Harvest Management of Sorghum Forages

By: Marc Sulc and Bill Weiss, OSU Extension

Many producers in Ohio have planted summer annual grasses this year to increase their low forage inventories. These include sudangrass, sorghum-sudangrass, forage sorghum, pearl millet, and teff grass. When should these grasses be harvested or grazed?

The general guidelines for harvesting or grazing these summer annual grasses as listed in the Ohio Agronomy Guide are shown in the table below.

Table 7-12: Harvest Information for Summer-Annual Grasses.

Forage Table

A recent research study sheds more light on these general recommendations, particularly in relation to mid-summer planting of the sorghum grasses. We planted a trail on July 19, 2019 near South Charleston, OH to evaluate the yield and fiber quality of a conventional sudangrass variety (hereafter designated “Normal”) and a sorghum-sudangrass hybrid carrying the BMR-6 gene for reduced lignin (hereafter designated “BMR”). Forage yield, neutral detergent fiber (NDF) concentration and NDF digestibility (NDFD) were measured on four dates after planting, with the forage being cut to a 4-inch stubble height at each harvest. The NDFD was measured after 30-hours of in vitrofermentation in rumen fluid plus buffer, followed by removal of microbial contaminants with neutral detergent solution.

The results were not surprising in that yield and NDF increased while NDFD decreased fairly sharply as the plants grew and matured (see figures below). There was a distinct advantage for the BMR hybrid over the non-BMR sudangrass variety (“Normal”) in terms of NDFD.

In general, diets can be formulated for different classes of livestock based on the fiber quality of the forage. For lactating cows using these forages, the amount of forage that can be fed will be limited by the NDF level. For example, if harvest was delayed in order to obtain highest forage yield, the NDF level was near 70%. At 70% NDF, the forage would probably have to be limited to 10% of the total diet of lactating dairy cows, on a dry matter basis.

For lactating cows, forage with NDFD levels of 50% are usually acceptable, and levels as low as 40% NDFD could probably work if necessary. However, higher producing herd or groups within herds are more sensitive to NDFD and require NDFD values greater than 50%. Based on these parameters, these grasses provided acceptable forage for lactating cow diets when harvested between 40 to 60 days after planting (30 to 50 inches tall). Heifer cow diets could utilize this forage harvested at about 60 days (50 inches tall).

Harvest of the BMR hybrid provided a longer window of acceptable forage. In this case, the forage could have been harvested almost to 80 days after planting (67 inches tall) and still be acceptable in a lactating or heifer cow diets. This provides opportunity for significantly greater forage yields.

Dry Matter Yield

NDF

Forage having NDFD levels as low as 35 to 40% with high NDF levels are acceptable for dry cows or beef cattle provided they are part of a balanced diet and their mineral concentrations are not excessive relative to requirements. Based on the results shown above, the forage harvested from 60 to 80 days after planting (50 to 67 inches tall) would have been acceptable for dry cows or beef cattle.

The results from the experiment shown here agree fairly well with a study conducted by researchers at Cornell University (Kilcer et al., 2005), who concluded that BMR sorghum-sudangrass has a relatively large harvest window to produce forage for lactating cow diets. However, they recommended that BMR sorghum-sudangrass be harvested for lactating cows when stand heights are about 50 inches (2-cuts possible with early June planting) because this will occur before the shift from vegetative to reproductive growth that lowers quality and it also reduces the amount of water that must be evaporated as yields increase. They did state, however, that with delayed planting into July, a second harvest may not be feasible, and delaying harvest to heights greater than 50 inches may be advantageous if extra forage is needed on the farm and the extra moisture can be dealt with.

In our study, we also investigated whether a 2-harvest system could provide similar forage yields with higher forage nutritive value compared with a single harvest after a mid-July planting date. The only combination of harvest dates that provided reasonable forage yields occurred when the first harvest was made at an 8-inch stubble height (to encourage faster regrowth) at 35 days after planting and the second harvest was made at a 4-inch stubble 48 days later (83 days after planting). That 2-harvest combination produced a total dry matter yield of 3813 lbs/acre for the BMR and 4870 lbs/acre for the normal variety, with an average NDF concentration of 65% and 48% NDFD for the BMR and 45% NDFD for the normal variety. Therefore, we concluded the 2-harvest system showed no significant advantage over harvesting once at 60 days when planting in mid-July.

In summary,  non-BMR sudangrass and sorghum-sudangrass planted in mid-July should be harvested between 40 to 60 days (30 to 50 inches tall) for lactating dairy cows, at about 60 days after planting (50 inches tall) for feeding heifers, and 60 to 80 days after planting (50 to 67 inches tall) for beef cattle or dry cows. The BMR hybrid provided a wider harvest window for lactating cows, with acceptable forage harvested nearly 80 days after planting.

Keep in mind that the sorghum grasses should be harvested or grazed prior to a frost, which can produce toxic levels of prussic acid in the forage. Details of this risk are available at https://agcrops.osu.edu/newsletter/corn-newsletter/2018-35/avoid-forage-toxicities-after-frosts.

Reference:

Kilcer, T.F., Q.M. Ketterings, J.H. Cherney, P. Cerosaletti, and P. Barney. 2005. Optimum stand height for forage brown midrib sorghum x sudangrass in North-eastern USA. J. Agronomy & Crop Science 191:45-40.