Northern Colorado Sheep Pasture Weed Identification Guide
A sheep pasture looks different after a full season of rotation and ours definitely looks night and day different, especially with the non-existent rainfall we experienced.
As we close out our first season moving sheep through grass this summer, we have been paying closer attention to the plants that show up. Some are harmless. Some are useful indicators of soil pressure. Some are a nuisance. A few deserve quick action because they can affect fleece, forage quality, or animal health.
When we first bought the property, before we acquired any livestock, we had a Weld County Agronomist through CSU come out to walk around and help us identify plant species and give advice on the health of our pastures and how to approach the first year. It was really helpful and eye opening and we will likely invite her back out to see how we are progressing. We found that we had one pasture that was in good shape with lots of established smooth brome grass, but the smaller pasture was over-mowed and fighting a lot of weed pressure, which made it ungrazable this year.
This guide is meant to help with field-level identification, especially when paired with clear photos from your own pasture. It is not a replacement for a county extension office (they are very useful if you can get an agronomist to visit your property!), veterinarian, or local range specialist, but it can help identify what you might be seeing or point you in the right direction!
Why weeds show up...
Rotational grazing changes plant pressure across a pasture. Sheep graze close, especially on tender new growth, and their patterns can expose areas where weeds already had an opening. Bare soil, overgrazed corners, wet spots near water tanks, and disturbed fence lines often grow the problem plants.
A few weeds also become more visible after sheep take down the grass around them. That does not always mean the weeds are new. It may mean they were hidden until the grass was grazed and the sheep left them behind making them obvious.
In northern Colorado, the big drivers for weeds are usually:
Dry summer stretches
Irrigation patterns or low wet spots
Windblown seed
Disturbed soil along gates and fence lines
Heavy use near water, mineral, and shade
Pasture rest periods that favor fast-growing annuals over perennial grasses
Common pasture weeds to watch in northern Colorado
Many weeds look alike when they are young. The best way to identify them is to look at the whole plant: leaf shape, growth habit, stem texture, flowers, seed heads, and where it is growing. I second guess myself for a few days on it until I've seen it a lot. If you're a livestock owner in the area, what am I missing or haven't seen on our pasture yet?
Weed or plant group (links to more information in blue) | What to look for | Why it matters in sheep pasture | |
   | Upright, bushy growth with narrow leaves, often in dry or disturbed soil. This is a big tumbleweed when it dries | Can spread fast and take over thin areas. Not palatable to sheep usually, but can be toxic in high volumes as it can be full of nitrates | |
  | Fine, prickly branches that form a tumbleweed when mature | Reduces forage quality and spreads seed easily. Has small spines that can irritate a sheep's mouth | |
 | Low vine with arrow-shaped leaves and small white or pink flowers that open and close depending on time of day | Hard to control once established because of deep roots. Our sheep love eating this, not toxic | |
 | Dock (curly) | Wavy-edged leaves and tall reddish-brown seed stalks | Can cause oxalate poisoning in high volumes, but not very palatable |
  | Arrow-shaped leaves with upper leaves clasping the stem. White clusters of flowers | A noxious weed that can cause irritation | |
 | Shrub with narrow leaves that bursts into yellow blooms in late summer. I like the look of the few we have so these may stay as they aren't harmful | Not harmful and unlikely to be consumed | |
  | Fine grasses with bristly seed heads | Seed heads can irritate eyes, mouths, and wool | |
 | Triangle shaped leaves growing 3-6ft tall. Interestingly, this can be foraged as "wild spinach" that humans can eat! | Can be toxic if it carries heavy nitrates and consume in high volume |
We've seen all of the above on our land so far, probably a few I didn't mention, too. If I had to tell you what we see the most, it is definitely Kochia and Lamb's Quarters.
CAUTION: I haven't seen this yet myself, but be on the look out for Poison Hemlock.
Which weeds need quick attention
Some pasture weeds are mostly management problems. Others should move to the top of the list to watch out for or remove.
Burr-forming plants deserve attention if you raise wool sheep. Burdock, cocklebur, and other sticky seed heads can make a mess of fleece. They often start near ditches, edges, and neglected corners.
Spiny plants such as thistles reduce usable grazing area. Sheep may eat some young thistle growth, but mature spines usually lower pasture quality. This year we had a sheep who decided to try some thistle and she had a sore mouth for a few days, while she was learning from her hard-learned lesson. We've decided to avoid including a large thistle patch in rotations and mowing or pulling it when we see it.
Deep-rooted perennials like bindweed and thistle are easier to manage when patches are small. Once they spread underground, mowing alone rarely solves the problem.
Questionable or toxic plants should be confirmed before animals keep grazing the area. Also, get familiar with what to look for if they are poisoned. If we know we are putting them in an area with some kochia, we make sure they have access to lots of good grass forage (and water) so that the pressure isn't so high that they resort to eating only kochia.
What pasture weeds can tell you about the pasture area
Weeds are not just invaders. They are management cues.
Dock weed often points to wetter or compacted ground. Kochia and thistle often show up where soil is dry, bare, or disturbed. Bindweed likes places where it can spread without much competition. Cheatgrass can fill in thin, stressed grass stands before perennial forage has a chance to recover.
During a first season of rotation, it helps to keep simple notes for each area:
Which weeds are present
Whether they are scattered or forming patches
When they flower or set seed
How the sheep respond to them
Whether rest improves grass competition
Where bare soil keeps appearing - this one is the most obvious to us because the kochia takes over here quickly!
Over time, those notes become more useful than memory. They also help you see whether your rotation is improving ground cover or simply moving pressure from one weak area to another. Our goal for rotational grazing is to strengthen the pastures that we have for our sheep, using our sheep. So we need to pay close attention to weed pressure so we can give the grass a better environment to thrive in.
Weed walks, a new hobby
Throughout the season, walk around slowly and mark the main weed patches. Pull or remove seed heads where practical and for larger patches, decide whether mowing, grazing timing, reseeding, targeted removal, or professional advice makes the most sense. Colorado has the remedy actions categorized into four actions dependent on the weed:
Cultural Treatment: Manipulating the environment to favor desirable plants and suppress weeds, such as establishing competitive vegetation, proper grazing, mulching, prescribed burning, and optimizing soil health.
Biological Treatment: Using living organisms, or biocontrol agents (e.g., specific insects or plant pathogens), to naturally suppress weed populations by reducing their vigor or reproduction.
Mechanical Treatment: Physical removal or disruption of weeds, which includes hand-pulling, mowing, tilling, grubbing, or girdling. This is mostly what we've been doing.
Chemical Treatment: Applying herbicides to kill or inhibit weed growth, typically through methods like foliar sprays, basal bark treatments, cut-stump applications, or soil treatments.
The biggest lesson from a first rotation season is that weed identification gets easier when it becomes part of your normal every day process, not a separate chore. We now try to observe as we rotate the sheep and mitigate any weed issues before the rotation by hand pulling or fencing around the weeds. Take photos. Compare plants at different stages. Watch what the sheep avoid or love. Keep notes because the seasons change quickly and the weeds may look completely different next month.
A good pasture does not need to be perfect. It needs steady cover and useful forage. Knowing the weeds when you see them is the first step!
KB
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