Harvesting Summer Annual Forage Crops

By Dennis Hancock

Summer annual forages (such as pearl millet, sorghum x sudangrass hybrids, etc.) are known for their remarkable ability to grow fast and produce several tons of forage in a short time period, even during periods of drought. Before they are planted, however, it is important to think through when and how these summer annuals will be harvested.

One major reason to consider the harvest strategy is that summer annuals are notorious for being difficult to dry down to a moisture level that is suitable for haymaking (15 percent), since their large stems do not give up their moisture easily. As a general rule, it is better to graze the forage than cut it for hay or baleage. However, for many producers, the primary objective is to make hay or baleage so that they can boost summer forage yield and carry more forage into the winter.

Grazing Management

By far, the most cost-effective way to harvest any forage is to graze the standing crop. However, unlike mechanical harvesters, grazers are selective. If the forage growth is too mature when grazing begins, cattle will eat only the more tender parts of the plant and will trample or waste the less desirable forage. In general, grazing of summer annuals should be initiated at the pre-boot maturity stage. The height at which these plants will reach pre-boot varies with species, whether or not the variety is a dwarf cultivar, and the growing conditions. As a result, the target heights listed in the table are merely “rules-of-thumb.” Maturity stage should be the primary indicator of when to start grazing a stand.

Approximate harvest (grazing or mechanical) and stubble heights of selected summer annuals

	Grazing management		Hay/baleage management
Species	Height	Maturity stage	Height	Maturity stage	Stubble height^*
	(inches)		(inches)		(inches)
Pearl Millet	18-22	Pre-boot	30-40	Early head	4-6
Sorghum-sudan	22-30	Pre-boot	30-40	Early head	6-8
Sudangrass	24-28	Pre-boot	30-40	Early head	6-8

^* Stubble grazed or cut below these height ranges may result in poor or no regrowth.

It is also important to turn out enough cattle on each paddock or strip to graze down the area within a few days or a week, at most. Continuous grazing, which is when cattle stay in the paddock throughout the growing season, will not work well for summer annuals. Animals must be rotated in and out of paddocks or strips within a paddock to allow rapid regrowth, minimize waste and prolong the stand.

Summer annuals are also prone to nitrate and/or prussic acid toxicity issues. Care should be taken to evaluate the forage for these problems if the stands have been subjected to high nitrogen fertilization, drought stress or frost. Since hydrogen cyanide (prussic acid) is volatile and will dissipate within a few days, do not allow cattle to graze sorghum, sorghum-sudan hybrids, or sudangrass (pearl millet does not accumulate prussic acid) for 7 to 14 days after a frost. To some degree, toxic nitrate levels can be managed by ensuring the animals are not overly hungry when grazing is initiated and by limiting the grazing time on affected forage. Nitrates in the grazing animal’s diet will also be reduced if the cattle are removed at an average stubble height that is 2 to 4 inches greater than the target specified in the table. As a reminder, a forage analysis is a far cheaper test of the forage’s toxicity than is treating or losing a poisoned animal.

Hay management

One common way that summer annuals are harvested is by cutting and baling the forage for hay. Despite the difficulty that thick stems pose to timely hay curing, hay production can be successfully achieved. The drying rate is greatly improved by using a mower-conditioner that utilizes intermeshing rollers that squeeze and crimp the thick stems as they pass through the machine. Adjust these rollers to squeeze the forage as tightly as recommended by the manufacturer. In my personal experience (no data), summer annuals that have been planted in 15-inch rows or wider tend to bridge over the stubble and allow more air movement through the forage. As a result, they seem to dry quicker than broadcast-established stands. Though tedding may help speed the drying time, tedding thick-stemmed summer annual hay is generally too hard on all but the most durable hay tedders.

Prussic acid poisoning is much less common when the affected summer annual is cut and baled as hay, since hydrogen cyanide (prussic acid) dissipates within a few days. However, toxic nitrate levels in hay will not go away, no matter how much time it is given. Thus, an analysis of any forage suspected of high nitrate levels is crucial in understanding how to deal with the affected forage. As with grazing management, nitrate levels would ideally be measured prior to harvest. If nitrate levels are found to be high, increase the cutter bar height to 2 to 4 inches above the target specified in the table so that the nitrates that are highly concentrated in the lower stalk are avoided.

Baleage management

In recent years, baled silage equipment has become more common. Wrapping and conserving summer annuals as baleage is an excellent option. Summer annuals are relatively high in soluble sugars, which enhances fermentation. In general, baleage is much more palatable than dry hay made from the same crop. In addition, well-fermented silage can often reduce nitrate levels by 30 to 60 percent. Despite this reduction, nitrate levels may still be dangerously high. There have been reports of summer annuals having nitrate levels well over 15,000 ppm. Even if these nitrate levels are reduced by 60 percent, the forage could still be over the critical level (greater than 4,500 ppm).

Baleage is an expensive way to harvest summer annuals. Bale wrapping equipment costs $15,000 to $40,000, and the plastic wrap will cost $2 to $4 per bale (depending on the style of wrapper). After adding in other variable costs such as labor and fuel, total costs may exceed $20 per dry ton. Care should be taken to focus on harvesting high-quality forage so that the baleage investment reaps the greatest reward.

Summer annuals can provide a substantial amount of forage in a short time and can fill a critical gap during a drought year. However, the way that summer annuals are harvested can greatly affect your operation’s bottom line and susceptibility to the risks.

Dennis Hancock is the Extension Forage Specialist, University of Georgia