Heterosis and inbreeding depression for forage yield and fiber concentration in smooth bromegrass

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 NDF 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 self-pollinated to create S1 families. The seven parent clones showed considerable diversity, as measured by 329 random amplified polymorphic DNA (RAPD) 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 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. Midparent 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. Midparent 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.