Pasture and Forage Minute: Managing Prussic Acid, Yucca and Fall Irrigation
Prussic Acid and Frost
With a warmer than usual fall so far, we’ve been able to graze later in the year with few concerns. However, cold temperatures are not far off and with our first freezes of the year comes the risk of prussic acid in sorghum species.
When temperatures drop, freeze-damaged members of the sorghum family, including sudangrass, sorghum-sudan hybrids, forage sorghum and grain sorghum (milo) release a toxic cyanide compound we know as prussic acid. Plants like pearl and foxtail millet that are not of the sorghum family don’t carry this risk. If digested, prussic acid interferes with the blood’s ability to transport oxygen, often resulting in death.
While deadly, prussic acid doesn’t stick around for long. After five to seven days, the toxin has dissipated enough that forages are once again safe to graze. However, this doesn’t mean the danger is over. Every time a new part of the plant receives damage from a frost, our timer must again be reset. This continues until the entire plant has been killed by frost.
So how do we stay safe? Pull animals from sorghum before a frost and keep them off for the five to seven days. Prussic acid also accumulates in new growth, so keep an eye out for new shoots and consider pulling animals until they reach 15-18 inches in height, or the plant dies.
Since prussic acid dissipates from dead tissues, haying sorghums won’t be a concern as long as moisture content is low enough. If harvesting for silage is an option, the proper ensiling will reduce prussic acid to safe levels.
Prussic acid is a real risk, but one easily dealt with by proper management. Don’t graze sorghums five to seven days after each frost until the entire plant is killed and keep an eye out for equally dangerous new shoots. If this prevents grazing from being an option, consider haying or silage as safe solutions.
Reducing Yucca in Rangeland
Yucca plants, which are also called soapweed, can be quite common on rangeland in western and central Nebraska. In some areas, they can be quite thick and significantly reduce grass production. There are ways, though, to reclaim those grazing lands.
Once established, yucca plants can increase on drier rangeland sites. They produce a deep taproot that competes aggressively for the limited water in these soils. With sharp leaves protecting the plant, cattle rarely eat it during summer.
Herbicides like Remedy, Tordon, Velpar or Cimarron Plus can control yucca, but only when each individual plant is sprayed directly. General broadcast spraying to control yucca on rangeland is cost prohibitive, although small patches can and should be controlled before they expand. Herbicides are most effective when applied in spring or summer.
When yucca covers too much land to spray, the only cost-effective way to reduce its impact is to winter graze. During winter, yucca often is the only green plant around. Sometimes cows actually will get down on their knees, lay their head sideways on the ground and chew through the base of the plant to get to the moist, tender parts. It has been observed, though, that it can take some time for animals to learn to graze yucca and there may be some animals in the herd that will not graze it, while others can be quite proficient. After several consecutive winters of grazing, yucca stands can be reduced so grass again thrives during summer.
Now that summer range is going dormant for the winter, grazing will do little harm to your grasses. This might be a good year to reclaim some of your rangeland back from yucca. Winter grazing is your best tool.
Fall Irrigation on Alfalfa
There have been many stressors on alfalfa fields this year, from early weevils to drought and hail and now fall armyworms and cutworms. Can fall irrigation help with stand stress?
For irrigated alfalfa, targeted fall irrigation may help the long-term strength of the stand. Although alfalfa is drought-tolerant with its deep roots, it uses a lot of water. With warmer than normal projections for the fall, alfalfa will continue to grow and use water, depleting the soil profile. Additionally, some moisture on the surface is necessary to prevent the roots from drying out and dying over the winter. With a full soil moisture profile headed into winter, soil temperature is better regulated, helping keep plants alive during the winter and initiating growth better in the spring.
Late season irrigation occurs during a time when evaporation is very low. This means little of what we supply will be lost, with most going to late season growth or filling the soil profile for strong spring growth. In many alfalfa fields, the water supplied during the growing season may never reach beyond four feet, when the roots go down eight feet. By padding the profile now, extra water will help plants better deal with stress during high heat and high water demands next summer. Alfalfa fields that have soils with low infiltration rates may not be able to absorb enough water during peak use periods of the year, even when supplemented with irrigation. Having a full profile going into next year for these fields to start out ahead instead of playing catch-up is one more reason for fall irrigation if available.
Watering dry alfalfa fields in the fall will help recharge water in the soil profile, combat winter stress and start plants growing strong during the spring of a dry year.