Submitted to: Plant Breeding
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/4/2005
Publication Date: 1/4/2005
Citation: Casler, M.D., Diaby, M., Stendal, C. 2005. Heterosis and inbreeding depression for forage yield and fiber concentration in smooth bromegrass. Plant Breeding. 45:44-50. Interpretive Summary: Dietary fiber is an important factor controlling feed intake by ruminants. Low fiber concentrations increase the intake potential of a forage crop. However, lower fiber also results in reduced forage yields, reducing the economic potential of low-fiber crops. Efforts to decrease fiber without also reducing forage yield have generally failed. The objective was to determine if hybrid vigor could be used to solve this problem. Smooth bromegrass hybrids have heterosis for forage yield, ie. the hybrid has a higher forage yield than the average of the two parents. If the hybrid has no change in fiber, then hybrids between two complementary low-fiber lines could be used to produce higher yielding low-fiber lines. Of 21 hybrids in smooth bromegrass, 15 showed significant heterosis for forage yield, but only seven showed heterosis for fiber. Furthermore, the yield heterosis averaged 14%, while the fiber heterosis averaged near zero. Several hybrids between low-fiber parents had significantly higher forage yield, but no change in fiber. Hybrids are a mechanism of delivering low-fiber, high-yield germplasm to forage producers.
Technical Abstract: Voluntary intake is generally considered to be the single most important factor limiting animal performance on high-forage diets. Neutral detergent fiber (NDF) is the laboratory variable most closely associated with voluntary intake potential. However, selection for low NDF generally leads to reduced forage yield. The objectives of this study were to estimate the correlation between forage yield and intake and to determine heterotic responses for both traits. Seven clones of smooth bromegrass (Bromus inermis Leyss.) were crossed in a diallel and four of the clones were selfed to create S1 families. The seven parent clones showed considerable diversity, as measured by 329 random amplified polymorphic DNA markers, mostly related to their pedigrees. The phenotypic correlation between forage yield and NDF decreased across generations, from 0.86 for parents, to 0.63 for GCA effects, and to 0.49 for the 21 cross means. The genotypic correlation between forage yield and NDF was reduced from 0.99 for parents to 0.71 for the 21 crosses. Forage yield heterosis effects averaged 14% with a range of -4 to 39%, with 15 of 21 values significantly different from zero. Mid-parent heterosis effects for NDF averaged -0.5% with a range of -3.1 to 2.6%, with seven of 21 values significantly different from zero. Mid-parent heterosis effects for forage yield and NDF had a moderate correlation of 0.44. The change in correlations across generations suggested that a part of the genetic correlation between forage yield and NDF is regulated by linkage. The general lack of heterosis for NDF indicated that losses in forage yield associated with selection for low NDF can be restored by heterosis for forage yield, following hybridization between complementary low-NDF populations. Recurrent selection for NDF to improve intake potential of smooth bromegrass must be conducted in multiple populations.