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Harvesting and Feeding Warm-Season Annuals for Forage

Harvesting and Feeding Warm-Season Annuals for Forage
By Leanna Duppstadt and Justin Brackenrich
This article is this week’s Agronomy Highlight, scheduled for Thursday, July 9 at noon. The Agronomy Highlight discussion is an opportunity to ask the author questions about the highlighted article, get updates from Penn State Extension Agronomy Educators around the commonwealth, share observations from your part of the state, and request content for the next issue of Field Crop News. Learn more about the weekly Agronomy Highlight discussion .
Whether you decided on using warm-season annual forages, such as sorghum, sudangrass, sorghum-sudangrass hybrids, or millets, in the spring and are now ready to harvest, or you are considering them as a cover crop after small grains, they provide an alternative feed source to traditional cool-season grasses, which slow production during the hot summer months. However, each of these forage types have their own qualities that need to be taken into consideration when it comes time to graze or to harvest and store. Often these qualities, good or bad, are overlooked until it is time for crop removal, but they are important to understand for proper management.
Prussic Acid and Nitrates
Major concerns of producers when talking about warm-season annuals are prussic acid and nitrate poisoning. Nitrate poisoning occurs when nitrates accumulate in the lower sections of plants, generally after a period of stress (drought, excessive heat, hail, etc.) or after an over fertilization of nitrogen. Prussic acid poisoning, much like nitrate poisoning, can occur as a result of a stressor, such as frost or a drought, but unlike nitrate, prussic acid is a physiological reaction of the plant resulting in the production of a toxic compound. Both conditions are more likely to occur in certain plant species and will be discussed more in the crop section. However, if you would like to learn more about them, read Reducing the Risk of Nitrate and Prussic Acid Poisoning in Livestock . Disclaimer- If you believe your livestock are experiencing any symptoms of nitrate of prussic acid poisoning, contact a veterinarian immediately.
For more information or needs on testing for prussic acid or nitrates, contact your local Extension office or questionable forage can be tested for prussic acid (HCN) at the Pennsylvania Department of Agriculture Summerdale Laboratory in Harrisburg (717-787-8808).
Warm-Season Annual Types
Forage sorghum is best utilized for silage but can be grazed or cut for hay if managed properly. Silages made from forage sorghum contain higher amounts of digestible energy compared to legume and cool-season grass silages but lower protein content than corn and cool-season grass or legume silages. Yields from forage sorghum are comparable to that of corn and it is also less likely to be fed on by deer , if that is an issue. Forage sorghum can be harvested for silage around the medium dough stage. It is important to note that forage sorghum does not regrow following harvest. Forage sorghum is susceptible to prussic acid and nitrate build up, but this is not usually a problem when it is harvested for silage. The greatest concern is when the crop has been stressed and is then grazed when the forage is less than 30 inches tall. Increasing cutting or grazing heights can reduce some risks associated with nitrates.
Sudangrass has declined in use with the introduction of sorghum-sudangrass hybrids. Sudangrass is susceptible to prussic acid buildup and poisoning, especially for grazing animals. For direct-cut silage, harvest at the early head to early bloom stage. Do not greenchop sudangrass until it is at least 18-24 inches tall and do not make hay or graze until it is 20-24 inches tall. If the plants have undergone a period of stress it is good to remember that the highest concentrations of nitrates are in the lower 1/3 of the plant, so leave behind more stem when harvesting. Or if possible, delay harvest until after the period of stress has passed. Ensiling nitrate accumulating forages can help to reduce nitrate levels by up to 50%. Concerns associated with prussic acid and nitrate poisonings have led to the crossings and productions of many sudangrass hybrids, which increased yields, but also slightly increased these toxic properties.
Sorghum-sudangrass hybrids were created to increase leafiness, thus increasing yield, and minimize seed head production. However, they tend to be higher in prussic acid at comparable growth stages. Sorghum-sudangrass may be best utilized as a greenchop forage and is only slightly more productive than sudangrass in pastures. Harvesting for silage can be done at the same stage as sudangrass – early head to early bloom. Do not graze, greenchop, or make into hay until sorghum-sudangrass is at least 30 inches tall.
Millets are a versatile grass, suitable for chopping, hay, and grazing. When discussing millets, Pearl, Foxtail, and Japanese are the front running species. They generally do not have the yield or quality of sorghum-sudangrass, although differences are slight. Pearl millet will regrow, but it will not be as fast as that of sudangrass or sorghum-sudangrass. On the other hand, Foxtail millet will not regrow. With some obvious drawbacks, millets have other beneficial qualities. Pearl and Foxtail millets, even after a killing frost or period of stress, do not contain enough prussic acid to be toxic to livestock, which means that they can be grazed or harvested at any time. Millets will perform better in cooler, wetter soils than other warm season annuals, making them good choices for the northern tier of PA. Since millets can tolerate cooler, wetter soils, have little prussic acid issues, and have limited regrowth, they are a quality candidate for cover crops after small grains.
Most warm season annuals will be in the range of 100-50-50 to 150-75-75 for full season fertilizer recommendations. As the season progresses and the length of the season reduces, so will fertilizer applications. One special consideration is nitrogen management to limit nitrate accumulations. Most warm season annuals will start with 100 pounds of N at planting and have 50 pounds applied after each cut. In the case of sudangrass and sudangrass hybrids (not to be confused with sorghum-sudangrass) the initial fertilizer application will start with only 50 pounds of N and then have 50 pounds applied after each cut. This is due to sudangrass’ strong proclivity to nitrate accumulation.
For faster recovery of sudangrass and sorghum-sudangrass, leave at least 4-6 inches of stubble and apply 50 pounds of nitrogen after each harvest. If rotationally grazing, it may be beneficial to mow afterwards to ensure even regrowth. Most annual forage crops are best for pasture or silage but, if field curing for hay, the use of a conditioner is recommended. It is also possible to wrap these forages as baleage which would have many of the same benefits as ensiling if done properly.
Often summer annuals are not managed to their full potential and are used just as an emergency summer feed. However, with proper management, harvest, and storage practices, they can be successful forage crops. Always remember to select a summer annual that best suits your management goals and production practices to optimize your forage use. 
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