Submitted to: Crop Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/27/1998
Publication Date: N/A
Citation: Interpretive Summary: Lack of persistence of white clover in managed permanent pastures of the northeastern U.S. often reduces profitability of dairy enterprises. Our goal is to increase sustainable profitability of U.S. dairy farms through identification of management practices that will increase longevity of white clover populations. White clover is generally the major source of legume protein and nutrients for grazing dairy cattle, and is a critical component of their nutritional intake. White clover populations may not be genetically diverse because the plant spreads and maintains itself primarily by vegetative propagation. We undertook a study to define the genetic variability (extent of genetic diversity) of white clover populations on 18 dairy farms in PA, NY, and VT. We used RAPD (random amplified polymorphic DNA) profiles, a procedure used in paternity tests, to provide data on the genetic makeup of those populations. Using sophisticated computer software to analyze molecular DNA marker variance i these populations, we found that the extent of genetic diversity was remarkable in every population. At the scale of individual dairy farms, all populations were different from each other; at the scale of states, the three groups of populations were indistinguishable. This reflects the ability of white clover to adapt to the similar ecology of the three states (similar topography and weather), and to the highly variable ecology that exists within each field (variable topography, soils, and neighboring plants). This adaptive response of white clover ensures that genetic diversity is maintained in intensively managed pastures.
Technical Abstract: Statistical analysis of molecular genetic markers for populations of white clover (Trifolium repens, L.) from 18 locations was undertaken to assess genetic variability of this obligately outcrossing species, which also spreads by vegetative propagation. Each population was located on a farm in Pennsylvania (PA), Vermont (VT), or New York (NY), in dairy pastures varying from 1 to 37 years of continuous intensive rotational grazing. Most of the locations had been seeded with white clover within one or two years of the sampling. Forty-eight individual white clover plants were sampled over two transects in 1 to 4 hectare fields in the summer and fall in four locations and in the fall in 14 locations. For each sample, genomic DNA was prepared and RAPD (random amplified polymorphic DNA) markers were amplified by the polymerase chain reaction to provide a data set of 54 RAPD markers (1056 white clover individuals). In each population, white clover genetic variability in a grassland sward was high and there was no evidence of clonal patches larger than 10 m across. A series of 28 overlapping "distance" data sets (genetic distances between pairs of individuals) was created and each data set was evaluated by AMOVA (Analysis of Molecular Variance software package). AMOVA apportioned the total variation among individuals within populations, among populations within states (PA, NY, or VT), and among states. The resulting proportion of variation residing between two populations was used as a measure of distance between them (interpopulation distance). A final interpopulation distance matrix of the 18 fall-sampled populations was constructed to calculate an "unweighted pair-group method, arithmetic average"-link tree (phenogram) using NTSYS (Numerical taxonomy and multivariate analysis