby and Dalrymple, 1987). With the exception of seasonal variation (see above), there is little evidence for variation in the endophyte levels within old pastures. There is no evidence that endophyte levels in a field vary by more than the expected sampling error of ±10% between successive years.
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