Combining Ability Effects for Forage Residue Traits in Grain Sorghum Hybrids 1

Combining ability effects of 10 female and 10 male inbred lines were evaluated for agronomic, grain, and forage residue traits in 100 grain sorghum [Sorghum bicolor (L.) Moench] F, hybrids for 2 years. Traits studied were height, flowering, tillering, seed weight, yield, protein percentage, protein yield of grain, leaves, and stems; and in vitro dry matter disappearance (IVDMD) percentage and digestible dry matter (DDM) yield of leaves and stems. General combining ability (GCA) effects exceeded specific combining ability (SCA) effects for all traits except height in females and grain and leaf yields in males. Both GCA and SCA were low for tillering and stem protein percentage. Genetic ratios, resembling heritability, were higher for GCA than SCA except for height in females and tillering in males. Grain yield generally had no extremely strong negative phenotypic correlations with any forage residue trait although grain yield correlated with stem protein and stem IVDMD percentages produced negative values that would warrant monitoring in a breeding program. Grain yield, stem and leaf yields, and stem and leaf DDM yields all had moderate to strong positive relationships with each other. A few potentially useful parents were identified having high GCA effects for forage quality characters. Data on protein and IVDMD percentages of leaf and stem fractions from grain sorghum breeding nurseries and hybrid yield trials should be routinely collected. Additional index words: Sorghum bicolor (L.) Moench, Yield, Protein, IVDMD, Livestock, Forage quality. GRAIN sorghum [Sorghum bicolor (L.) Moench] is planted in the United States primarily for grain production, and hybrids normally are chosen for agronomic performance, pest resistance, or grain quality. Although substantial amounts of sorghum plant residues are utilized by livestock after the grain is harvested, little attention has been given to the crop's forage attributes by either the farmer or the plant breeder. Combine-harvested grain sorghum fields traditionally have been pastured, but there is a trend toward removal of all residues from the field for use in beef or sheep wintering rations. High feed costs have encouraged increased use of sorghum residues, and improved mechanical technology has permitted baling, stacking, and ensiling of these materials as wit~ conventional hay and forage crops. Obviously, not all reSidues from the grain sorghum crop are utilized because not all sorghum growers are livestock producers, and economics preclude hauling large amounts of bulky material great distances. Also, stubble often is purposely left in the field for moisture conservation and erosion control. Nonetheless, improvement in the feeding quality of that portion which is used as forage should directly benefit the livestock producer and, indirectly, the consumer. Certain line cultivars, like 'Midland', were noted for their post-harvest pasture value; such types often had a juicy stalk and high stem sugar content (Swanson, 1951). However, 1 Contribution from USDA-ARS and the Nebraska Agric. Exp. Stn. Published as Paper No. 6837, Journal Series, Nebraska Agric. Exp. Stn. Received 1 Apr. 1982. 2 Geneticists, USDA-ARS; George Holmes professor, Dep. of Agronomy, Univ. of Nebraska, Lincoln, NE 68583; former graduate student, now Dessert Seed Co., P.O. Box 181, EI Centro, CA 92244; former graduate student, now Western Agric. Res. Stn. , P.O. Box 169, Kakamega, Kenya; and former graduate student, now deceased. 97 studies that compare forage quality and forage yield of more than one grain sorghum hybrid in the same experiment are limited. In those few experiments, variability existed for leaf and stalk crude protein content, in vitro dry matter disappearance (IVDMD), and residue yield (Martin and Wedin, 1974; Ross et aI., 1980; Rutto, 1978; Smith, 1977). The results suggest that improvement of forage traits in hybrid grain sorghum might be possible through plant breeding with short-term gains accomplished by crossing appropriate inbred lines and long-term gains through recurrent selection. Numerous combining ability studies have been conducted with grain sorghum hybrids, beginning with Kambal and Webster (1965). Others are cited by Ritter (1978), and without exception, these reports dealt only with agronomic and grain traits. Only recently has the study of combining ability for forage traits been undertaken (Ross et aI., 1980; Rutto, 1978). In these instances, general combining ability (GCA) was relatively high compared to specific combining ability (SCA) for leaf and stem protein, IVDMD, and forage yield in single-year tests, but correlations of forage traits with agronomic and grain traits were inconsistent. Our investigations supplement those of Rutto (1978) who studied GCA and SCA effects, genetic ratios, and correlations of forage, agronomic, and grain traits in a large number of F 1 hybrids. MATERIALS AND METHODS The experimental materials were 100 grain sorghum F 1 hybrids made by crossing 10 females with 10 males using a modified diallel described by Griffing (1956) based on the Method II mating system of Comstock and Robinson (1950). Both parental groups included standard and experimental inbred lines and represented a range of germplasm adapted to eastern Nebraska (Table 1). The experiment was planted 26 May 1977 and 3 June 1978 Table 1. Parental lines used in combining ability study of grain sorghum hybrids.