Heterosis and Inbreeding Depression for Forage Yield and Fiber Concentration in Smooth Bromegrass

take potential (Van Soest, 1994). The concentration of NDF is heritable in several forage species, and rates of gain Voluntary intake is generally considered to be the single most from selection for reduced NDF concentration of herbimportant factor limiting animal performance on high-forage diets. Neutral detergent fiber (NDF) is the laboratory variable most closely age are reported to be as high as 12 g NDF kg 1 DM associated with voluntary intake potential. However, selection for low year 1 (Casler and Vogel, 1999). NDF generally leads to reduced forage yield. The objectives of this In perennial forages, selection for reduced NDF constudy were to estimate the correlation between forage yield and NDF centration generally leads to reduced forage yield (Casand to determine heterotic responses for both traits. Seven clones of ler, 1999; Surprenant et al., 1988). As an approach to smooth bromegrass (Bromus inermis Leyss.) were crossed in a diallel ameliorate this positive correlation, combined selection and four of the clones were self-pollinated to create S1 families. The for low NDF and high forage yield reduces the correseven parent clones showed considerable diversity, as measured by lated responses for low forage yield, but also reduces 329 random amplified polymorphic DNA (RAPD) markers, mostly the genetic gains for reduced NDF concentration (Surrelated to their pedigrees. The phenotypic correlation between forage prenant et al., 1988). Observed selection responses for yield and NDF decreased across generations, from 0.86 for parents, to 0.63 for GCA effects, and 0.49 for the 21 cross means. The genotypic forage yield, following selection for NDF concentration, correlation between forage yield and NDF was reduced from 0.99 for suggest that linkage and/or pleiotropy are the cause of parents to 0.71 for the 21 crosses. Forage yield heterosis effects avergenetic correlations between these two traits (Casler, aged 14% with a range of 4 to 39%, with 15 of 21 values significantly 1999). The distinction between linkage and pleiotropy different from zero. Midparent heterosis effects for NDF averaged is important because linkages can be broken by selection 0.5% with a range of 3.1 to 2.6%, with seven of 21 values signifiand recombination, whereas trait–trait associations caused cantly different from zero. Midparent heterosis effects for forage yield by pleiotropy cannot be broken. and NDF had a moderate correlation of 0.44. The change in correlaSegregating progeny are required to separate the eftions across generations suggested that a part of the genetic correlation fects of linkage from pleiotropy. If pleiotropy governs between forage yield and NDF is regulated by linkage. the genetic correlation between forage yield and NDF concentration, the relationship should not change across generations, such as F1 or S1 progeny. However, linkage V intake is generally considered to be the between loci controlling forage yield and NDF concensingle most important factor limiting animal perfortration would result in changes to the genetic correlation mance on high-forage diets (Fahey and Hussein, 1999). between the two traits (Lande, 1984). Physical distension of the rumen is the major factor limIn addition, random drift can also explain some of iting voluntary intake of high producing ruminants on the reduction in forage yield associated with selection high-forage diets (Mertens, 1994). For most high-forage for low NDF concentration (Casler, 1999). Drift is a podiets, intake of fibrous bulk generally causes rumen fill tentially reversible process when selection for low NDF and satiation before the ruminant has maximized its is conducted in multiple and diverse germplasm pools. caloric intake, resulting in a reduced plane of nutrition Hybridization of diverse low-NDF populations could (Van Soest, 1994). Voluntary intake can be increased restore forage yield potential via heterosis, possibly reby reducing bulk volume of the feed, which increases taining the low-NDF trait unless the heterotic response intake before satiation, or by increasing fiber clearance of NDF is similar to that of forage yield (Casler, 1999). from the rumen, which reduces the time required to The objectives of this study were (i) to determine the stimulate appetite. relationship between forage yield and NDF concenCell wall concentration, estimated as NDF, is a meatration in parents and segregating progeny of smooth sure of fibrous bulk, which is negatively associated with bromegrass and (ii) to determine the heterotic responses voluntary intake (Van Soest, 1994). The concentration of forage yield and NDF in smooth bromegrass F1 hyof NDF is generally considered to be the single laborabrids. tory variable most closely associated with voluntary inM.D. Casler, USDA-ARS, U.S. Dairy Forage Research Center, 1925 MATERIALS AND METHODS Linden Dr. West, Madison, WI 53706-1108; M. Diaby and C. Stendal, Dep. of Agronomy, Univ. of Wisconsin, Madison, WI 53706-1597. Germplasm and Crosses Research supported by the Univ. of Wisconsin, College of Agric. and Seven smooth bromegrass clones were selected for their Life Sciences by Hatch formula funds. Mention of a trademark or susceptible (three clones) or resistant (four clones) reaction brand name does not imply endorsement over any other product to Puccinia coronata Corda (Delgado et al., 2000). The seven by the USDA-ARS. Received 17 Mar. 2004. *Corresponding author ([email protected]). clones were selected from four populations: three from Alpha, Published in Crop Sci. 45:44–50 (2005). © Crop Science Society of America Abbreviations: DM, dry matter; NDF, neutral detergent fiber; RAPD, random amplified polymorphic DNA. 677 S. Segoe Rd., Madison, WI 53711 USA