- Author: Dan Macon
I suppose it's a reflection of the time of year, but in my first 6+ weeks as the livestock and natural resources farm advisor in Placer, Nevada, Sutter and Yuba Counties, I've fielded multiple calls - and made multiple ranch visits - focusing on smutgrass in irrigated pastures. Based on all of this interest from local ranchers (and based on the smutgrass I'm seeing in my own irrigated pastures), I've turned to an outstanding publication by a number of my UC Cooperative Extension and UC Davis colleagues - Managing Smutgrass in Irrigated Pastures (Davy et al. 2012).
A native to tropical regions, smutgrass is a warm-season perennial. On our pastures near Auburn, I typically don't begin seeing smutgrass until late June or early July - after we've made one or two passes over our pastures with the sheep. Mature plants have a tall, spiky seed-head and low, sparse leaves. Livestock generally avoid grazing smutgrass - perhaps due to its coarse leaves or some kind of secondary compound that reduces palatability. Left ungrazed (which usually happens, since livestock don't like smutgrass), the nutritional value of the plant (especially protein) declines significantly during the late summer.
In our operation, I've noticed that our mature ewes will graze smutgrass (mostly the leaves). As the paper suggests, we may get better utilization with our ewes because we manage the grazing intensively to reduce their selectivity - in other words, short grazing periods with greater livestock density seems to help us overcome some of the lack of palatability. We also pay attention to our rest periods, adjusting them to allow for sufficient regrowth of our desirable plants before grazing again. In May, our rest periods on our irrigated pastures might be 25 days; in August, we try to let pastures rest 35-40 days between grazing bouts. I think this allows the more desirable forage species to develop stronger root systems and greater canopy cover, which helps reduce the ability of smutgrass to take hold. Shorter rest periods, on the other hand, may favor smutgrass establishment.
As with many weed problems, smutgrass can be a symptom of other management issues. Smutgrass needs bare ground and sunlight to germinate. Grazing management or other disturbances (like pasture harrowing) that result in bare ground, therefore, can allow smutgrass to become established. Josh Davy (the lead author of the paper) and Betsy Karle (an area dairy farm advisor with UC Cooperative Extension) found that proper irrigation can have a favorable impact on reducing smutgrass - in their experiment, pastures that were irrigated on a 14-day rotation had more smutgrass than pastures that were irrigated on a 7-day rotation. Our pastures are set up on a 10-day rotation, and I definitely see more smutgrass on the sections of our hillside pastures that don't get as much water.
Some producers use a rotary wiper to apply herbicide directly to smutgrass. The Davy paper indicates that herbicide treatment (with glyphosate) is most effective after smutgrass has flowered, when the plants are storing sugars back to the roots (in late summer or early fall). Desirable plants should be grazed as short as practical so that only smutgrass is exposed to the wiper. Several local producers have used a rotary wiper or spot-treatment techniques with some success. As with all herbicide applications, timing and equipment calibration are critical. The paper provides detailed information about herbicide concentration and timing.
For more detailed information, be sure to check out Managing Smutgrass on Irrigated Pastures!