Reduce Weeds by Friending Grasses

Author: Susan Kerr, WSU Regional Livestock and Dairy Extension Educator Reviewed by Dr. Tim Miller, WSU Weed Scientist

Publish Date: Summer 2017

If Extension educators had a penny for every time someone asked for help with weed control, we would never have any budget worries!

Are weeds a never-ending challenge for you? If so, consider looking at them from a new angle. Instead of spraying or digging weeds out year after year, concentrate on making your grass happy. The factors that promote optimal grass health and production work against weed growth. What is good for grass is not good for weeds.

Why Care about Weeds?

Weeds are any plant out of place. They take resources away from desired plants and can become aggressive invaders of home and agricultural spaces; many are toxic to livestock, as well. This article will give you several arrows for your quiver of weed control tools.

Start with a soil test on your pastures. Information about soil testing, analytical laboratories, and amending soil is at West side soils in WA and OR tend toward acidity. Grasses do not thrive in acid soil, but many weeds do. Applying lime to fields takes several months to raise soil pH, so it is often done in the fall to benefit next season’s pasture growth. Liming to raise pH will make grass happy.

Match grass species and soil type. Some grasses will do better in soils with better drainage and others are more tolerant of heavy clay soils. Consult with NRCS and Conservation District personnel for recommendations about which grasses are the best fit for your soils.

Encourage biodiversity. Having multiple grass species and some legumes will hedge bets against all kinds of weather, fertility, disease, and insect problems. Some species are better at withstanding wet soils (birdsfoot trefoil, clovers, timothy, fescue) and others are more drought tolerant (alfalfa, sainfoin, fescue, ryegrasses, wheatgrasses, teff). Some plants will emerge earlier in the grazing season, others will appear later, and livestock should have something to eat through the grazing season. Also, legumes will add nitrogen to the soil, which will benefit grasses.

Fertilize pastures as recommended by a soil test. Fertilizer (especially nitrogen) makes grass happy. Many weeds have deep taproots and are able to scavenge soil nutrients better than grasses can. Phosphorus is a limiting soil nutrient in some areas as well and grasses respond well to supplementation when needed. Micronutrients such as boron or sulfur could be deficient locally; plants could be expected to respond dramatically when soil nutritional deficits are corrected. Commercial testing laboratory personnel and Extension educators can give recommendations on fertilization rates based on soil test results.

Irrigate wisely. If you are legally able to irrigate your fields, timely applications of water will benefit grass. Grasses tend to have more shallow roots than weeds and are therefore more quickly stressed by dry soils. One of the aims of good grass management is to encourage a deep and healthy root system. Proper grazing duration and timing will aid this, as will less frequent but prolonged irrigation cycles. Frequent application of small amounts of water will promote shallow grass roots.

Do not turn out too soon in the spring. Wait until pastures are at least 6 to 8” tall before turnout, no matter how sick you are of winter feeding chores. Even if grass is tall enough, don’t turn animals out if you can pull grass out of the ground (the “pull test”)—if you can, will they. Don’t turn livestock out onto muddy or spongy ground, either, or you will get pugged and compacted soils just right for weed incursions (Photo 1).

Use best pasture management practices. This means never turning livestock out onto pastures that are less than 6 to 8” tall and never grazing lower than 3 to 4.” “But I don’t have enough land for that!” you say. If that’s the case, you have too many animals for your land. Your options are to sell some animals, buy/lease more land, or confine some/all in a sacrifice area and feed hay until there is sufficient regrowth for grazing. You might need to do that before pastures are ready for spring turn-out, and again during the summer pasture growth slump.

Many recommendations say not to re-graze a pasture or grazing paddock sooner than every 21 days, but even that may not be enough—use the 6 to 8” minimum height rule. Grazing too low or too frequently makes grass sad and promotes weed establishment.

Dividing pastures into grazing paddocks can be done inexpensively using portable electric fencing. Design cells so animals will be in them no more than 4 to 5 days for two reasons: this will help prevent animals from ingesting infective internal parasite larvae and it will prevent a second bite of a plant within 5 days (this is very stressful to a plant recovering from grazing or mowing). Constant re-grazing (Photo 2) stresses grasses and doesn’t let them make and store energy; dead grass, bare soil, and weeds follow.

Use best grazing and mowing practices. Grasses respond to being cut; most weeds do not. Planned grazing or mowing will keep grasses in their vegetative state and they will send out more tillers, resulting in more leaf area, lusher growth, and more vigorous and nutritious plants (Photo 3). Unmowed grasses head out and go dormant, decreasing their digestibility and offering little competition for weeds (Photo 4). Weeds generally do not respond well to mowing; mowing before seed setting is an effective non-chemical way to control and even eliminate annual weeds. Mowing after grazing will make pastures more uniform and prevent un-grazed weeds from setting seed. Desirable annual pasture grasses should be allowed to go to seed at the end of the grazing season so the next crop can establish itself.

Limit fall grazing. The pasture season begins in the fall, when grass plants establish next year’s growing points. It is critical to not graze below 3 to 4” during this time. Resting plants from mid-September to mid-October, if possible, will allow plants to set growing points and store plenty of sugar in their lower stem to get off to a good start next year.

Minimize soil compaction. Due to their root system, grasses have a hard time thriving in compacted soils; weeds do not. Compaction happens due to the weight and/or timing of equipment or livestock impact on soil. Every pass of “heavy metal” (field equipment) has the potential to compact soil, which is one of the reasons for interest in no-till (a.k.a. high residue) farming practices. If livestock are turned out too early in the spring or fields are worked too soon, wet soils can readily compact. Compacted soils can be aerated and management practices changed to minimize recompaction.

Increase soil organic matter. Organic matter in soil increases the soil’s water holding capacity and helps keep soils aerated vs. compacted. These factors benefit soil macro- and micro-organisms, thus contributing to nutrient cycling. A carbon sequestration study on California grasslands ( documented that the application of compost to grasslands boosted soil fertility and water holding capacity for three years post-application. More soil fertility + more water retention + more aeration = happy grass.

Monitor animal behavior and condition. If animals are constantly eating and never resting, chewing cud, sleeping, playing, etc., that means they are always hungry and not getting enough to eat. They will graze too low and re-graze plants too often, initiating the death-spiral for grasses and opening the door for weeds. Such animals will have low body condition scores and/or not be growing or lactating up to their potential; they need additional food such as hay, grain, or more productive pasture. On the other hand, if animals’ body condition scores are too high, they are eating more than they need. This is mostly a problem with horses; horses on good pastures may not need 24/7 access to grass and could be pulled off to a sacrifice area (daytime in summer, nighttime in winter for animal comfort) to reduce grass trampling, soil compaction, and plant stress.

Prevent bare soil. Overseed weedfriendly bare areas with desirable plants if the conditions causing bare areas have been corrected. Use winter cover crops if transitioning in or out of pastures to control weed establishment.

Use herbicides wisely. Identify weeds to control and seek advice on legal and effective chemical control agents. When initially trying to control weeds on severe pasture infestations, chemical control will speed the process considerably; grass promotion and grazing management may allow for just spot-treatment in the future.


Overgrazing grasses stresses and kills desirable plant species, creating bare areas easily invaded by undesirable species. Using the above practices to support and encourage grass production will help you manage away from weeds and reduce the need for chemical control considerably.

For additional reading

• PNW Weed Control Handbook

• Pastures for Profit: A Guide to Rotational Grazing

• Pasture and Grazing Links

• PNW Pasture and Grazing Management

• Irrigation