Dr. Mark Sulc, Professor and Extension Forage Specialist, Department of Horticulture and Crop Science and Dr. Bill Weiss, Professor and Extension Dairy Specialist, Department of Animal Sciences, The Ohio State University
Summer annual grasses, such as sudangrass, sorghum-sudangrass, forage sorghum, pearl millet, and teff grass, are being used as additional sources of forage on dairy farms. This article discusses harvest and grazing management of these grasses.
The general guidelines for harvesting or grazing these summer annual grasses as listed in the Ohio Agronomy Guide are shown in Table 7-12.
Table 7-12: Harvest Information for Summer-Annual Grasses.
We planted a trial on July 19, 2013 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 4 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 vitro fermentation 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 sharply as the plants grew and matured (see figures below). The varieties were similar in yield and NDF, but there was a distinct NDFD advantage for the BMR hybrid over the non-BMR sudangrass variety (“Normal”).
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 herds or groups within herds are more sensitive to NDFD and require NDFD values greater than 50%. Based on these parameters, the “Normal” sorghum-sudangrass provided acceptable forage for lactating cow diets when harvested between 40 to 60 days after planting (30 to 50 inches tall). Heifer diets could utilize this forage harvested at about 60 days (50 inches tall).
The BMR hybrid provided a longer window of acceptable forage for dairy cows. In this study, the forage could have been harvested almost 80 days after planting (67 inches tall) and still be acceptable in a lactating or heifer diet. This provides opportunity for significantly greater forage yields.
Dry matter yield and total fiber (NDF) and 30-hour fiber digestibility (NDFD) of two
varieties of summer annual grasses planted on July 19, 2013 near South Charleston, OH.
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 with a study conducted by researchers at Cornell University (Kilcer et al., 2005), who concluded that BMR sorghum-sudangrass has a larger harvest window for producing forage for lactating cows. 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 earlier harvest reduces the amount of water that must be evaporated for ensiling as yields increase. The Cornell researchers stated that if planting were to be delayed into July, a second harvest may not be feasible, and delaying harvest to heights greater than 50 inches might be advantageous if extra forage is needed on the farm and good drying conditions exist to get rid of the extra moisture.
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 35-days after planting with an 8-inch stubble height (to encourage faster regrowth) 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 lb/acre for the BMR and 4870 lb/acre for the normal variety, with an average of 65% NDF for both varieties 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 plantings are made 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. Harvesting should occur 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, because toxic levels of prussic acid can be produced in the forage after a frost. Details of this risk are available at https://forages.osu.edu/news/be-alert-late-season-potential-forage-toxicities.
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.