Weed Management in Pastures

Jerry Doll

Weed management means using a wide range of tactics and practices to keep weeds in check. In this lesson, we’ll review what these are and how you can apply them to your pastures. Let’s start with preventing new weeds from appearing in your pastures. This is easy to understand but harder to accomplish. Here are examples of practices that prevent introducing new weeds into your pastures.

1. Buy & Plant Clean Seed
2. Buy Clean Feed & Bedding
3. Clean Machinery when Leaving Weedy Fields
4. Check Custom Equipment Entering Your Fields
5. Prevent Seed Production in Fence Rows, Field Borders & Roadsides
6. Keep Informed of New Weeds in Your Area

Sometimes it may be possible to eradicate a weed from a given pasture, fence row or barn area. This requires diligence and persistence and is seldom the goal of land operators. Not that it would not be good to eradicate weeds, but the reality of most situations is that "the complete elimination of all live plants, plant parts and seeds from an area" is rarely feasible. Thus we are left with trying to contain and suppress the weeds we have.

Weed management can mean different things in specific situations. One definition is that it is doing what need to be done when it needs doing and doing it well. Many consider management to be limiting or reducing the weed infestation to tolerable levels (thresholds). The number of weeds per unit area is seldom that practical in pastures as weeds are often not randomly arranged but then to be thicker in some areas than others. Also pasture weeds cause more damage than just the loss of feed for the horses; some have spines and some are poisonous.

Nearly all agree that the best approach to weed management is to use a skillful combination of prevention, eradication, and control strategies and practices to manage weeds successfully. This view implies a shift away from reliance on control of existing weed problems and places greater emphasis on preventing propagule (seeds, spreading roots, etc.) production, reducing weed establishment in a pasture and minimizing weed competition with the crop. Here is a view of what those management practices to minimize the importance of weeds in pastures are.

General Pasture Management and Weeds

The above diagram lists several practices that may not seem to be related to weed management. The reason a well-managed pasture has relatively few weeds is that a competitive mix of well-adapted forage species that are growing vigorously is able to "defend" itself from weed encroachment. In fact, weeds in pastures are often a symptom of other problems (inadequate soil fertility, not the right pasture species, overgrazing, etc.). So the first step in developing a weed management plan it to make a weed a careful assessment of why weeds are present. This will avoid implementing weeding practices that have minimal or short duration effects. If weeds persist in spite of a sound pasture management program, then specific weeding practices should be launched as described below.

Matching Management Practices to Your Species

The primary reason to do a pasture weed inventory is to know what species are present. Based on this, you can determine how important they may be. Some "weeds" are of little concern in terms of forage quality. These include quackgrass, white cockle and dandelions. That is why they were not included in the pasture weed ID checklist. Others present physical injury and loss of useful grazing areas because of spines. This is true of bull, musk, plumeless, Canada thistle and horse nettle. Some weeds are very unappealing to animals because of odor (many mustards) or high fiber content (spotted knapweed and giant ragweed). Some weeds present health risks to animals and humans (stinging nettle, horsetail, wild parsnip and hoary alyssum).

The weed ID checklist is organized by life cycle because the best management practices for a given weed complex will be based on this characteristic. Let’s see what management practices are best suited to each weed life cycle.

Annual Weeds.

Serious annual weed problems are rare in pastures and are indicative of declining health of desirable species. These paddocks may be candidates for improvement or renovation. Annual weeds are common in areas where soil disturbance occurs. Examples include corals, feed lots and watering sources. The repeated hoof action is concentrated areas is detrimental to perennial forages and actually encourages seed propagated species to germinate and develop as the soil is often disturbed and there are few other plants to compete with the weeds. There is little need to worry about annual weeds in these areas unless they are spreading into adjacent sites or are poisonous or spiny.

Most annual weeds are often successfully managed with a timely mowing program. Try to mow as the first signs of flowering are evident. Mowing annual weeds once will usually control them if the pasture is healthy and exhibits rapid regrowth. Some annual weeds will regrow after mowing so check the pasture to see if and when additional mowings are needed.

Biennial Weeds.

Plants in these life cycles only reproduce by seed so if we prevent seed production for many years, we can approach eradication. Biennials form a rosette of leaves the year seeds germinate and must undergo winter to shift from vegetative to reproductive growth. Plants flower the following summer and die at the end of summer or with the first frost. Preventing seed production is the key to managing biennial thistles.

Mowing biennials when they are in the bud to early flower growth stage is certainly helpful and some species, such as bull thistle, seldom try and flower after mowing. Others, such as wild parsnip, plumeless and musk thistle, and wild carrot, will certainly regrow and flower again. Walk your pastures often and mow as often as appropriate and feasible.

Another way to kill biennials without using herbicides is to cut them off below the soil surface. This is feasible when infestations are localized and not too dense. A heavy, sharp hoe or spade can be used to accomplish this. Cut plants at least one inch below the soil surface to prevent them from regrowing from buds in the crown region (junction of the root and above ground tissues).

Several herbicides effectively and economically control biennial weeds. The key to success is application timing. Treat when plants are in the rosette stage. This is the entire growing season in the year they germinate and up to the time the begin forming the flower stalk (bolting) the next season. Most biennials will begin bolting in mid to late May so you will need to spray the plants in their second year of growth in early to mid May for best results. The herbicide effectiveness drops rapidly when plants start to flower. At this point, mowing the a much better option.

Late summer and early fall is an ideal time to spray to kill biennial weeds. At this time, all plants of concern in rosette stage so there is no concern about being too late. At the end of the season biennials are actively translocating sugars and starches to crown/roots in anticipation of winter; the herbicides we apply flow with these carbohydrates and reach their intended target (the roots and crown buds) with great efficiency.

Herbicides that kill broadleaf weeds are generally very safe to forage grasses. Unfortunately there are no herbicides that kill broadleaf weeds like bull and plumeless thistle without also killing forage legumes like clover and trefoil. In areas that have legumes and the biennial weed infestation is not too thick, a backpack sprayer could be used to treat individual plants. Only the crown area would need to be treated.

Another way to prevent injuring legumes in weedy pastures is to use a selective applicator like a rope wick or roller applicator. These tools wipe a concentrated herbicide solution (20 to 33%) onto weeds taller than the grasses and legumes; the herbicide usually used is glyphosate. We have had satisfactory results using a rope wick applicator to prevent seed production in both bull and plumeless thistles in pastures. Some farm cooperatives have selective equipment available to rent and there are handheld versions available at many lawn and garden centers.

Perennial Weeds.

Perennial weeds like Canada thistle, milkweed and goldenrod regrow via a spreading root system. Others like hoary alyssum and chicory have a branched or simple taproot. In both cases, killing the root is essential to prevent regrowth. Perennials also produce seed and occasionally infestations start from seed so mowing to prevent seed production is encouraged. However, mowing will rarely eliminate a perennial weed infestation. In sites that will be renovated and reseeded, repeated soil tillage with a field cultivator or disk for and entire growing season can approach eradication of spreading-root perennials like Canada thistle. Such frequent soil disturbance prevents plants from replenishing the carbohydrates in the roots and nearly all plants die during the winter. Taprooted perennials can be killed more easily by tillage but they have a higher likelihood of reinfesting from seeds in the soil so check carefully for young plants after tillage.

Perennial species require a different management approach than annuals and biennials. Mowing once will not control these weeds. Mowing on an interval that allows these plants to regrow to 8 to 12 inches between mowing will eventually kill these plants. Keep in mind that mowing this often will likely weaken desirable species as well. Herbicides may suppress or perhaps eliminate a perennial weed infestation. Timing of the application should coincide with the appearance of the first flowers for most species. This is often in June or early July. We also recommend that pastures with perennial weeds be mowed when the first flowers appear and then treat the infested areas with an appropriate herbicide when the weeds regrow 18 to 24 inches tall (usually 25 to 40 days after mowing). Herbicides will be effective well into September and early October in many years. In all cases, the best results will occur when the weeds are actively growing when treated. Try to avoid spraying when plants are suffering from unusually hot, cold, wet or dry conditions.

Herbicide Use.

We will not give specific recommendations on herbicides here. Consult the UW Extension Bulletin A3646 (Pest Management in Wisconsin Field Crops 2001) or visit with your county agricultural extension agent or other ag profession for advice on selecting the proper product, rate, time and method of application. Follow all label guidelines and precautions regarding proper mixing and spraying, preventing drift to adjacent areas, and grazing restrictions following application.

In summary, here are the keys to success to minimize and manage weeds in your pastures.

• Soil test and fertilize accordingly
• Use appropriate seeding rates
• Select adapted species combinations
• Maintain proper soil pH
• Control perennial weeds prior to seeding
• Practice rotational grazing
• Walk pastures to catch emerging weed problems
• Check fence rows and the edges of woods for weeds that may be invading into your pasture
• Control developing weed problems by hand cutting , mowing or herbicide application
• Consider renovating pastures that are beginning to develop serious weed problems