Yield and Digestibility of 22 Summer Annual Varieties.

A trial conducted at Virginia Tech’s Southern Piedmont Agricultural Research and Extension Center evaluated the yield and digestibility of 22 varieties of summer annual grasses including conventional and BMR forage sorghums, sudangrasses, and sorghum-sudangrass hybrids, and pearl millet.  The study was planted on June 1, 2009 and all plots received 84 kg ha^-1 nitrogen per acre at seeding and 67 kg ha^-1 nitrogen per acre after the first harvest.  Plots were harvested on July 27, September 10, and October 21, 2009.  At the first harvest, yields for species-variety combinations differed by 3762 kg ha^-1, ranging from 3908 to 7670 kg ha^-1.  In vitro true digestibility also varied at the first harvest, ranging from 540 to 745 mg kg^-1.  The first harvest represented approximately 70% of the total yield.  The total yield for the growing season differed by 3784 kg ha^-1, ranging from 5500 to 9284 kg ha^-1.  A weighted average for digestibility ranged from 589 to 752 mg kg^-1 for the 2009 growing season.  What was most interesting is that the highest yielding variety in the trial was also one of the most digestible.  This indicates that high yield and digestibility may not be mutually exclusively traits.  In fact, when digestibility was graphed against yield for the species-variety combinations used in this trial, no relationship was found.  To better understand how to use yield and digestibility data when selecting or recommending a summer annual grass species-variety combination for livestock operations, the difference from average for the yield and digestibility was graphed and the graph was divided into four quadrants.  The upper right hand quadrant includes varieties that have above average yield and digestibility.  These species-variety combinations would be the most desirable to include in a forage production system.   Varieties in the upper left hand quadrant have above average digestibility, but below average yields.  While digestibility is good and animals may perform well on these varieties, dry matter production is lower.  In the bottom left hand quadrant, species-variety combinations with below average yield and digestibility are found.  These varieties would likely be the last choice for including in forage production programs.  The final quadrant located in the bottom right hand side of the graph, includes varieties that have above average yield and below average digestibility.  These varieties might be suitable for operations focused primarily on yield with less emphasis on animal performance.