Toxic effect from ryegrass studied

Press, 30 December 1988, Page 9

Toxic effect from ryegrass studied

By

JOHN HARFORD

A chance remark in a hotel led to research which showed white clover suffered a toxic effect when grown with high endophyte ryegrass.

The research by the D.S.I.R.’s Grasslands Division at Lincoln was begun when Mr Lee Sutherland, a technician with the D.5.1.R., heard a farmer say his crop had suffered a drop in yield after the soil had grown a mixture of endophyte ryegrass and white clover.

A dryland trial was begun in 1985 to test the suspicion that endophyte ryegrasses reduced the performance of associated white clover. Although the endophyte protected the pasture from attack from Argentine stem weevil and increased its persistence, the performance of sheep dropped and the animals ran the risk of developing ryegrass staggers.

Three reasons were considered to be the most likely causes of reduced clover performance: selective grazing from sheep which found high endophyte ryegrass to be less palatable than white clover; increased competitiveness of endophyte ryegrass; and a possible toxic effect on the clover from the endophyte.

The clover levels were not affected during the early stages of pasture growth when measured in

December. By mid-Janu-ary the clover population declined more rapidly. The difference between the endophyte and nonendophyte pastures remained the same until June when 56 per cent fewer clovers survived under the endophyte ryegrass. The clover which did survive suffered a 40 per cent reduction in vigour.

The difference in clover survival was the same for mowed and grazed plots, discounting selective grazing as the cause of reduced clover performance.

Further research showed the greater the yield from high endophyte ryegrass, the less clover that survived. Mr Sutherland said the figures supported the argument that endophyte ryegrass outcompeted white clover but also indicated a biochemical interaction between the two species. “About half the’difference in clover survival is due to ryegrass out-com-peting clover and the other half due to a biological or poisoning effect of the endophyte. We don’t fully understand the toxic effect yet but research is underway,” he said.

In further trials, wheat seedlings grown in soil which had previously grown white clover and non endophyte ryegrass produced 29 per cent more grain than wheat in soil where endophyte ryegrass had been grown previously with clover.

But in soil where clover had not been part of the pasture mix, there was no difference in grain yield from wheat.

Mr Sutherland said a drop in nitrogen levels, which would accompany a decline in clover numbers, appeared to be the cause of the difference in wheat performance.

“We concluded that the endophyte effect on the clover component of the pasture has a significant effect on soil nitrogen fertility, which in turn reduces subsequent crop yields. Any other residues or toxins which may be produced by endophyte ryegrass do not remain in the soil to affect following cereal crops.”

The effect of endophyte ryegrass on clover could have a substantial economic significance to New Zealand’s economy, he said. White clover was sown in about 80 per cent

of pastures and its value to the economy, including its contribution to soil nitrogen fertility, was about $2 billion a year. “Any reduction in the vigour of clover due to increased, and in some areas unwarranted, use of endophyte ryegrass should be of concern. “The on-farm cost of the endophyte effect will have to be measured against the short term increase in pasture production and persistence. In areas where weevil attack can be severe, this endophyte effect might be a small price to pay,” said Mr Sutherland. Other researchers at the D.S.I.R. have concentrated on providing alternatives to ryegrass. Mr Tom Fraser said they wanted a new grass which would “take the ryegrass management” and not change the way a farm was run.

Four main species had been advocated as alternatives to ryegrass in Canterbury. Roa tall fescue could be used on medium to good soils but could also suit dry conditions. In some trials, stock performance had lifted 15 per cent when grazed on fescue. Wana cocksfoot could be used on ground with

slightly lower fertility and Maru phalaris could be used with Wana or Roa on the lighter end of soils, said Mr Fraser.

The three were the major alternatives for dry areas because they were tolerant to drought and insect attack. Mr Fraser was confident they could match ryegrass in production and offer greater persistance. The fourth option was Matua prairie grass which performed well under drought conditions but required a more careful management. It was a

particularly good winter growing grass which would produce two to three times the amount of dry matter during winter than ryegrass. Mr Fraser said the change to alternative grasses was accelerating after 25 years of ryegrass and white clover domination of pastures, and would continue. “Some are slower to establish but once they

are they would accept normal ryegrass management, except for Matua.” Seed growers were responding quickly to the change. While very little Roa tall fescue seed was grown two years ago, enough is now produced to plant hundreds of hectares. The drought in Canterbury could mean a shortage of alternative pasture seeds next year, he said.

Maru was the only grass which was resistant to grass grub, although other grasses had varying degrees of tolerance. It was unclear if mixing Maru with other species would keep grass grubs away from the other grasses.