Horsenettle Control in Grass Pastures and Hayfields

At Ag Progress Days last week, the most frequently asked question at our booth was about how to control horsenettle in pastures and hayfields. With a wet spring and a dry summer, it has thrived and is causing problems in grass forages. Below are some details about this stubborn weed and some suggestions to manage it.

Biology: Why It's So Aggressive

Horsenettle is native to the United States and is an aggressive warm-season creeping perennial. It actively grows from May to early September, and its shoots sprout from roots during mid-spring and continue to emerge through summer. It can also germinate from viable seed. Horsenettle can thrive in hot weather and is drought-resistant because of its deep root system. It can tolerate poor soil conditions (including low fertility) and can grow in many different soil types. Horsenettle prefers sunny environments but can tolerate moderate shade and is commonly found in overgrazed pastures. Its foliage is sensitive to cold temperatures, so its top growth is usually killed by the first frost.

Toxicity Problems in Forage

Horsenettle affects forage quality and stocking density for a few reasons. Due to its sharp prickles, it can cause irritation and injury to soft tissue in the mouth and throat of livestock if consumed. It also has certain toxic qualities, but this can widely vary depending on environmental conditions. Green berries are usually more toxic than mature berries of horsenettle, but the whole plant can be poisonous if ingested by livestock or humans. Toxicity is usually higher in late summer/fall compared to spring. It is difficult to find good toxicity data for horsenettle specifically, but there are some general guidelines for weeds in the Nightshade family, of which horsenettle is a part. Solanine, the poisonous compound, is toxic at 42 mg/kg (LD50) or 0.1 to 0.3% of total body weight (e.g., 1 lb per 1000 lb cow). Solanine can remain toxic even in dry hay. Also, 12 to 36 hours after mowing, horsenettle releases sugars, making it more palatable to livestock; thus, if overconsumed, it can cause serious illness or sudden death. However, if desirable forage is ample and accessible, livestock tend not to consume horsenettle.

Management in Pastures and Hayfields

As with any perennial weed, multiple tactics are necessary to manage it for the long term. It is always best to incorporate good agronomic practices such as using correct forage varieties, maintaining good soil fertility and pH, using good grazing and haying management, and keeping fencerows and field edges clean. Aside from these, occasional mowing and using effective herbicides at correct application timings are important for good horsenettle control. Mowing can impact the growth of horsenettle, but if used alone, it will not fully control this weed. If mowing is repeated too often, horsenettle can adapt to the cutting height and eventually grow below the height of the mower deck. It is best to mow at least once or twice during the growing season, followed by an effective systemic herbicide application in mid to late summer (July through early September). After mowing, allow horsenettle to regrow to about a foot tall and flowering (prior to berry formation) before applying the herbicide.

Herbicide Options

The choice of herbicide will be influenced by the setting – hayfield or permanent pasture (i.e., no hay or manure is removed from the field). Herbicides such as 2,4-D, dicamba, and metsulfuron will only provide about 70% control of the weed and suppress it for a limited time, and can be used in either setting. If it is a hayfield, the best option is Crossbow at 3-4 quarts/acre, which will provide about 80% control. If it is a permanent pasture, then the best herbicide option is GrazonNext HL at 1.5 to 2.6 pints/acre, which provides about 85% control. The reason for using different herbicides in different settings is because the GrazonNext label has restrictions concerning the use and management of plant residues (hay, straw, mulch, compost) and manure that may contain aminopyralid residues. These include important restrictions concerning the movement and sale of hay products treated with aminopyralid. Therefore, be certain you understand and can follow these label restrictions before using this product. On another note, whatever herbicide is used, applications too late in the year may be impacted by poor leaf uptake due to leaves eaten by flea beetles and Colorado potato beetles. If leaves are too tattered by insect feeding or if the plant is starting to show signs of going dormant (e.g., yellow or browning leaves), then the herbicide application will likely not be effective. Otherwise, if the weed is still green and actively growing, then it can be sprayed into the fall season.

Grazing and Haying Restrictions

Whenever using herbicides, make sure to follow any grazing and haying restrictions for each of the products. Some have no or minimal waiting periods between herbicide application and crop utilization, while others have periods of up to a month or more. Each product can differ depending on whether the forage is being grazed or harvested for hay. Furthermore, the restrictions can vary upon the kind of livestock and if they are lactating.

Where to Find More Information

More details on any of these topics covered above can be found on the respective product labels or in the 2025 Penn State Agronomy Guide. In particular, refer to Table 2.6-11 (herbicide effectiveness vs. weeds table), Table 2.6-12 (comments on herbicide use), Table 2.6-14 (grazing and haying restrictions), and Table 2.6-17 (optimal time to spray herbicides on specific weeds).

In summary, if horsenettle populations have been growing for many years in a field, it will take a few years or more to get them under control with these repeated measures annually. Effective control in grass hay/pastures is primarily based on routine mowing, timely systemic herbicide applications, a thick forage stand, and not overgrazing. Over time, these tactics can drastically reduce its negative impacts.