Switchgrass (Panicum virgatum L.) is a perennial grass native to the North American tallgrass prairie and broadly adapted to the central and eastern USA. Transfer of germplasm throughout this region creates the potential of contaminating local gene pools with genes that are not native to a locale. The objective of this study was to identify structural patterns and spatial variation for molecular markers of switchgrass populations from the northern and central USA. Forty‐six prairie‐remnant populations and 11 cultivars were analyzed for random amplified polymorphic DNA (RAPD) markers. Although there was significant population differentiation, little of this variation was associated with geographic regions. A small amount of population differentiation was associated with hardiness zones and ecoregions, suggesting that a recent proposal to use these two criteria for defining plant adaptation regions has merit for defining gene pools and seed‐transfer zones of switchgrass. Cultivars of switchgrass cannot be differentiated from prairie‐remnant populations in the northern and central USA on the basis of RAPD markers, indicating that they are still highly representative of natural germplasm. Seed sources of switchgrass can be moved considerable distance within hardiness zones and ecoregions without causing significant contamination, pollution, swamping, or erosion of local gene pools.
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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.
Neutral detergent fiber (NDF) is the most effective and measurable predictor of animal intake. About 70% of the variation in animal production is attributed to differences in intake potential of livestock feed. Sixteen molecular markers associated with NDF in four divergently selected smooth bromegrass (Bromus inermis L.) populations were selected from a previous study for further examination for possible use in a marker-assisted selection (MAS) breeding program. The objectives of this experiment were to confirm the association of previously identified markers to NDF concentration in smooth bromegrass populations and to develop a marker-based selection index. Marker and NDF data were analyzed and marker indices were constructed using data from the current and previous studies. Index scores were used to rank genotypes and create selection differentials on the basis of phenotypic data. Marker frequencies calculated on a subpopulation basis between the previous and current studies were highly unrepeatable. Nevertheless, of the 14 groups of marker indices, seven of which accounted for pedigree structure, one marker index appears to provide the greatest potential for use in MAS to reduce NDF across all four populations of smooth bromegrass. Where pedigree structure is known, selections made using this index would lead to the largest expected response per year and eliminate the need to collect phenotypic data for as long as linkage relationships remain intact. Where pedigree structure is unknown, a general marker index may be used, or phenotypic data can be utilized along with a marker index. However, inclusion of phenotypic data would necessitate a cost-benefit analysis.
Resources are always limited in plant breeding programs, limiting the number of plots or plants that can be utilized in recurrent selection. Replication of selection units in space or time provides a mechanism to improve the precision of measurements. For smooth bromegrass (Bromus inermis L.), neutral detergent fiber (NDF) is the most effec-
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