Counter herbicide resistance with crop rotation

Herbicide resistance is a major problem facing grain producers in the Western Cape.
Long-term trials conducted in the province by Dr Mark Hardy of the Western Cape agriculture ­department ­reveal that the most effective crop rotation system for weed management is grain crops ­followed by broad-leaved crops or legume pastures. Wouter Kriel reports.
Issue date 1 June 2007

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Herbicide resistance is a major problem facing grain ­producers in the Western Cape. If not addressed as an integral part of farm management, the problem will increase and result in lower grain yields and increased input costs, placing the already low margins under even more pressure. Herbicide resistance has however not developed only due to poor management practices on the part of grain producers. The local grain production market is small if ­compared on global terms, and in relation to the costs agricultural chemical companies must incur to register herbicides for the industry. Few new herbicides have therefore been registered in recent years. As a result producers have little choice in the herbicides available to them and tend to consistently use the chemical that works for their farming system.
Monocropping of grain crops has also not helped in specialised selective herbicides being used for controlling grass weeds in these crops. Unfortunately, the inevitable result of the constant use of these ­chemicals, and of selective grass herbicides in ­particular, has resulted in the ­development of herbicide resistance in several weed ­species, with ryegrass being one of the main problem species. The bad news is that “new” herbicides are unlikely to become available in the near future, and the current move towards conservation tillage does not allow for weed management through ­mechanical soil disturbance. Producers must therefore develop alternative strategies to combat weed infestation and herbicide resistance through alternative production practices.

Long-term research conducted by Dr Mark Hardy, specialist researcher for the Western Cape Department of Agriculture, clearly shows that weed infestation and ­herbicide resistance problems can be controlled through various practices in ­conservation farming systems. Crop ­rotation forms the basis of weed control. “Also, if ­herbicide resistance has not yet developed on the farm, implementing an ­appropriate crop rotation system will be effective in ­preventing future herbicide resistance from ­developing,” Dr Hardy says.

Employing crop rotation

In grain production systems of the Western Cape, crop rotation involves alternating the crops planted on a particular land from one year to the next. This provides ­opportunities for weed control measures not available when the same crops are monocropped. The most effective crop rotation for weed control is when a grain crop (such as wheat – a grass species) is followed by a broad-leaf crop (such as canola) or a legume pasture (medics, clovers or lucerne). When deciding on an appropriate crop rotation system, the farmer must ­understand that the different herbicides available for weed control may have the same or different modes of action. ­
Herbicide resistance develops when herbicides with the same mode of action are frequently used. It is therefore important to also ­alternate between herbicides with ­different modes of action from one year to the next on the same land. Crop ­rotation allows for the use of a greater range of herbicides (and therefore modes of action) than does monocropping with grain crops, says Dr Hardy. Registered herbicides that still work, such as those with the active ingredient ­Trifluralin, must be used with care to ­prevent ­resistance. ­According to Dr Hardy, this can be achieved by using a crop rotation that only requires the use of Trifluralin every second or third year. It is also important to know, for each land on the farm, if any of the weed species have shown resistance to a ­particular herbicide (and to a particular mode of action) or even a range of herbicides. The crop rotation the farmer uses to ­control weeds on a particular land depends on what crops or pastures can be ­productively and economically grown on that land, the weeds that are found on that land, and the herbicide resistance status of those weeds. Ryegrass was the most ­important weed in Dr Hardy’s long-term crop rotation trials.

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Swartland trial results

The Swartland trials have been active for 11 years. Severe herbicide ­resistance has ­developed in ryegrass in all ­rotations where wheat follows wheat, particularly in monocropped wheat systems. In these systems, the higher the ryegrass ­population, the lower the grain yields. During the 2006 ­season (with an extremely ­favourable ­climate for ­maximum yield) yield ­differences above 500kg per ­hectare were recorded within the ­monocropping wheat ­systems, depending on ryegrass ­densities. Where the ryegrass was ­effectively ­controlled in ­rotations where wheat ­followed canola, lupins or medic pasture, wheat yields ranged from 1 000kg/ha to 1 500kg/ha higher than the best yields recorded in the wheat after wheat systems, according to Dr Hardy.
In a dry season, ryegrass weed control becomes even more difficult. Water stress results in herbicides being less effective, so where ryegrass populations are high, as in monocropped grain production systems, weeds competition results in very poor grain ­production. The Swartland drought of 2003 was such a season. Ryegrass could not be controlled in wheat-on-wheat systems, while ryegrass populations in wheat-on-medic ­systems were low. Grain yields in wheat-on-wheat systems ranged from 0kg/ha to about 500kg/ha, whereas grain yields of wheat-on-medic systems were above 1 500kg/ha. These results clearly show the long-term advantage of ­­­medic-wheat rotation when compared with the monocropped wheat production s­ystem. ­Cropping systems where herbicide ­applications and modes of action could be varied, such as those where canola and lupins are planted in rotation with wheat, also showed consistently higher wheat grain yields than did the monocropped wheat, according to Dr Hardy.

The advantage of lucerne

The same observations were made in the long-term cropping trials in the ­southern Cape. The advantage that ­farmers in the southern Cape have over their Swartland counterparts is lucerne, which is grown in long-rotation with cereal, and protein and oil seed crops – six or seven years of lucerne followed by six or seven years of these other crops. Ryegrass and other weed species can be effectively controlled during the lucerne phase using broad-spectrum ­herbicides such as Paraquat and ­Glyphosate, and ­grazing management options. By ­controlling weeds during the lucerne phase, weed seeds are prevented from building up in the soil seed bank so that when the ­cropping phase starts, weed seed densities in the soil are low and more easily controlled than high weed seed ­densities. Also, by not using selective herbicides during the lucerne phase, the chance that herbicide resistance will develop is low.
As described earlier, it is important to plan the cropping phase so that herbicides with different modes of action can be used ­during the six- to seven-year ­cropping phase. “With good planning, the farmer should be able to manage weed and ­herbicide resistance with the available ­herbicides,” Dr Hardy says.
Dr Hardy’s long-term cropping systems research trials also demonstrated the value of using grazing pressure, mowing and ­“pasture topping” to manage ryegrass ­problems in addition to using herbicides. ­Initial ryegrass populations (measured above 5 000 ryegrass seedlings per square metre) were dramatically reduced by mob grazing a pasture when the ­ryegrass started to flower. At high grazing ­pressure, the sheep tend to remove most of the ­flowering stems.
Mowing at the same time has the same effect but is obviously a more ­expensive operation. Pasture topping involves ­applying a low dose of Glyphosate when ryegrass seed is at the soft dough stage of development, ­preventing the ­ryegrass plants from ­producing viable seed. “All these actions lower weed seed ­production in the pasture phase, ­therefore lowering the potential for high weed ­populations in the cropping phase,” Dr Hardy explains. “In a farming operation where the ­producer requires hay or silage, or where there is a market for these conserved feeds, a crop such as oats can be introduced in the rotation. Weeds are also removed during the hay-making process. This is one of the most effective ways of preventing weeds from seeding, and it results in a dramatic ­reduction of weed densities,” he says.

Contact Dr Mark Hardy on (023) 614 3839 or e-mail [email protected]. |fw