|Thoughts About Summer Annual Grasses|
By Don Ball
Drought conditions in Alabama this year have put many livestock producers in a bad position. Pastures have been grazed short and we are off to a poor start with hay production. Not surprisingly, many producers are thinking about planting some acreage of a summer annual grass such as pearl millet, sudangrass or a sorghum-sudangrass hybrid. These species have the potential to make a substantial amount of good quality forage growth in a short period of time, but one should to be aware of the problems or potential problems associated with these species as well.
Planting into an extremely dry seedbed is very risky business. A light rain may cause the seed to germinate, after which seedlings quickly die due to lack of moisture to sustain them. April or May is the best planting time as this allows a long growing season and multiple cuttings. However, if moisture is readily available (a big "if"), sorghum-sudangrass hybrids or sudangrass can be planted as late as early August and still make enough forage growth for one hay cutting or grazedown. (Browntop millet can also be planted in August, but is a relatively low yielder even if soil moisture is good.) Pearl millet should be planted by early July. Summer annual grasses are always more expensive to grow than perennial grasses and the lower production associated with late plantings makes the cost even greater than usual.
Summer annual grasses require fairly high levels of management for good results, with fertility being one important consideration. Phosphorous, potassium and lime should be applied to the area to be planted according to soil test recommendations. Nitrogen (N) should be applied at the rate of 60 pounds/acre at, or soon after, planting time. Additional applications of 60 pounds of N/acre should be made after each cutting or graze-down up to September 1.
Grazing should be initiated for various major summer annual grasses when they have reached the following heights: pearl millet 18-24", sudangrass 18-24" and sorghum-sudangrass hybrids 22-30". If the growth is too tall when grazing is begun, the animals will selectively graze and will waste and trample much of the forage. The plants should not be grazed lower than 6-8". If a field is not grazed uniformly, it should be mowed to a 6-8" stubble height.
Continuous grazing (keeping livestock on the area all the time) is not a good method to use with summer annual grasses. One-third to one-half more acreage is required with this system to obtain the same benefit as areas where other grazing systems are used. Rotational grazing (rotating animals from one area to another every 7 to 10 days), strip grazing (allowing animals access to a small strip of pasture by use of an electric fence), or limit grazing (allowing animals access for only a few hours in any given day) are much more efficient and economical.
The summer annual grasses mentioned should normally be cut for hay when they reach a height of 30 to 40 inches. While waiting longer than this to cut for hay will increase yield, it will lower quality significantly. The summer annual grasses should never be allowed to mature past the boot stage (just before seedhead emergence). As with grazing, stubble of 6 to 8 inches should be left with each hay cutting. This high stubble height allows quicker regrowth and also holds cut forage off the ground, allowing faster drying and baling.
It is almost essential to use a hay conditioner when cutting summer annual grasses for hay. Otherwise, it may take 5 or 6 days for the large stems to lose enough moisture to permit baling. With the use of a conditioner, length of time from cutting to baling is usually about half what it would otherwise be. Even if baled in large round bales, hay of summer annual grasses should be put under shelter.
Two livestock disorders associated with summer annual grasses need to be mentioned. Toxic levels of nitrates are possible in summer annual grass forage, especially in hay. Unfortunately, nitrate levels degrade little with time, so hay can cause deaths months after having been baled. In addition, extremely young forage growth and/or extreme stress can result in dangerous levels of prussic acid in plants in the sorghum genus, which includes sudangrass and sorghum-sudangrass hybrids. The single most dangerous time with these species is immediately after a killing frost. Fortunately, prussic acid degrades with time and is not a danger once the forage has dried.
Don Ball is an Extension Forage Crop Agronomist with Auburn University.