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

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 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.

Genetics of Inbreeding Effects in Smooth Bromegrass

“…Although studies on other plant species have reported enhanced performance in outcross mating relative to parents (McDonald et al, 1952;Hufford and Mazer, 2003;Berdahl and Ray, 2004), Casler et al (2005) reported negative mid-parent heterosis effects for neutral detergent fiber (averaged −0.5% with a range of −3.1 to 2.6%). However, there was variation among genotypes.…”

“…However, there was variation among genotypes. Although studies on other plant species have reported enhanced performance in outcross mating relative to parents (McDonald et al, 1952;Hufford and Mazer, 2003;Berdahl and Ray, 2004), Casler et al (2005) reported negative mid-parent heterosis effects for neutral detergent fiber (averaged −0.5% with a range of −3.1 to 2.6%). The lack of significant difference between S 1 and OP, and the lower performance of OP compared with parental clones for some traits in the present study, suggests low heterosis for these traits.…”

“…Christie and Kalton (1960) evaluated inbred segregates of 10 representative clones of bromegrass (Bromus inermis Leyss) to find that seed weight, fertility, and seed yield exhibited substantial inbreeding depression, averaging 21, 20, and 53% respectively from S 0 to S 1 for seven families and 19, 24, and 66% respectively from S 2 to S 3 for three families. In smooth bromegrass, Casler et al (2005) reported that S 1 family means were 18 to 33% lower than their parent means for forage yield and were 1.5 to 3.6% lower for neutral detergent fiber. On the other hand, cross-breeding between populations often has positive effects on fitness-related traits (Sprague, 1983;Levin, 1984).…”

“…They observed that the greatest inbreeding depression within a population was generally observed either early, during seed maturation, or late, during the growth and reproduction stages. In smooth bromegrass, Casler et al (2005) reported that S 1 family means were 18 to 33% lower than their parent means for forage yield and were 1.5 to 3.6% lower for neutral detergent fiber. Mitchell-Olds and Waller (1985) stated that inbreeding depression usually appears in early stages of the life cycle but is purged from later stages.…”

Evidence for Heterosis in Italian Ryegrass (Lolium multiflorum Lam.) Based on Inbreeding Depression in F2 Generation Offspring from Biparental Crosses

“…Dry matter yield in tall fescue declined by 6% from the Syn1 to Syn2 generations of four-parent synthetic [29]. In smooth bromegrass (Bromus inermis L.), the average level of heterosis for herbage yield in diallele crosses between seven parents was 14% with a range of −4 to 39%, and the level of inbreeding depression based comparison of selfed progenies with parents was 18%-33% [30]. The difference between species may be attributable to the number of genotypes used for crossing and different mechanisms of genetic control.…”