Tips for sorghum forage management

Posted on April 14, 2020 in Forage Foundations
By Dr. Matt Akins, University of Wisconsin-Madison assistant scientist
For many forage growers, last year’s growing season was difficult with significantly wet conditions, and several producers made the decision to try sorghum forages with varying success.  If you are looking to continue growing sorghum forages this year, keep these tips in mind:

1.  Establishment
Soil temperature is important when it comes to planting sorghum.  Wait until the soil temperature is between 60 and 65 degrees F and isn’t excessively moist.  In the Midwest, this occurs in early- to mid-June in northern regions, and one to two weeks earlier in southern regions.  Planting into wet, cool soils will result in poor germination and weed issues.  Avoid areas that stay wet and use other forage options better adapted to those soils.

Seeds should be planted at a depth of 0.75 to 1 inch in heavier soils and between 1 to 1.5 inches in sandy soils.  Avoid planting two to three days prior to heavy rains due to soil crusting.

Seeding rates will vary depending on the type of seed you decide to plant.  For sorghums, a seeding rate of 60,000 to 80,000 seeds per acre, or about 5 to 7 pounds per acre, at 15- to 30-inch spacing is preferred; higher rates will result in thinner stems and greater lodging.  For sorghum-sudangrass hybrids and sudangrass, the preferred seeding rate is 15 to 20 pounds per acre at a narrow spacing of 6 to 7 inches for multiple harvests, or wider spacing of 15 to 20 inches for a single harvest.

2.  Harvest management
Multiple harvest management for sorghum-sudangrass or sudangrass increases forage quality for lactating cows but can reduce yields by as much as 50% compared to a single harvest.  A BMR variety is suggested to increase fiber digestibility.  In central Wisconsin, yields have been around 3 to 5 tons of dry matter (DM) per acre.  The first harvest should be taken at about 45 days after planting (typically in late-July in Wisconsin) or when the plant is 30 to 36 inches tall.  A second harvest should be taken either in early- to mid-September or seven to 10 days after a killing frost.  An earlier harvest allows for a cover crop and manure application but may disrupt corn silage harvest.  Leaving 6 inches of stem after the first harvest will allow for faster regrowth and elevate the swath.

A single harvest of non-BMR forage sorghum or sorghum-sudangrass maximizes yield and provides moderate quality forage for bred/pregnant heifers.  Yields in central Wisconsin have been around 5 to 8 tons of DM per acre, with higher yields in some cases.  A single-harvest BMR forage sorghum also works well for lactating cows, but yields have been 10% to 15% less than non-BMR varieties.  If planted by mid-June, most varieties (except photosensitive) will be at a maturity (soft-hard dough for forage sorghum) or moisture level (30% to 40% DM) to direct harvest by mid-October.  If you are cutting and wilting a single-harvest crop, merging or raking will not likely work due to long stems.  Direct pick-up is recommended in this situation and can be successful by using the frost to dry the crop for at least one to two weeks.

3.  Nitrates and prussic acid after frost
Nitrates accumulate in the stalk base due to the plant’s inability to convert them into amino acids, and they do not dissipate after a frost.  To reduce the nitrate content, raise the cutting height by 6 to 12 inches and allow the silage to fully ferment for 30 days.  It is best to test the crop for nitrates prior to harvest to make harvest management decisions, as well as before feedout.

Prussic acid accumulates in the leaves when cells burst and convert dhurrin to prussic acid.  To help dissipate the amount of prussic acid after a frost, it is suggested to dry the crop with a seven- to 10-day delay (the leaves will be dried/brown).  Testing can help minimize risk, but you should contact the lab for suggested sample preparation to reduce volatilization during shipping, which can alter results.

Category: Crop varieties
Feed quality and nutrition
Forage Foundations
Forage harvesting