Optimal nitrogen and sulfur rates for annual forage grasses

BOZEMAN - Annual cool and warm season grasses are important livestock forages in Montana. Now Montana State University researchers are determining optimal nitrogen rates for these forages and the role of sulfur in forage quality.

"Warm-season grasses are more tolerant of low soil pH, low fertility and drought than most cool-season grasses," said Marilyn Dalen, assistant research professor at the Northwestern Agricultural Research Center (NWARC) in Creston. Diversifying forage production by growing annual warm season grasses can increase forage availability and improve animal nutrition.

Dalen and collaborators Jessica Torrion, MSU associate professor of crop physiology, and Eeusha Nafi, former postdoctoral researcher at NWARC, measured yield, crude protein, and fiber content of four annual warm season grasses under rainfed and irrigated conditions. The grasses were pearl millet, sudangrass, teff and sorghum-sudangrass. These grasses reached optimal yields with around 100 pounds total available nitrogen per acre to a 3-foot depth. This includes nitrogen available from fertilizer and nitrogen released from both the prior faba bean crop residue and soil organic matter.

Under rainfed conditions, May to September rainfall was 6.7 and 3.8 inches in 2020 and 2021, respectively, at NWARC. Forage yield declined at total soil nitrogen levels greater than 150 pounds of nitrogen per acre. Increasing nitrogen from 100 to 150 pounds per acre did not increase yields. Yields were about 18% higher with irrigation.

Increasing total available nitrogen above 112 pounds per acre did not consistently produce more forage under irrigated conditions.

"The possible higher yields are probably not economically feasible considering fertilizer and irrigation costs," said Dalen.

Pushing yields with high nitrogen and more than 8 inches of irrigation can be risky in some situations. Clain Jones, Montana State University Extension soil fertility specialist, added, "There is also increased risk of high forage nitrate levels, nitrate leaching, and soil acidification with higher nitrogen rates if forage yield goals are not reached."

According to the research, among the warm-season forages, sorghum-sudan types produced the highest yield under irrigation in 2020. There were no differences among forage species in their response to available nitrogen level.

Forage crude protein increased, and fiber decreased, with increasing soil nitrogen under both rainfed and irrigated conditions. However, forage nitrate increased with increasing soil nitrogen under irrigated conditions to around 2,000 parts per million with 225 pounds total available nitrogen per acre.

Under rainfed conditions, forage nitrate increased to a maximum around 1,300 ppm at 150 pounds nitrogen per acre with no further increase with higher nitrogen. When forage nitrate content is greater than 1,500 ppm, it can be toxic to pregnant livestock unless blended with at least 50% low-nitrate forage.

Amanda Grube, a recent Montana State University graduate student in the Department of Animal and Range Sciences, studied whether adding sulfur to the soil can reduce nitrate accumulation and improve barley forage quality. Hayes Goosey, Grube's major adviser and MSU Extension forage specialist, led the study with assistance from Torrion; Kent McVay, Extension cropping systems specialist, and Pat Carr, associate professor of cropping systems. All are in the Department of Research Centers.

"In general, as available soil sulfur increased, forage nitrates decreased," said Grube.

At NWARC in that study, broadcasting and incorporating 20 pounds sulfur per acre prior to planting reduced forage nitrates to below toxic levels.

"However, there is a limit to how much added sulfur can help lower nitrates," said Goosey. At moderate soil nitrogen (25-35 pounds of nitrogen per ton of forage), sulfur helped lower nitrates, however if available nitrogen was high (greater than 35 pounds per ton), sulfur fertilization reduced nitrate but did not reduce nitrates to non-toxic levels.

According to the research, forage quality is largely determined by soil nitrogen. If there is too much nitrogen, then forage quality declines. It is best to manage soil nitrogen, rather than sulfur, to limit forage nitrates.

Grube and Goosey suggest sacrificing a little in yield by using less nitrogen to reduce the risk of nitrate toxicity. The producer should collect soil samples yearly, prior to seeding, to reevaluate nitrogen levels and amend with correct fertilizer nitrogen amounts. In fields that continually produce higher nitrate forage, producers should aim for the lower 25 pounds nitrogen per ton level and add 10-20 pounds sulfur per acre to minimize forage nitrate levels.

The 25 to 35 pounds nitrogen per ton recommendation agrees with the warm season grass research done by Dalen, where 100 pounds soil nitrogen per acre produced around 4 tons per acre in rainfed conditions and 112 pounds soil nitrogen produced around 5 tons of forage per acre under irrigation.

For more details on the study of annual warm season grass nitrogen rates, see Fertilizer Fact 84, https://landresources.montana.edu/fertilizerfacts/index.html. For results from the hay barley study, see Amanda Grube's thesis at https://scholarworks.montana.edu/handle/1/19148.

Additional MSU Extension resources on soil fertility are available at landresources.montana.edu/soilfertility/.