Forage Sorghum For Dairy Cattle

Forage sorghum is a warm-season annual that is used for silage production and fed to dairy cattle in many regions of the United States. Compared with corn plants, it uses water more efficiently and is more heat- and drought-tolerant, which is important in areas where irrigation is limited or there is a greater chance of drought. In the semitropical regions, two crops of forage sorghum may be produced, or it can serve as a second crop following corn for silage. The feeding value of forage sorghum is influenced greatly by variety planted and stage of maturity at harvest. While forage sorghum silage is not a complete replacement for corn silage, it can be successfully used in rations fed to growing or lactating dairy cattle.

Forage Sorghum Production

Compared with corn, the cost of production for forage sorghum silage is lower, primarily because of lower seed and irrigation costs. Water requirements are reported to be 30% to 50% lower than for corn, which is an important consideration in areas that are dependent on irrigation to produce a crop. Forage sorghum will tolerate lower soil fertility than corn and still produce reasonable yields, but forage sorghum also responds well to fertilization. Forage sorghum can be planted later in the growing season than corn and still produce similar yields. Yield will typically be lower than corn silage, depending on soil fertility and water availability.

Varieties

Considerable variation exists in the days to maturity, plant height, yield, nutrient content, and lodging potential among forage sorghum varieties. Compared with typical varieties, brachytic dwarf varieties have shorter internodes, resulting in shorter plants that have a higher percentage of leaf relative to stem and are less prone to lodging. Brown-midrib (BMR) varieties have lower lignin concentrations and higher fiber digestibility compared with typical varieties, providing more energy to support heifer growth or milk production.

Figure 1 illustrates the variation in yield and NDF digestibility of 34 varieties of forage sorghum evaluated in the Texas Panhandle between 2008 and 2011. In vitro NDF digestibility (48 hour) of the BMR varieties averaged 64.8% compared with 56.1% for non-BMR varieties. Average yield (35% DM) was 22.8 and 20.5 ton/acre for non-BMR and BMR varieties, respectively, in this trial. Lodging scores ranged from 0% to 46% among varieties evaluated and should be considered when selecting a variety for planting. Given the variation in yield and quality among varieties, it is important that producers select varieties based on variety test data within their region.


Figure 1. Relationship of yield and NDF digestibility (NDFD) of forage sorghum varieties with (∆) or without (■) the BMR gene.

Toxicity Concerns

In the early stages of growth (less than 18 inches in height), forage sorghum can have high concentrations of prussic acid or hydrocyanic acid which is extremely toxic to cattle. Concentrations decrease as the plant grows and are typically not a problem at harvest under normal conditions. However, prussic acid concentrations increase after a frost. Prussic acid concentrations will decrease in the standing forage if harvest is delayed for approximately seven days after frost. Ensiling for a minimum of three weeks further reduces prussic acid concentrations. If there are any questions about the potential presence of prussic acid, a sample should be submitted for analysis before harvesting.

Nitrate toxicity is not as common in forage sorghum as in other forages in the sorghum family, but it can occur during a drought if large amounts of nitrogen fertilizer have been applied. Allowing the plant time to transform nitrogen to ammonia or protein after a rain will reduce the risk of nitrate toxicity. If there are concerns about the potential for nitrate toxicity, a sample should be taken and submitted for analysis. Ensiling will reduce moderate nitrate concentrations to a safe level in most, but not all, cases.

Harvesting Forage Sorghum

The recommended stage of maturity for harvesting forage sorghum is when the grain reaches the early to late dough stage to optimize DM content and nutrient quality. Harvesting earlier during the late vegetative or early head stage of maturity will result in silage with very low DM content (<25% DM), which will result in excess seepage and an undesirable fermentation including higher concentrations of acetic acid and ethanol. If harvesting is going to occur at this stage, wilting is recommended. If harvesting at the hard dough stage of maturity, the forage will have higher DM content, but the grain will be more mature and less digestible. Kernel processing forage sorghum that is in the late dough or hard-grain stages of maturity will improve grain digestibility and support improved animal performance.

Chemical Composition

In general, forage sorghum has higher concentrations of protein, fiber, lignin, and ash but lower concentrations of starch compared with corn silage (Table 1). The variation in the chemical values, as measured by the standard deviation, reflect differences in sampling, varieties planted, stage of maturity at harvest, fermentation, storage, and other factors that affect forage quality. Average 30-hour NDF digestibility was 60.7% for forage sorghum silage compared with 57.6% for corn silage.