12 May 2015

4 minute read

Dr Jaymelynn Farney, beef systems specialist, Kansas State University, writes that there are four primary categories of sorghum including: grain sorghum, forage sorghum, sudangrass, and sorghum-sudangrass hybrids.

These summer annuals yield tons of forage that offer energy and protein that make a quality forage for cattle whether grazing, haying, or ensiling.

For all categories of sorghum, and for some millets, prussic acid poisoning can be an issue for cattle, writes Dr Farney.

 In regards to ranking prussic acid toxicity sudangrass has low levels of HCN and rarely kills animals, sorghum-sudangrasses are intermediate, and sorghum has the highest levels of HCN and is the greatest risk to animals.

Cattle are the species most susceptible to prussic acid poisoning, but sheep can also be affected. Additionally, sorghums have issues with nitrate toxicity, so test the forage prior to turn-out or haying. Millets such as foxtail and proso have issues with nitrate toxicity and prussic acid.

Pearl millet has nitrate toxicity concerns yet is not a concern with prussic acid. Even though sorghums and millets have some toxicity issues, the benefits of these forages for cattle production have led to the development of management practices to minimize risks.

Grazing Considerations

These forages should be grazed while the quality (energy level) of the forage is high. Quality declines as the plant matures, so harvesting needs to occur before the boot stage.

To manage for prussic acid (highest content in new growth), it is recommended that grazing occurs once the sorghum plant reaches 24 inches or taller but for sudangrass, sudanhybrids, and millets grazing can begin at 18 inches of height.

Dr Farney adds that, when grazing sorghums during the summer, a rotational or strip grazing system is a way to manage for prussic acid and improve utilization. In a rotational grazing system cattle will only be rotated onto the sorghum paddocks once the plant is taller than 24 inches and allowed to graze until the plant is 8 inches tall.

The 8 inch remaining stubble will allow for re-growth in varieties that have regrowth potential. Additionally, not allowing the cattle to graze these forages lower than 8 inch will decrease the risk for nitrate toxicity since most of the nitrates are found at the base of the plant.

Ideally the cattle should consume the sorghum to the 8 inch height within a couple of days (maximum of 10 days) to minimize cattle consuming regrowth that is high in prussic acid. An example rotational grazing system for sorghums include grazing when the forage is 24 inches tall and stocking at 5-6 AU/acre (AU=animal unit; 1 AU is a 1000 pound animal) in other words 5000-6000 pounds per acre. The grazing length is approximately 7-10 days in that paddock.

Allow the paddock to rest for about 25 days before grazing again, or whenever plant height reaches 24 inches.

An example of rotational grazing for sudangrass, sudangrass-hybrids, or millets includes beginning grazing when plants are 18 inches of height and grazing for 7 to 10 days with a stocking rate of 4-5 AU/acre. Allow for a rest period of about 21 days or turn back into the paddock once the plants are 18 inches in height. Generally, if adequate moisture is available sorghums and millets that have regrowth potential will have 2-3 grazing opportunities. Remember this is a rough estimate for a rotational grazing situation.

Depending on varieties, weather, and moisture levels your forage yields might lead to a different grazing strategy than the examples listed above. See page 5 for more details on estimating pounds of forage and stocking rate.

Haying Considerations

When harvesting for hay there is a tradeoff between maximum tonnage and peak quality.

Harvesting at boot stage or just prior optimizes both the quality and quantity. Make sure when cutting for hay that you give the plant sufficient time to dry down and help avoid issues from mold development.

Higher seeding rates will decrease stalk size and speed drying. Even though nitrates and prussic acid are issues with sorghums, when harvesting as a hay product, nitrates are the primary issue. The curing process removes the prussic acid (prussic acid is very volatile), however, the nitrate remains in the plant as it dries.

There are harvesting techniques that minimize nitrate issues in sorghums (raising the cutter bar) and testing the plants for nitrate prior to harvesting. If the nitrate test is high, delay harvesting to allow the plant to continue growing (more growth, more maturity = less nitrates).

Variety Considerations

There are many different varieties of sorghums, sudan-hybrids, and millets so it is important to discuss with forage experts the varieties that meet your operations objectives prior to purchasing the seed. Some common questions that you need to be prepared to answer include:

•What do you plan to do with this forage?

•Do you need the forage to have regrowth potential?

•Are you concerned about prussic acid and high nitrates?

There are several variety studies conducted by Kansas State University at research centers throughout the state. Prior to making your determination about the variety of summer forage use your area’s data about quality and tonnage to maximize the benefits of these summer forages.