Agronomic and silage quality traits of forage sorghum cultivars in 1995

Agronomic and silage quality traits were measure d for 37 forage sorghum cultivars and three grain sorghum hybrids. The 1995 growing season was characterized by above average rainfall in the spring and early summer, and a hard freeze on September 22. At the time of the freeze, 20 cultivars had reached the early-milk to earlydough stage, 12 were in the bloom stage, and the remaining eight were still in the earlyto late-boot stage . The late planting date and low plant populations resulted in below-normal whole-plant D M and grain yields. Plant height s for the grain sorghums were near normal, but the forage sorghums were well below expected plant heights. The preensiled, whole-plan t DM contents of the 37 forage sorghums ranged from 23.0 to 39.9%. As expected , the silage nutritive value traits of CP, NDF, and ADF were most favorable for the three grain sorghum hybrids and least favorable for the eight forage sorghum hybrids that were still in the boot stage when the freeze occurred.


Introduction
Forage sorghu m is an important silage crop for beef and dairy cattle producers in the High Plains region of the United States.Sorghums have greater drought tolerance, better ability to recover from drought, and lower production costs than corn.Kansas livestock producers harveste d about 80,000 acres of sorghum for silage in 1995, which yielded approximately 800,000 tons.
Results from earlier studies indicated that cultivar and growing season have a tremendous effect on agronomic and silag e quality traits of forage sorghums (KAES Report of Progress 678, page 13; and KAES Report of Progress 727, page 68).Our objective was to continue documenting agronomic perform a nce and silage quality traits over a wide range of forage sorghum cultivars currently available in Kansas.

Experimental Procedures
Thirty seven forage sorghum cultivars and three grain sorghum hybrids were selected to represent a wid e range of phenotypic characteristics and season lengths .All were grown under dryland conditions in 1995 near the Kansas State University campus.The forage and grain sorghum plots were planted o n July 3, and each cultivar was a ssigned randomly to each of three replications .The six-row plots were in a Reading silt loam soil with anhydrous ammonia applie d at 80 lb of nitrogen per acre.Rows were 27 ft long with a 30-inch spacing, and plots were thinne d to a uniform stand of 26,000 to 28,000 plants per acre.
The three grain sorghums and 11 of the 17 forage sorghums that had reac h ed the early-milk to early-dough stage before a hard freeze on September 22 were h arvested between September 26 and October 6.T he remaining 26 forage sorghums were harvest ed on October 19, which is near the average annual first-freeze date for the Riley County location of the plots.
The two outside rows in each plot were prote ctive borders.All heads in two inside rows were hand clipped, and the heads were dried in a forced air oven for 2 weeks.The dried heads were threshed with a stationary mach ine, and the grain yield was adjusted to a 14.5% moisture basis.Whole-plant DM yield was measured by harvesting the two remaining inside rows with a FieldQueen precision forage harvester.The chopped mat erial from each plot was sampled for whole-pla nt DM determination and ensiled in 4 × 1 4 inch PVC laboratory-scale silos.All silos were packed to similar densities using a specially designed hydraulic press.The PVC silos were opened after approximately 90 days of storage.All silages were analyzed for pH and DM, crude protein (CP), neutral detergent fiber (NDF), acid detergent fiber (ADF), and ash contents.

Results and Discussion
Agronomic performa nce of the 40 sorghum cultivars is shown in Tables 1 and 2. Days to half bloom for the 16 grain-producing forage sorghum cultivars th at reached the early-milk to early-dough stage before September 22 ranged from 56 to 67 days.Plant heights for all 37 forage sorghums were below normal.As expected , the three grain sorghums were the shortest overall.Tw o of the late-season hybrids (Mycogen Red Top Kandy and Casterline Supersile ) were the tallest forage sorghums; Pioneer 841F, DeKalb X585, and Golden Harvest H-45 were the shortest.
The preensiled whole-plant DM contents of the 37 forage sorghums ranged from 23.0 to 39.9%.The three grain sorghum hybrids averaged 37.5% DM, whereas the eight late-season forage sorghum hybrids that wer e harvested in the boot stage averaged only 24.1% DM.The average pH of 3.9 indicated an extensive fermentation phase, which was prima r ily a function of the low D M content of most cultivars.Forages ensiled with less than 30% DM can produce large amounts of effluent during initial storage.Who le-plant DM yield was highest for two late-season hybrids (Century II Hygrachop and Casterline Supersil e) and the middle-season DeKalb FS-5, whereas Pioneer 8771 grain sorghum and Early Sumac forage sorghum had the lowest whole-plant DM yields.Grain yields were below normal for the thr e e grain sorghums and the 16 forage sorghums that produced grain.Pioneer 8771 grai n sorghum and DeKalb X489 forage sorghum had the highest grain yields, and Earl y Sumac variety had the lowest grain yield.Surprisingly, none of the 40 sorghum cultivars lodged before harvest.
The early freez e on September 22 coupled with the late planting date a nd low plant populations resulted in below-normal whole-plant DM and grain yields f or all 40 cultivars.The silages of the 1 6 grain producing forage sorghums that reached the early-milk to early-dough stage before the freeze ha d an average DM content of 28.3%, whereas the silages of the 20 forage sorghum s with little or no grain fill had an average DM c ontent of 23.1%.In addition to a suitable DM content, the early-and middleseason cultivars that produced grain also had higher silage quality traits than the 20 lateseason cultivars, as evidenced by a higher average CP content (8.6% vs. 8.3%) and lower average contents of NDF (48.9% vs. 54.4%)and ADF (29.6% vs. 33.8%).
These data indicate that in th e 1995 growing season, the late-season fora ge sorghum cultivars generally produc ed more whole-plant DM yield than the grain sorghums or early-and middleseason forage sorghums.However, the lateseason cultivars had the lowest nutritive values and would be prone to produce excessive effluent and undergo an unfavorable fermentation because of their low DM content.The LSD (least significant difference) is valid only within a column. 1

Table 1 . Mean Agronomic Performance and Silage Quality Traits of the Grain Sorghums, Forage Sorghums that Produced Grain (w/grain), and Forage Sorghums that Did Not Produce Grain (w/o grain)
1Mycogen Greenleaf Sterile was not included in the calculation of the mean grain yield. 2

Table 2 . Agronomic Performance of the 40 Sorghum Cultivars
1 bl = bloom stage, hd = heading stage, and bt = boot stage. 2 Adjusted to a 14.5% moisture basis.3TheLSD (least significant difference) is valid only within a column.4