An integrated approach to nutgrass control : SRDC final report NFS002

Abstract

This project aimed to formulate and promote an integrated approach to nutgrass control in sugarcane cropping systems. The project was a Grower Group Research Project undertaken from July 2007 to June 2010 by the NSW Farming Systems Group Inc. and BSES Limited extension officers in NSW.

Trials evaluating the effect of nutgrass on plant-cane yield, ratoon biomass and soybean biomass were undertaken. These trials showed cane yield losses of around 30% in both plant and ratoon cane where nutgrass was allowed to grow without any control. Even allowing nutgrass to grow for 4-8 weeks after planting or ratooning resulted in a reduction in cane growth. Quantification of nutrient uptake and removal of soil moisture by nutgrass provided reasons for the large effect of nutgrass on cane yield. In moderate to heavy infestations, nutgrass tops took up 25-45 kg N/ha that would otherwise be available to the cane crop. Similarly, nutgrass tops took up 45-50 kg K/ha (equivalent to the K applied in many planting mixes). A thick sward of nutgrass removed the equivalent of 11-12 mm rain from the plough layer in 4-8 days.

The key to long-term control of nutgrass is to reduce the number of viable tubers and the literature clearly shows the effect of gylphosate in reducing the number of viable tubers. However, the effect of other nutgrass-specific herbicides registered for cane on tuber viability had not been evaluated. A large-plot, replicated trial assessed Hero®, Krismat®, Sempra®, and ‘double-knock’ treatments of 2,4-D followed 2-4 weeks later by one of the former nutgrass-specific herbicides for their effect on nutgrass tuber viability. All herbicide treatments improved cane growth by reducing nutgrass density and all significantly decreased tuber viability.

Trials were established to evaluate some ‘best-bet’ options for nutgrass control. These trials aimed to assess ‘packages’ of successive treatments all aimed at reducing the viability of nutgrass tubers. Results from these trials were used to extend options for long-term nutgrass control to cane producers.

The trial work undertaken and associated economic analysis showed that it is highly economic to control nutgrass in cane. Total control of nutgrass resulted in large dollar benefits ($350-450/ha) and trials evaluating ‘one-off’ herbicide treatments for nutgrass also showed net benefits of $200-400/ha. This indicates that, even where nutgrass is patchy, growers could outlay $60-100/ha for nutgrass control and still obtain a substantial net benefit.

A factsheet/decision support package titled ‘Managing nutgrass in cane’ was produced and circulated throughout the Australian sugarcane industry. This factsheet emphasises that nutgrass can only be managed using a long-term integrated approach aimed at reducing the number of viable tubers. No single ‘one off’ treatment will control nutgrass and repeated treatments are the only sure way of controlling nutgrass. A single ‘one off’ treatment will reduce nutgrass competition in the short term but, because of its ability to rapidly propagate under suitable conditions, repeated treatments are imperative. The factsheet provides options for nutgrass control at various stages in the cane crop cycle