Grazing horses on warm-season annual grasses

Grazing during the summer slump

Pastures in the Upper Midwest mainly consist of cool-season grasses. However, many cool-season grasses go through a summer slump, resulting in reduced pasture productivity and forage quality during July and August.

Annual warm-season grasses have the potential to provide forage during the summer slump and can be used as emergency forage should cool-season grasses suffer winter injury. However, annual warm-season grasses are known to have high fiber content and lower crude protein (CP) and nonstructural carbohydrates (NSC) when compared to cool-season grasses. Most warm-season grasses are prone to accumulating nitrates, which can be toxic to grazing livestock at high concentrations.

Horse preferences

Researchers grazed four adult horses on ‘Summer Lovegrass’ teff, ‘PCS 3010’ sundangrass, ‘PCS 202’ sorghum sudan BMR, Japanese millet, ‘Red’ Siberian millet, and ‘Jumbo’ annual ryegrass. Annual ryegrass served as a cool-season grass control.

Horses grazed the forages in rotation between June and September. Horse preference was determined by visually assessing the percentage of available forage removed on a scale of 0 (no grazing activity) to 100 (100 percent of the existing vegetation grazed down to 4 inches).

• Most preferred: Annual ryegrass, teff and sudangrass
• Least preferred: Sorghum-sudangrass and Japanese and Siberian millet

Yield

• Sudangrass was consistently among the highest-yielding grasses (more than 6 tons per acre).
• Siberian and Japanese millet and annual ryegrass were among the lowest-yielding forages (less than 2.8 tons per acre).

Neither Siberian nor Japanese millet survived the entire growing season, which may have contributed to their lower overall yields. All other annual grasses resulted in three or more grazing rotations during the summer and fall months.

These results suggest that annual warm-season grasses, except for Siberian and Japanese millets, have the potential for high yields when grazed by horses. However, warm-season forages should not be grazed lower than 4 to 6 inches for optimal regrowth in a grazing system.

Nutritional values

Annual ryegrass consistently had higher crude protein and nonstructural carbohydrate levels, as well as lower neutral detergent fiber (NDF) content when compared to warm-season grasses. While warm-season grasses had a lower nutritive value, these forages met or exceeded the nutrient needs for adult horses at maintenance.

The lower NSC values in the warm-season grasses suggest they have the potential for owners managing horses diagnosed with metabolic conditions (such as equine metabolic syndrome).

It is important to note that an inverted calcium to phosphorus ratio (Ca to P) was observed in most forage species (the ideal Ca to P ratio is 1:1 to 3:1). If Ca to P is less than 1:1, horses will need a calcium supplement. 

Nitrates

Some forages can accumulate high levels of nitrates (NO3-N) as a result of overfertilization or environmental stress. Nitrate toxicity is considered rare in horses. Previous research shows that horses can consume forage with nitrates as high as 4,600 ppm before nitrate poisoning is observed. 

While all forages, except annual ryegrass, were at acceptable levels in 2014, they all exceeded 6,430 ppm NO3-N in 2015. Excessive drought and heat were not factors; therefore, the mineralization of soil nitrogen from past manure applications to the site may have contributed to the high nitrate values.

Nitrate toxicity was not observed, likely because the horses grazed these forages for short periods of time and in rotation with other forages. Nitrate concentrations should be determined before grazing warm-season grasses. 

Warm-season forage recommendations

Based on maximizing yield, forage nutritive values and preferences, teff and sudangrass show potential as annual warm-season horse pasture forages.

Sudangrass can lead to prussic acid poisoning, cystitis syndrome and abortions in horses. While these conditions are more commonly observed under stressful environmental conditions, including high temperatures, drought, and frost, they increase management concerns for horse owners. 

Teff may be used in Midwest horse pastures as an emergency forage or to maximize grazing during the summer slump.

The low nonstructural carbohydrate content of warm-season annuals shows potential for these forages to be grazed by horses diagnosed with metabolic conditions. But due to the inverted calcium to phosphorus levels and higher nitrate concentrations, teff should be tested before grazing.