Fall is a great time for managing troublesome perennial and biennial weeds found in pastures.

 Perennial weeds like Canada thistle or horsenettle will prepare for winter by moving their energy reserves from their aboveground growth to their root system below ground. Biennial weeds will survive for all or part of two years, like musk thistle, wild carrot, and wild parsnip. These plants will be in rosette stage (their first-year growth) as they are also moving resources from above ground to their root systems below ground. Fall is typically the best time of year to treat many perennial and biennial weeds, as long as the plant foliage is in good condition.

 To help ensure the success with fall weed control of perennial and biennial weeds in pastures, follow the steps below:

1.    Scout your pasture to identify target weeds, their location, and the physical condition of their foliage.

a. Some pastures may require a broadcast herbicide application while other pastures may only warrant spot spraying.

b. For the herbicide to be effective, it is important to have healthy foliage that can absorb the herbicide. Some species, like horsenettle, may be in poor condition by fall and are better targeted during early flower.

2.    Target making herbicide applications on days with sunny conditions and when daytime temperatures are above 50-55 F.

3.    Herbicide treatments, especially those containing growth regulators, will kill legumes that have been interseeded with grasses.

4.    Always read herbicide labels carefully to determine weeds controlled, appropriate rates, and any restrictions for areas treated with the herbicide.

a. Residues of certain products (picloram, clopyralid, aminopyralid) persist in the foliage of plants in treated areas and can be moved with forage/hay bales or in waste (e.g. manure) of animals consuming the forage. These herbicide residues can damage sensitive plants (such as soybean) if moved from the application area with forage or livestock.

What happens if there is a frost before you can spray your pasture this fall? This is always a big question we get, and the answer is herbicide applications can still be quite effective after one or even several frost events. However, it is important to pay attention to the principles above to create the best conditions for effectiveness, especially making sure weed foliage is in good condition and applications occur on warm, sunny days. Read more in this article “Effect of frost on pasture weed control.”

Finally, from a long-term perspective it is important to determine why the weeds are present in the pasture. The last couple years of drought have resulted in pastures being over-grazed, which creates an opportunity for weeds to become established. Focusing on improving and managing the forage stand so it is healthy and vigorous is a big part of managing weed pressure in pastures. This involves evaluating soil fertility and stocking rates and determining the need for overseeding or interseeding to thicken up a forage stand. This article discusses some considerations for pasture improvements. Herbicides can be valuable tools for improving pasture quality; however, they are not a substitute for good management practices.

Though we recognize the importance, soil fertility and health often get put on the backburner as producers juggle day to day tasks caring for their land and livestock. However, to improve pasture health, productivity, and longevity, an effective first step is to get a baseline understanding of soil fertility. Soil sampling cropland and hay acres is commonplace, but pasture poses extra challenges due to topography and grazing utilization patterns. Here are five tips to help you collect representative soil samples of pasture to improve forage productivity through the soil.

Plan ahead. Identify a certified soil testing laboratory that completes the desired analyses. Utilize the expertise of the lab and/or your area extension specialist to identify the appropriate analysis package to best meet your goals and track soil parameters over time. For example, if the primary goal is to use soil tests to make fertilization decisions, soil pH, organic matter (OM), phosphorus (P) and potassium (K) are critical measurements. If you have some carbon or microbial activity related goals, other soil test packages may be more appropriate. Establish the depth soil cores will be taken at and make sure the lab is aware of core depth at submission. Selected core depths, whether 6 inch or 2-3 inch should be selected based on measured parameters and what guidelines are being used for subsequent fertilization decisions.

Make a game plan. Consider how the soil test results will be utilized to make management decisions. Rather than collecting a whole pasture sample, consider taking samples based on similar management zones or slopes, paddocks, or grid sampling. For areas inaccessible to fertilize, don’t sample. Other areas to avoid sampling include within 150-feet of limestone roads where road dust may influence the soil and areas of concentration like feeding areas, livestock waterers, or loafing areas where significantly higher levels of manure and urine are deposited.

Collect a representative sample. Start with clean sampling materials. Walk in a random, zig-zag pattern in the sampling area and take 10-15 cores. Discard and re-sample a core if it looks or feels significantly different from other cores in the composite sample. Remove the grass tuft on top and compile the cores in a plastic bucket. Thoroughly mix, and then use the mixed cores to fill the soil sample submission bag to the appropriate level. Each soil sample bag submitted should represent 10 acres or less. To speed up the process, draw a line at desired core depth on the soil probe so a ruler will not be needed to maintain consistency.

Prioritize limiting nutrients. It is often not feasible to address all nutrient shortfalls in one year, so assess limiting nutrients and prioritize which nutrient applications will provide the most return. The first priority to address is soil pH. Grass-based pastures will tolerate a wider pH range (6.0 - 7.0) while legume-mixed pastures require a minimum pH of 6.5. Legumes have the greatest response to improved pH, so lime application is necessary to increase or introduce legumes into pasture. Correcting soil pH in pasture also favors natural introduction of new grass and legume species. While N is often the first nutrient applied because of the obvious yield response, inadequate levels of P and K can be limiting factors in yield boosts. Correcting P and K from low or very low levels to optimum level can improve pasture yield by 65% and 80%, respectively, but application above optimum level typically doesn’t warrant an additional forage yield.

Be consistent. Ideally, sample each pasture every 3-5 years to monitor changes in soil fertility and health. Collect at a consistent time of year to reduce the variations driven by environment. For example, the same area sampled in the fall will look different in the spring. Avoid sampling in wet or very dry conditions. Time soil sampling to occur 6-months before desired fertilization to get the best return on investment from the fertilizer input. There are apps available to help track and summarize soil changes over time, but a pen and paper or spreadsheet will do the trick as well.

Do you have a new seeding this year? Usually, we talk about evaluating forage stands, especially alfalfa, in the spring for winterkill, but we can also take some time here this fall to evaluate how well those stands established and to determine if any action may be warranted before the 2025 growing season.

Stand evaluation: Scout your new alfalfa seeding to first get a visual of what is present. Are you seeing a lot of weeds? Are the species you planted present? It is also important to do some stand counts.

 As a rule of thumb, in the establishment year of a new seeding there should be 30-35 or more healthy and robust alfalfa plants per square foot. Next spring, in the first full production year, the target is to have 15-25 plants per square foot.

Take stand counts at different parts throughout the field to get a good picture of what the whole field looks like.



What do you do if you have a less than desirable stand? If your stand evaluations are less than desired, there are some options on how to thicken the stand up. Generally, we do not recommend seeding alfalfa into an existing alfalfa stand due to the autotoxicity. If the alfalfa stand is less than 12 months old, re-seeding alfalfa into the existing stand, especially if there are areas that are extremely thin, may be an option. However, a better option would be to consider interseeding a forage grass like orchardgrass or a legume like red clover to thicken the stand up. Since it is recommended to have at least 6 weeks of growth before a killing frost, we are getting late to do any late summer or “fall” seedings anymore this year. Instead consider frost seeding (works best with red clover) or doing interseeding next spring. Read more on those methods here.

Virtual Fence Pasture Walk

A pasture walk will be hosted by producers actively utilizing virtual fence technology in their operations. Few have firsthand experience utilizing this new resource for graziers, and through peer learning, other producers can decide if and how this technology may fit into their operation. In addition to virtual fence, fall weed control in pastures will be discussed by the ISU forage specialist. The pasture walk will be held September 10^th, from 5 – 7 PM, at 11879 Birch Ave, Breda, IA.

The pasture walk is free to attend, thanks to the Iowa Forage and Grassland Council, and a light meal is included. To RSVP, call the Carroll County extension office at 712-792-2364.

Interseeding Sudangrass into Thin Pastures

Kirkwood Community College will host a field day on September 12th, featuring pasture plot interseeded with Sudangrass for a project funded by the State Soil Conservation & Water Quality Committee. This is year two of a three-year study. More information can be found at the following link: Interseeding Sudangrass | Iowa Beef Center.

Montgomery County Pasture Walk

Join us for a pasture walk on Thursday, September 12th at 5:30 PM hosted by ISU Extension and Outreach, NRCS, and Keith and Karen Klocke. There is no cost to attend, and a meal will be provided by the Mills-Montgomery County Cattlemen. Pre-register by calling the Montgomery County Extension office at 712-623-2592 or signing up online: https://iastate.qualtrics.com/jfe/form/SV_b2yj0QBtpO38PWu

Fencing and Grazing Clinic

The Iowa Beef Center will host a fencing and grazing clinic on September 20th at the Western Research and Demonstration Farm from 9 AM to 4 PM. The farm is located at 36515 Hwy E34, Castana, IA. There is no cost to attend, and lunch will be provided. Preregistration is required by September 16th. Contact 515-294-2333 or beefcenter@iastate.edu and leave your name, email, and phone number to register. Program details can be found in this IBC press release

Corn Residue Field Day

Southwest Community College in Creston will host a workshop on September 26th from 10 AM to 3 PM addressing updated recommendations for grazing corn residue. The field day will feature results from a project funded by the Iowa Beef Checkoff comparing the feed value of corn residue harvested with or without a chopping corn head, as well as other recent data to consider when determining feed value and stocking rates of today’s corn residue. If you have questions or would like to register, additional information can be found in this IBC press release. 

Education Funds Available

Planning a forage-related meeting and looking for sponsorship dollars to offset meeting expenses? The Iowa Forage & Grassland Council provides an Education Grants Program which is designed to assist with funding forage education activities in Iowa including forage field days, informational meetings and pasture walks. As of January 1, 2024, the limit has been raised to $250! Learn more about the grant on the IFGC website.

ISU Grazing CRP Project

Researchers at Iowa State University, in coordination with the Farm Services Agency, are looking to understand how cattle grazing may impact environmental outcomes on land enrolled in the Conservation Reserve Program. Project leaders are looking to collaborate with landowners and cattle farmers throughout the state to conduct this study. Visit the project website to learn more about the basics of the study and how to get involved!