One of the most economically important pests that reduce crop yield each year are unwanted plants that interfere with crop growth, development, or harvest. These plants, called weeds, compete with the crops for water, light, and soil nutrients to reduce crop yield. Some weeds are capable of naturally releasing substances into the soil that are allelopathic, or toxic, to the crop. Weeds can serve as hosts for some crop diseases. Weeds also provide shelter and serve as a food source for insects and diseases that overwinter or provide habitat for unwanted wildlife species such as voles that reduce crop stands or yields.
A number of decisions must be considered in developing a successful weed control program. To assist in weed management decisions, a producer must be able to properly identify the specific weed problems in each field along with other aspects and factors that might influence weed emergence and growth. It is also important to understand the life cycle of weedy plants, their growth habit, and their potential competitiveness or impact on the crop.
The life cycles of weeds can be grouped into three major categories:
Annuals complete their life cycle in one growing season and reproduce only by seed.
Biennials are capable of completing their life cycle during two growing seasons. The first year normally consists of vegetative growth, whereas the second year involves both vegetative and flower development. Biennials, such as musk thistle, reproduce only by seed. Sometimes these plants may complete their life cycle within one year.
Perennial plants are capable of existing for more than two years. Reproduction can be by seed and by vegetative structures such as rhizomes, stolons, tubers, taproots, or creeping roots.
Warm-season annuals and perennial weeds have become of increasing concern as no-tillage practices have increased in Kentucky's crop production systems.
Proper weed identification is an essential component of any successful weed management program. It is even more critical in no-tillage systems because herbicides are the primary method of weed control. Training and a skilled eye are often needed to properly identify weeds during early vegetative growth stages.
In fact, an effective post-emergence control strategy for weeds often depends on proper identification when weeds that are less than 4 inches tall. Field scouting should begin within at least 2 weeks of planting and continue at weekly intervals for 8 to 10 weeks into the growing season. Scouting methods recommended for weeds in corn can be found in Kentucky Integrated Crop Management Manual for Field Crops available at the county Extension office.
A history of previously known weed problems in a field greatly aids in preparing an overall weed control strategy at the beginning of the growing season. Knowing the previous field history can also provide insight on their identity when weeds emerge. A good method for developing a field history of weed problems is by mapping weeds from previous and current field scouting reports and from observations made at harvest. A detailed weed map for each field will provide information on the location of weed infestations and help monitor changes in these infestations from year to year.
Herbicides are the primary method of weed control in production agriculture. They are particularly important for combating weed problems in no-till or conservation tillage production systems. Herbicides are generally considered to be either soil active or foliar active. Soil-active herbicides are generally applied to the soil surface since they are most effective shortly after weed seed germination, whereas foliar-active herbicides control weeds after they have emerged from the soil; thus, they are applied post-emergence to the weeds.
In no-tillage systems herbicides are usually needed for vegetation control prior to crop emergence. In many cases, the green vegetation present among the previous crop residue consists of cool-season annuals and perennials, along with some emerging summer annual weeds.
In recent years there has been greater reliance on post-emergence herbicides. Certain weeds, especially warm-season perennials, may not be readily controlled by pre-emergence application, weed escapes (due to resistance or environmental conditions not conducive to weed control) must be treated with post-emergence herbicides. Post-emergence herbicides also provide the benefit of allowing the use of a more integrated weed management approach since herbicides are applied only when needed.
More specific information on herbicides and their use in corn can be found in University of Kentucky Extension bulletin Weed Control Recommendations for Kentucky Farm Crops (AGR-6). For more information contact the Pulaski County Extension office. Information Gathered from UK Publication "Weed Management" by JD Green/James Martin.