Changes in Neotyphodium coenophialum infestation levels in tall fescue pastures due to different grazing pressures

Infection of tall fescue (Festuca arundinacea Schreb,) with the mutualistic fungal endophyte Neotyphodium coenophialum (Morgan-Jones & Gems) Glenn, Bacon & Hanlin results in numerous biochemical and physiological changes that increase host persistence. This study was conducted to determine effects of steer (Bos taurus L.) grazing on changes in endophyte infestation levels of tall fescue pastures, Eighteen 1.2-ha pastures were seeded in 1992 in 20-cm drill rows involving combinations of high, medium, or low grazing pressure and 'Ky 31' tall fescue infested with N. coenophialum at four levels ranging from endophyte-free (E-) to 80% infestation (E+), Pastures were grazed for about 10 mo. each year from fall 1993 until summer 1996, although animals on some of the high grazing pressure pastures occasionally had to be removed because of insufficient forage, Endophyte levels were monitored yearly with protein-A sandwich enzyme-linked immunosorbent assay (PAS ELISA). Stand density of tall fescue remained satisfactory throughout the study, After 2 yr, endophyte infestation levels in low grazing pressure pastures were at the same level as at the beginning of the study, and E+ levels remained constant in all pastures that started at high E+, regardless of grazing pressure. However, in high and medium grazing pressure pastures, E+ levels increased by 20 to 30%. Based on these data, we conclude that moderate and high grazing pressures influence endophyte infestation level of pastures and must be taken into account when designing pasture management systems.