This article was originally published in 2021 in the Milk Producer Magazine produced by Dairy Farmers of Ontario.
Written by Christine O’Reilly
Sorghum species—including forage sorghum, sudangrass and especially their hybrid sorghum-sudangrass—are gaining popularity in Ontario as forage crops. To increase palatability and digestibility in sorghum species, many producers have been increasing the seeding rates they use.
A decade ago, sorghum species for forage use were commonly sown at 20 to 25 pounds per acre. More recently, seeding rates have climbed, and sorghum species are frequently seeded at 30 to 40 lb/acre. Higher plant population decreases stem diameters and lignin content. While producers knowingly accept the increased risk of lodging, it also increases the risk of allelopathy.
Any mention of sorghum species and allelopathy in extension resources usually indicate it is not an issue unless seeding rates are increased above what is normally used for forage. But the publishing dates on those articles show they were written before seeding rates started to climb. Awareness of allelopathy will help producers avoid issues from sorghum species in their forage production systems.
WHAT IS ALLELOPATHY?
When a plant is described as “allelopathic,” it means the plant has an impact on the germination and-or growth of other plants. The most familiar allelopathic crop to dairy producers is alfalfa. Autotoxicity is a specific type of allelopathy where the chemicals released by the crop only affect other plants of the same species. As experience with alfalfa shows, allelopathy is not a deal-breaker for forage production—it just requires awareness and good management.
Research on sorghum species and allelopathy has found negative effects on wheat, corn and alfalfa. Small-seeded weeds seem to be particularly susceptible to sorghum allelopathy (Weston et al., 2013). In addition, grain and forage sorghums seem to release the greatest concentrations of allelopathic compounds. Researchers have reported significant differences between varieties within a species in terms of their ability to affect weeds and crops (Tibugari et al., 2020).
Key Points for Sorghum Allelopathy:
- Allelopathy is not a deal-breaker for forage production—it just requires awareness and good management;
- Seed size of sorghum species and varieties varies hugely;
- To avoid excessive plant populations and allelopathy issues, calculate the appropriate seed rate rather than using lb/acre (kg/ha);
- Avoid planting the next crop in the old sorghum rows.
WHY DO SORGHUM SPECIES AFFECT OTHER PLANTS?
Sorghum species exude several allelopathic compounds from their roots. Dhurrin is the best known because it’s also present in the leaves of these plants and breaks down into prussic acid. The other water-soluble compounds sorghums produce are phenolic acids. Much of the allelopathy research being done on sorghum involves extracting the water-soluble allelochemicals and using them as an organic weed suppressant spray.
The most significant allelopathic chemical produced by sorghum species is called sorgoleone. Sorgoleone is not water-soluble and persists longer in the soil than the other compounds. It interferes with seedlings’ photosynthesis. Sorgoleone production is linked to sorghum root hair development. Maximum secretions occur at temperatures between 25 to 35 degrees Celsius, especially with high humidity and plenty of available oxygen.
HOW TO AVOID ALLELOPATHY FROM SORGHUM SPECIES?
Seeding rate is likely to have the biggest effect on risk of allelopathy. The seed size of sorghum species varies hugely, from 11,000 to 45,000 seeds per pound. Even varieties or hybrids within a species lack consistency in seed size. A seeding rate based only on weight can result in very inconsistent plant populations. As populations climb, so does the risk of accumulating allelopathic compounds to levels that might affect the next crop.
Most sources suggest a population of 80,000 to 100,000 plants per acre for forage sorghum. Although seed rates for sudangrass and sorghum-sudangrass are typically higher than forage sorghum, there is a lack of consensus as to an appropriate plant population. More research is needed in this area since much of the ongoing sorghum allelopathy work is focused on weed suppression rather than forage crop rotations.
To avoid an excessive plant population, producers can calculate the appropriate seeding rate. Seeds per kilogram (or seeds per pound) should be stated on the seed tag or bag, along with the percent germination. The seeding rate can be determined using the formula in Table 1.
Table 1: To avoid an excessive plant population, producers can calculate the appropriate seeding rate using this formula.
Research suggests tillage can increase the rate at which allelopathic compounds degrade in the soil. Roth et al., 2020, found incorporating sorghum residue delayed wheat emergence but had no impact on grain yields, while wheat seeded into no-till sorghum residue emerged normally but had reduced yield. Light-textured soils seem to increase the risk of allelopathy.
Forage harvest removes above-ground residue, which removes one source of allelopathic chemicals. Avoid planting the next crop in the sorghum rows since the highest concentrations of allelopathic compounds are often found there.
Rather than increasing seeding rates, other strategies exist to help producers maximize quality. Timely harvest is critical to prevent the crop from becoming overmature. Sudangrass has finer stems and more leaves than forage sorghum or sorghum-sudangrass, which may increase palatability to livestock. Brown mid-rib (BMR) varieties of sorghum species are available, which reduces the amount of lignin the plant produces. Unlike corn, the BMR trait in sorghum species does not seem to significantly decrease yield compared with conventional varieties.
Allelopathy is a complicated subject. Managing seeding rate to reduce the risk of sorghum species affecting other crops in rotation is important as acreage increases in Ontario.
References:
Roth, C.M., J.P. Shroyer, and G.M. Paulsen. 2020. Allelopathy of sorghum on wheat under several tillage systems. Agron J. 92:855-860. Tibugari, H., C. Chiduza, and A.B. Mashingaidze. 2020. Farmer knowledge, attitude and practices on sorghum allelopathy in five sorghum producing districts of Zimbabwe. South African Journal of Plant and Soil. 37(2):152-159.
Weston, L.A., I.S. Alsaadawi, and S.R. Baerson. 2013. Sorghum allelopathy – from ecosystem to molecule. J Chem Ecol. 39:142-153.
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