With the rapid spread of herbicide-resistant weeds, it has become increasingly difficult to control weeds in no-till systems and many people are considering going back to tillage. Before taking this drastic step, consider enlisting cover crops as an ally against weeds.
Cover crops can provide weed control benefits through three main mechanisms:
- The first mechanism is simple competition. Rap-id-growing, large-leafed cover crops such as buck-wheat, okra, sorghum-sudan, and Florida broadleaf mustard can simply outgrow and shade most weeds. Crops like cereal rye that grow before winter annual weeds get started also give excellent weed suppres-sion. Plant diversity is important so that the mix has multiple levels of canopy to intercept all of the sun-light before it can get to emerging weeds. It is amazing how effectively a diverse cover crop blend can prevent weed growth. Many customers report that planting a cover crop in wheat stubble eliminates the need for several weed control passes.
- The second mechanism is nitrogen sequestration, a particularly useful concept if the desired subsequent cash crop is a legume, such as soybeans or peas. A well fertilized corn crop often leaves as much as 25% of applied nitrogen in the soil after harvest. If this nitrogen is still present in the soil when a legume crop is planted the following year, the nitrogen will stimulate weed growth and delay nodulation. A winter cover crop, such as cereal rye or winter barley, can take up and sequester nitrogen and hold it in the residue, becoming available later in the year when the residue decays. Many weeds, pigweeds in partic-ular, require free nitrate in order to germinate and do not grow well in low nitrogen conditions. The picture below shows the weed control from cover crop cereal rye (left half) versus no cover crop (right half) prior to planting soybeans. The rye out-competed the weeds as well as sequestered all available nitrogen, thus suppressing weed growth.
- The third weed control mechanism is allelopathy, or the secretion of chemicals by plants that suppress the germination or growth of other plants. Rye produces an allelopathic compound that is highly effective against marestail and pigweeds, while oats are highly allelopathic to kochia. Other allelopathic plants include sorghum-sudan and many brassicas, particularly mustards.