BMR Forages Offer Dairy Ration Flexibility
Written by Ev Thomas and Dan Wiersma*
Alfalfa may be the queen of forage crops while corn silage is king, but brown midrib (BMR; bm-6 gene) summer annuals also have a place in many dairy forage programs. These include the BMR versions of sudangrass, sorghum-sudangrass, forage sorghum and brachytic dwarf sorghum. BMR forages are higher in quality due to reduced lignin content and higher NDF (neutral detergent fiber) digestibility, and for this reason have largely replaced non-BMR summer annuals.
BMR sudangrass and sorghum-sudangrass hybrids have been on the market longer so we know more about their performance, but the results of recent research on brachytic forage sorghum are encouraging. This species has shortened internodes resulting in a shorter plant with a greater leaf-to-stem ratio.
It’s Summer for a Reason
When planting any summer annual it’s important to note the word “summer”: These crops don’t like cold soils, so the soil temperature should be at least 60°F and warming at planting, about 10° warmer than for corn planting. Uniform stands can be established using a grain drill with smooth seed tubes (not corrugated ones). Warm-season annuals grow better than corn under dry and hot conditions, but a reasonable amount of rainfall is needed for high yields.
Fertilize according to soil analysis, and don’t spare the nitrogen. Manure can supply much of the nitrogen needed, but if manure isn’t applied, most summer annuals will respond to at least 100 pounds of fertilizer N (nitrogen) per acre for each cutting (where more than one harvest is made). When more than one harvest is made, observe the recommended harvest height, which differs among the species.
Most summer annuals are harvested for silage but they can also be harvested as dry hay or used for grazing. Yields are higher with forage sorghum than with sudangrass or sorghum-sudangrass, but this species is seldom harvested for hay because the cut stems are coarse and dry very slowly.
Don’t Cheat Regrowth
Stubble height is critical for summer annuals where a second (or third) crop is expected: leave at least 6 inches of stubble since much of the nutrition needed for early regrowth is in the bottom portion of the stem. If ensiled, use a research-proven silage inoculant because these forages can be difficult to pack properly and are very high in sugars, in some cases over 20 percent sugar on a dry matter basis.
Compared with sorghum-sudangrass, sudangrass is faster to emerge from the soil and is usually the summer annual of choice for grazing. Sorghum-sudangrass is taller and is typically used for silage because it has wider leaves and larger stems and therefore takes longer to dry.
“On the stem” dry matter content for most summer annuals is about 10 percent; the rate of drying is often slow for the first hours after mowing, accelerating quickly as the crop approaches 35 percent DM. Wide windrows will greatly speed the drying rate while preserving plant sugars.
The forage quality of sudangrass and sorghum-sudangrass varies with harvest timing and variety planted. According to DairyOne forage lab data, the silage of both crops averages about 12 percent crude protein, 62 percent NDF, and 58 percent NDF digestibility with very little starch. (It’s not known what proportion of the DairyOne samples represent BMR versus conventional varieties but this would have more impact on fiber digestibility than on starch, protein or NDF). Forage sorghum silage is lower in crude protein and NDF, but its 10 percent starch content is several times higher than that of sudangrass or sorghum-sudangrass.
Contrast these forages to processed (non-BMR) corn silage, which averages 8 percent crude protein, 44 percent NDF, 33 percent starch and 51 percent 30-hour NDFD. Net energy (lactation) averages about 0.50 for most summer annuals compared to 0.72 for corn silage. Therefore, the ability of these crops to make milk is due primarily to their high sugar content while starch plays a much bigger role with corn silage.
A new crop for most farmers, BMR brachytic dwarf sorghum has short internodes and is intended to be harvested once each growing season at the soft dough stage, which usually occurs at about the same time of the season as corn harvested for silage. At this stage of maturity, whole plant dry matter may be high enough that it can be direct chopped at 30 percent or more dry matter without any wilting. Narrow rows (15 inches or less) result in somewhat higher yields than when planted in 30-inch rows. We’re still learning about this crop, but based on limited research it appears to have similar milk-producing capability to good quality corn silage.
A Few Cautions
Most summer annuals can be highly toxic if harvested with less than 18 inches of growth and for a few days after a hard frost. Freezing breaks plant cell membranes, allowing the chemicals that form prussic acid (also called cyanide) to mix together and release this compound.
Dairy cattle eating recently frozen summer annuals can get a high enough dose of prussic acid to be fatal. This can be avoided by waiting three to five days after a freeze before grazing summer annuals. Baling recently frosted summer annuals reduces prussic acid content by about 75 percent. If the crop is ensiled, wait about three weeks for the prussic acid to dissipate as a gas.
*Ev Thomas is retired from the William H. Miner Agricultural Research Institute and president of Oak Point Agronomics Ltd. Dan Wiersma is the alfalfa business manager with DuPont Pioneer.
Used by permission from the March 10, 2016, issue of Hoard’s Dairyman. Copyright 2016 by W.D. Hoard & Sons Company, Fort Atkinson, Wisconsin.