Many agronomic management implications arise from the sequence in which crops are sown. Planning crop rotation in advance minimises disease and insect problems and maximises crop fertility. With disease, insects and fertility managed optimally, crops become more competitive against weeds. The implementation and/or effectiveness of some weed management strategies rely on specific crop type and variety, or the sequence of cropping.
At the same time the ability to control a target weed in a specific crop may be so limited that growing that particular crop should be avoided in paddocks where the target weed is a problem. For example, winter pulses should not be grown in paddocks where black bindweed or wireweed are a problem.
Successful weed management relies on implementation of the best available agronomical practices to optimise crop environment and growth. Basic agronomy and fine-tuning of the cropping system are the first steps towards weed management. Using the best available agronomy will increase the benefit gained from the weed management procedures employed.
Some ‘agronomy for weed management’ decisions may seem obvious or ‘second nature’. These include optimising competitive ability of the crop by sowing on time and placing the seed at the ideal depth. Other decisions, such as growing a ‘new’ crop to enable implementation of a weed control method (eg sowing field peas to enable non-selective crop-topping) require significant change and greater planning.
Another example of the importance of crop and variety choice (when implementing a weed management approach) relates to ‘in-crop seed-set control’ methods. These methods are much less detrimental to crop yield and quality where the crop variety matures prior to the weed species. Knowledge of relative competitiveness, sowing time, maturity, and difficult to control weeds is important. Similar information about specific varieties should be sought on a local basis. The ability to compete with weeds varies between crop types and between varieties within a crop type. In high weed pressure paddocks, growing a competitive crop will enhance the reduction in weed seed-set obtained through employing weed management procedures. It will also reduce the impact that surviving weeds have on crop yield.
Sowing bread wheat or barley is recommended to maximise crop competition. For example, in areas where summer crops can be grown successfully, a winter fallow – summer sorghum rotation prior to wheat is a very effective way of managing wild oats and paradoxa grass.
Crop sequencing to minimise soil- and stubble-borne disease and nematodes
A healthy crop that is not constrained by disease is far more competitive with weeds and less affected by them as a result. Sound rotation of crops and varietal selection can minimise the negative impact of soil- and stubble-borne diseases and nematodes on crop yield and seedling vigour.
Crops with dense canopies such as canola and mustard act as more effective break crops. The canopy development of mustard is the fastest while chickpeas do not reach full canopy closure until much later in the season. The denser canopy enhances microbial decomposition of cereal residues which harbour the crown rot fungi.
Selecting sound crop sequences and varieties to deal with the significant pathogens and nematodes of the paddock in question is good management.
When selecting varieties there is usually a trade-off between tolerance to specific diseases on the one hand and desirable crop traits on the other. Thus, when selecting a variety it is important to conduct a risk–benefit analysis for all diseases and significant yield, quality and agronomic traits for the individual paddock and crop varieties in question.
Weeds are alternate hosts to some pathogens. Grass weeds are alternate hosts for fungal pathogens which cause crown rot and take-all in winter cereal crops. Broadleaf weeds can also act as alternate hosts for sclerotinia, which can affect a wide range of pulse and oilseed crops. Practylenchus neglectus nematode will multiply readily in wild radish and exceptionally well in wild oats. Similarly, barley grass acts as a suitable host for P. thornei.
Use of crop sequencing as a disease break is only effective if alternate weed hosts are controlled during the fallow and in-crop.
Rhizoctonia can affect seedling crop growth, leaving the crop at greater threat from weed competition. The use of tillage to remove plant growth for a period prior to sowing can significantly reduce the level of Rhizoctonia inoculum in the soil. Tillage to 10 cm depth immediately prior to sowing also physically disrupts fungal hyphae and suppresses the disease in the short term. In a no-till system, using modified sowing points that provide soil disturbance below the seed can also limit the occurrence of Rhizoctonia. Be aware of Rhizoctonia, and understand when and where it is likely to occur in your region so that appropriate management steps can be implemented.
Article: CulturalWeedControl199 (permalink) Date: 9 January 2013; 8:46:01 PM AEDT