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Sorghum forage harvest considerations

Matt Akins
Sorghum forage plots harvested on November 1, 2017 in Waupaca County.  Greg Blonde, left, former Waupac Co. Extension agricultural agent, and Greg Peterson, of the Waupaca Co. Land and Water Conservation, are harvesting a forage sorghum plot (hard-dough stage).  Note the differences in maturity as varieties without a seed-head are photoperiod sensitive (won’t head out until day-length is less than 12 hours and 20 minutes).

Warm-season annual forages including sorghums (forage sorghum, sorghum-sudangrass, sudangrass), are useful forage crops within a rotation such as after a cereal grain forage harvest or after a perennial forage winter-kill. These forages can perform well with good establishment management (soil temperature over 65°F usually in early to mid-June; and proper seeding depth of ¾ to 1” in heavier soils or 1.5” in sandy soils) and fertility (45-60 lb N/acre per cutting, or 90-120 lb N/acre for single cut crops). 

Generally, these do better during warmer summers and in well-drained soils so avoid planting in areas that stay wet. Recent work with sorghum forages in Central Wisconsin (Marshfield, Hancock and Waupaca) has shown that harvest management plays a key role in optimizing forage quality for specific animal groups and has large effects on potential forage yields. 

Use of a single or multiple harvest system (likely only 2 cuttings in Wisconsin) are both options with sorghum forages. A single harvest will maximize yields with 1.5 to 2 times the yield compared to doing two harvests, however multiple harvests will result in higher forage quality (at least for sorghum-sudangrass and sudangrass). 

Forage Sorghums

Forage sorghums are better adapted to a single harvest due to lower seeding rates and less regrowth potential. Yields in Central Wisconsin have ranged from 5-10 tons DM/acre with higher yields from excellent establishment and warm weather. Forage sorghums can be harvested similar to corn silage as they will develop a large grain head which contributes a significant amount of starch if allowed to mature to the soft to hard-dough stage. Harvest will usually be in late September to mid-October when moisture is 30-35% DM. 

Yields of forage sorghum or sorghum-sudangrass using single (mid-late October) or multiple (late July/early August and mid-October) harvests at Marshfield and Hancock ARS.

Use of a variety with the brown mid-rib trait (BMR) will improve fiber digestibility and improve its usage in lactating cow rations.  If feeding heifers, a conventional variety will work well.  Chopping at 3/8 to 1/2 inch length chop and processing will help minimize large stem pieces and break some of the grain particles.  Seeding at a wider spacing (20-30 inches) and at about 75,000 seeds per acre will help minimize lodging that can occur when seeded at higher rates.

Sorghum-sudangrass and sudangrass

Sorghum-sudangrass and sudangrass are versatile forages and can be harvested using a single or multiple harvests. Yields with 2 harvests have ranged from 3-6 tons DM/acre.  For lactating cow forage, make sure to use multiple harvests with cuttings taken when the crop reaches 30-40 inches tall which will result in greater leaf content and smaller stems.  The first cutting is typically in late July or early August, and the second cutting in early/mid-September or possibly after a killing frost to maximize yield.  This forage can work well as either chopped silage or baled silage.  If the crop becomes tall and stemmy, chopped silage is suggested as animals will not consume the long, coarse stems in baled silage.  A BMR variety is suggested to improve fiber quality and intakes.  The suggested cutting height is 6 inches to improve regrowth potential, but good regrowth can occur from lower cutting heights and tillering from the plant base.

Sorghum-sudangrass plots harvested on August 7, 2019 at the Hancock ARS (approx. 36-48” tall).  Note the difference in height due to the left variety having the brachytic dwarf trait.

Using a single harvest can also work well for sorghum-sudangrass with a couple harvest options.  Yields have ranged from 5-10 tons DM/acre.  This forage will work well in dairy heifer rations to control feed intake and growth of pregnant heifers, and possibly dry cows depending on potassium, prussic acid, and nitrate content.  Conventional varieties work well as the goal is to have a higher fiber/lower energy forage.  A cut/wilt/chop harvest can occur in September during a warm, dry spell with 3-4 days wilting likely needed.  Make sure to chop finely (3/8 to 1/2 inch) to minimize long stems that will not be eaten.  Another option is to allow the crop to freeze and then dry for 1-2 weeks to dry the leaves enough for the crop to reach 30% dry matter.  After the crop is dry enough, direct harvest the crop with a forage harvester equipped with a multi-directional cutting (kemper) head.

Sorghum forages can accumulate toxins (prussic acid and/or nitrate) following dry conditions or after a frost.  Allowing the crop to stand for at least 1 week will help lower nitrates in the stem following dry conditions and prussic acids from leaves after a frost.  Testing of the forage prior to feeding is suggested if concerned about these toxins and allow for feeding adjustments.

Matt Akins
UW Extension

Akins is an Assistant Scientist and Extension Dairy Specialist with the UW-Madison Department of Animal and Dairy Science