Roundup cotton boosting soil conservation: researchers

Australia's cotton farmers are following a trend in the US industry to adopt conservation tillage practices as herbicide-tolerant cotton varieties revolutionise weed control.

Since their commercial release in season 2000-01, there has been a rapid uptake of Roundup-Ready cultivars, according to CSIRO farming systems expert Grant Roberts, of the Australian Cotton Cooperative Research Centre in Narrabri, NSW.

Roundup Ready cultivars, engineered for tolerance of the herbicide glyphosate, already account for 83,000 hectares of cotton, or about 38 per cent of the area planted.

The area of Roundup Ready cotton has already overtaken the area planted to transgenic cultivars protected against insect attack by the Bt insecticide gene but Roberts said the lower level of Bt adoption was mainly due to an industry cap on the Bt gene to protect it from pest resistance

He said some cultivars were "stacked" with both Roundup Ready and Bt genes; others contained one or the other gene. Industry protocols for managing pest resistance currently limit the total area of the crop that can be planted to Bt cultivars to 30 per cent.

But Roberts said the Roundup Ready cultivars had merely enhanced farmers' weed-control options. Many growers -- particularly dryland farmers -- had already adopted conservation tillage to limit wind and water erosion by controlling weeds with shielded sprays that prevent spray drift onto unprotected cotton plants.

"Roundup is very useful in dryland situations, and irrigated farms growing Roundup Ready varieties are beginning to use it as a substitute for residual herbicides," he said.

All Roundup Ready cultivars currently use Monsanto's proprietary Roundup Ready gene, but other herbicide tolerant genes are being developed by other companies.

In the future Roundup Ready genes will be available with Bollgard II cultivars, which will be protected by two independently acting Bt-toxin genes; these may cover a substantial proportion of Australian cotton production.

Growers in the US and Australia are now awaiting the advent of cultivars containing Monsanto's new Roundup Flex gene, which will extend the period of tolerance to Roundup from the four-leaf stage to around the 14-node stage or greater -- around the time of peak flowering.

Roberts said that while Roundup Ready cultivars were reducing the time and labour involved in controlling weeds by cultivation, savings to farmers depended on the weed pressure in the field as the licensing fees attached to the technology often replaced the cost of the herbicides dropped out.

A study conducted for the National Cotton Council in the USA, published this week, found that reduced- and zero-till cotton areas in the United States now accounted for 59 per cent of the total areas. The zero-till acreage has almost doubled, to 29 per cent of the total area since the first herbicide-tolerant cultivars were planted in 1997, while the reduced-till area has more than doubled, to nearly 30 per cent of the total area.

The change is most prevalent in the mid-south US states, where 74 per cent of the area is zero- or reduced-till.

Dr Andrew Jordan, director of the National Cotton Council's technical services department, said the new study "merely confirms what most of us suspected".

"Weed control is critical for a good cotton crop and biotechnology is giving growers another weed control tool while allowing them to move to more cost-effective, environmentally sound methods of cotton production," he said.

Growers surveyed in the study reported that conservation tillage methods reduced normal fuel and labour costs by an average of $US20.13 per acre ($US49.74 per hectare).

Roberts said savings which exceeded the licence fee applied in the US were encouraging more growers to plant herbicide-tolerant cotton, hence the trend towards conservation tillage.