Contamination and persistence of endophyte-free ryegrass pastures established by spray-drilling, and intensively grazed by dairy cows in the Waikato region of New Zealand

The contamination of endophyte-free ryegrass (Lolium perenne) pastures with volunteer endophyte-infected ryegrass, and the persistence of endophyte-free ryegrass, were studied over three years at the Dairying Research Corporation, Hamilton, New Zealand. Plots were sprayed with glyphosate herbicide in mid March 1996 (sprayed: S), or mid March and again in mid April (double sprayed: D). A sub-plot treatment allowed the effects of the presence or absence of white clover (Trifolium repens) to be studied. All plots were direct-drilled with endophyte-free perennial ryegrass in late April 1996, and were subsequently rotationally grazed by dairy cows. Sources of contamination were endophyte-infected ryegrass seed from reseeding of existing ryegrass, seed consumed by cows and deposited in dung, and seed buried in the soil, with endophyte infection levels after germination of 66, 38, and 75%, respectively. incomplete kill of existing endophyte-infected ryegrass after herbicide applications was another possible source of contamination with a 50% endophyte infection level. Contamination of D plots was consistently less than in S, averaging 25 and 58%, respectively, after three years, but was <10% of plants in D plots over the first two years. Reseeding of existing endophyte-infected ryegrass was considered the most important source of contamination and the soil seedbank the least. Double spraying reduced competition for establishing endophyte-free ryegrass, which consistently improved ryegrass survival compared with S, reflecting a higher pasture ryegrass content for 12 months, but no improvement in herbage accumulation. White clover level did not influence contamination. The trial showed that an endophyte-free ryegrass pasture could be maintained for at least 3 years showing similar trends in yield and persistence as high-endophyte based ryegrass pastures previously monitored at the site.