Weed Control In Forage Crops
Introduction
The importance of weed control in forage production should not be overlooked, especially when you consider the high investment cost associated with their production. Weeds reduce forage yield by competing for water, sunlight, and nutrients. In addition to yield losses, weeds can also lower forage quality, increase the incidence of disease and insect problems, cause premature stand loss, and create harvesting problems. Some weeds are unpalatable to livestock or, in some cases, may be poisonous.
Weed management strategies in forages should focus first on cultural practices and then on chemical weed control practices. Vigorous, dense-growing forage stands have fewer weed problems. Thus, cultural and management practices that promote a highly competitive forage stand may prevent many weed problems. These practices include:
- fertilizing fields based on soil test recommendations
- seeding well-adapted, vigorous, long-lived varieties
- buying weed-free seed
- cutting forage at proper timing intervals or growth stages
- timely control of insect and disease problems
- rotating fields with other crops to interrupt the buildup of certain forage related weed species.
Because of the aggressive nature of some weed species, they can become established despite preventive efforts. Therefore, herbicide treatment might be necessary to combat some weed problems. The specific herbicides and control strategies available for use will depend on the type of forage you grow (alfalfa, alfalfa/grass mixtures, clovers, forage grasses, etc.), whether your stand is a new seeding or an established stand, and the crop growth stage (dormant, non-dormant, between cutting). Before using a herbicide, always read and follow product label directions.
Weed Control In New Seedings
Weed control is more critical during the first year than any other period of forage production. Forage seedlings grow slowly and are easily overcome by rapidly growing weeds. Research has shown that some broadleaf weed seedlings are capable of growing five times more rapidly than certain legume seedlings. Because alfalfa stands gradually decline with age, it is important to start with a good stand. A uniform, dense stand is more likely to survive longer and have fewer weed problems than a thin stand.
Forage Crop - Site Selection
Consider field histories when you select a field for forage production. It might be difficult to establish and maintain a weed-free stand in fields known to be infested with weeds such as thistle, dandelion, and quackgrass. In addition, some herbicides that are applied in previously grown crops have the potential to carry over and cause injury to newly seeded forages. More information on herbicides that have a potential to injure alfalfa and other forages can be obtained from MAFRD offices and directly from the Product label.
Fertilization
Adjusting soil nutrient levels according to soil test recommendations is important during the establishment phase and throughout the life of the forage stand. The objective is to achieve a competitive stand that is capable of suppressing weed emergence and growth. Proper fertility is not effective at eliminating established weeds, especially in areas where the forage stand is poor.
Cutting - New Stands
Clipping or mowing can be an effective option for controlling weeds, such as common cocklebur, in forage stands. This method controls weeds by removing the leaves and lateral buds that develop new growth. Annual broadleaf weeds have buds that develop above the soil surface; they are more easily controlled with clipping or mowing than grasses, which have crown buds near the soil surface. Mow as low as possible to be effective. Because alfalfa and other legumes have crown buds, they can tolerate low clipping. When you clip new seedings, be careful not to smother the forage with heavy residues. Remove clipped vegetation when weed infestations are heavy.
Herbicides For New Seedings
Herbicides used for new seedings are designed to eliminate or reduce competition from rapidly growing weeds during the establishment phase. In some instances, herbicides that aid establishment have also contributed to higher yields in subsequent years and greater longevity of stands. During seedling development, forage grasses usually are susceptible to injury from herbicides used in legume establishment. Subsequently, no herbicides are registered for new seedings of legume grass mixtures. Refer to the Field Guide to Crop Protection (Forage Crops Chart – Recommended Herbicides) for further information.
Maintaining Established Stands
Established forages are capable of growing rapidly and competing against many weed seedlings during the growing season. However, weeds gradually invade fields where forage stands decline with age. Timely mowing and the use of herbicides may aid in weed control and prolong the life of the stand. If you have a weed problem that occurs in field borders, along fence rows, or in adjacent fields, you should mow or spray to prevent production and spread of weed seed from these areas into forage fields. This is particularly important for such weeds as thistle and dandelion, which are capable of producing a large number of seeds that easily spread to new areas.
Clipping Established Stands
The routine mowing of forages for hay is sometimes effective in controlling some perennial weeds by reducing food reserves and plant vigor. However, in grazed forages, livestock often selectively graze and may leave particular weed species. Mowing soon after livestock have been removed from the field can help control these weeds and prevent seed production and further spread of infestations.
Herbicides For Established Stands
Several herbicide options are available for established forage stands, refer to the Field Guide to Crop Protection (Forage Crops Chart – Recommended Herbicides) for further information. You can use many of the same herbicides available for new seedings. Furthermore, the deep root system of established plants such as alfalfa enables them to tolerate certain herbicides that are not suitable for new seedings. When selecting herbicides for forages, you should consider such factors as: whether the herbicide can be applied as a dormant season, non-dormant, or between cutting treatment; effectiveness on weed species to be controlled; feeding and grazing limitations; rotational crop restrictions; and cost of treatment.
Scouting Methods For Forage Crops
Scouting for weed problems early is an effective tool for identifying and controlling weed problems before they develop into situations that cannot be easily managed. This requires a trained eye and the ability to identify weeds in their early growth stages. Winter annual weeds, such as stinkweed and flixweed, usually germinate in late fall and are present in early spring, whereas, the summer weed complex, which includes weeds such as wild oat and lamb’s quarters, will be present after the first harvest through a killing frost in the fall.
Weed infestation levels or weed density should be determined by estimating the percentage of ground cover occupied by weeds. This can be accomplished by randomly selecting one site for every 10 acres within a field. A minimum of three sites should be selected in fields with fewer than 20 acres. At each field site, an area approximately 30 feet by 30 feet should be used to determine the percentage of weeds present. Keep in mind that fields that appear almost weed free could have a 5% weed density. Only in extremely poor forage stands will weed infestations in excess of 50% occur. At each site, record the predominant species and its size at the time of sampling.
The above information was in part based on an article produced by J.D. Green, M.W. Marshall, and J.R. Martin of the University of Kentucky.
