For most forage managers weeds are a fact of life. So, if they are present, how did they get here? In newly establishing stands weed types may appear where these species were never seen before. In that case we can only presume they came with the new forage seed we bought and planted. If they were present before in small amounts, how can they suddenly be so abundant? In this article I will share thoughts on how weeds are coming into stands and how can we prevent a weed problem from even starting. Also, once weeds are present how can they be effectively managed, controlled or eliminated?
In new establishing stands purchased forage seed is often blamed as the initiating source of weeds. This is possible and I will explain why. There are tolerances of various Noxious and Nuisance weeds in perennial forage designations of (Ranked in Order from Highest to Lowest class) Foundation, Registered, Certified, Blend, and Common seed classes. Less weed seeds are tolerated in the Foundation class and most at the lowest Common No. 2 class. To ensure this forage seed fields are acceptable for seed production, they may be walked by a forage seed inspector prior to harvest. If they are, or even if they are not once this field is harvested and cleaned, random samples are taken, and a specific seed lot number given to this field. The samples are inspected for any contaminants and germination tests are done. The sample designation of class is on this certificate and could be lowered based on contaminants, weed seed types and amounts, and germination. The contaminants, weeds, and germination are recorded with this seed certificate number for any buyer to look over and know what they are buying. This happens for every different seed lot or blend you may buy from a forage seed company. It is important to note that Primary noxious weeds are not allowable in forage seed of any of the designated classes except in Common No. 2 and then only at a certain level. Primary noxious weed examples are: Canada thistle, Quackgrass, Knapweeds, Toadflaxs, Leafy Spurge, Tansy Ragwort, Perennial Sow-thistle, etc.
My thoughts for this article are mainly on Secondary Noxious weeds as their spreading is increasing. Secondary Noxious weed examples are: Scentless Chamomile, Ox-Eye Daisy, White Cockle, Downey Brome, etc. The tolerances for Secondary Noxious weed seeds in perennial forage seed ranks from least in Foundation classes, to more in Common seed classes. The highest tolerance is in the Common No. 2 class.
My advice is that whenever you are buying perennial or even annual forage seed “plan ahead of time”. Approach the organization you plan to buy from asking for a Seed Certificate for that species you want to buy or that Blend components if in a mix. The company head office has those on file and can fax or email a copy of it to you.
After this discussion on perennial forage seed and weed tolerances allowed in forage designated classes, The reality is it is very, very seldom that problem weeds come in purchased perennial forage seed. All of my experience of over 30 years in forage extension, research support, and key industry networking has shown that as the case. Companies are too competitive and do not want to lose your business. But…always check the seed lot certificate before buying.
The majority or maybe even all the time I will boldly say that I have found weeds in perennial forage stands come from being here already. To get to the field beforehand weeds come through a variety of means. Imports of grain, straw, hay, wildlife, birds, water, wind, and machinery are ways most common. Some weed seeds can be survive in the soil for a short time frame of just a few years and others can survive as a viable seed in the soil “seed bank” for over 100 years. Dormant weed seeds wait for the best circumstance for their survival and take advantage of a situation that is most promising for establishment. This opportunity is highest where there is little competition and adequate nutrients. Slow growing forage seedlings, seedlings lacking nutrients to be competitive, places where seeding error occurred, cover crops outcompeting forage seedlings, bare areas between plants, drought are all opportunities for weeds to fill a void.
In established stands weeds find opportunities usually due to a lack of fertility, forage cultivar lack of longevity, over grazing, or supplemental feeding on this landscape. These are all opportunities for weed seeds to arrive or grow. Once established many weeds are prolific seed producers. It does not take much time for a weed invasion to go from unnoticed to be significant.
The importation of weeds is where most of our weed control efforts need to be. Purchased feed is a key area to focus on. Asking the seller questions about the presence of weeds and even looking at the field before purchasing feed is the first steps to prevention. Feeding in a specific area, that may be cultivated or on a grass pasture will give the most flexibility in weed control if a problem does arise. Monitoring areas where imported feed is fed should always be done. Prevention of seed set on new establishing weeds is number one in importance.
Control of new problem weeds needs to be quick and thorough. Hand rogueing, herbicides, mowing, animal consumption, and ensiling are all options that need to be done in timely manner. For example, once petals of Scentless Chamomile start to droop, up to 300 seeds/flower may be viable. Oxe-eye Daisy can produce 26, 000 seeds/plant. In general, if weeds are not allowed to set seed in 4-5 years 90-95% of weed seeds will be reduced in the soil. However, if one year is missed these efforts are undone.
Mowing to a 4-8 Inch height works well for annual weeds. Herbicide options even for legume/grass stands are available but should be used early in weed growth. Hand roguing and removal of weed plants must be done prior to weed seed drop.
Silage is an excellent tool to use in weed control. Silage fields when weeds are vegetative before they set seed. Fortunately ensilaging and digestion can reduce viability of almost all weed seeds. Of the two processes silage is more effective than an animal’s rumen for destroying viability of weed seeds. Grass type seeds are the most easily damaged by either biological process. For different species of broad-leafed plants ensiling and digestion varies from highly effective to particularly good control. One exception that survives both ensiling and digestion is Round Leaf Mallow. A caution I have is that with round or square bale silage, the lower moisture content and higher pH at stability may not create as much damage to a weed seed as a chopped silage. I have not found research that has tested weed seed survival in bale silage.
In summary your best weed control in perennial forages is a vigorous and competitive forage stand. When purchasing new seed to establish a stand start early and buy after you have looked over the seed certificate that comes with that forage seed. Purchase forage cultivars or mixes best suited to your planned use. Establish that forage stand using best management practices so it can be most competitive. Monitor new and established perennial forage stands in a timely manner and use weed control methods quickly to prevent weedy plants from setting seed. If purchasing feed know what you are buying and feed it in a restricted area for best options for future control. Do field monitoring of these areas through the next growing season and even year or two after. Applying fertility or controlling animal grazing will keep the forage stand highly vigorous and competitive and limit invading weeds from getting a foothold.