Additive Genetic Correlation Estimates Among Alfalfa Traits
Heathcliffe Riday and E. Charles Brummer.
Raymond F. Baker Center for Plant Breeding, Agronomy Dept., Iowa State University
Medicago sativa subsp. falcata (particularly western Eurasian germplasm) forms a heterotic
pattern with elite subsp. sativa breeding material (Riday and Brummer, 2002a, 2003a). Two weaknesses
of falcata germplasm relative to sativa are decumbent growth habit and slower regrowth after cutting
(Riday and Brummer, 2002b, 2003b). A few falcata populations have been noted with moderately
acceptable growth habit (Riday and Brummer, 2003b). Disappointingly, however, no falcata populations
have been studied that show regrowth after cutting near sativa performance levels. To fully utilize falcata
germplasms, improvement of these two character traits will be necessary. A concern we had was that
there might be negative additive genetic correlations (i.e., correlations of breeding values [Falconer and
Mackay, 1996]) between biomass yield and regrowth or/and height. The objective of this study was to
develop methods to compute these correlations and to analyze them among yield, height, and regrowth.
In alfalfa, reported genetic correlations (rG) are usually based on total genetic variances (σ2G). To
calculate additive genetic correlations (rA), additive genetic variances (σ2A) are needed. These variances
are more difficult to calculate in autotetraploids than in diploids and require halfsib family, parentoffspring, and parent data to estimate (Levings and Dudley, 1963). Even more complicated is calculating
the additive covariances between two traits (T1, T2; σA;T1,T2). We present SAS codes to calculate additive
covariances in autotetraploids and their standard errors based on work presented by Holland (2003).
Based on 107 alfalfa genotypes from 30 populations (2-5 genotypes/population) we estimated
average additive genetic correlations between biomass yield, plant height at harvest, and regrowth. This
experiment was planted at two locations and data was collected during two years post establishment.
Yield was measured three times during both years. Height was measured three times at harvest and once
in the late autumn (before frost) during each year. Regrowth was measured during spring emergence and
after each of the three harvests during both years (Riday and Brummer, 2003b).
Yields at different harvests were highly correlated with each other. The same was observed
between heights, except autumn height. Regrowth measures, except in the spring, were highly correlated
with each other. Few additive genetic correlations between height and yield were observed. Third
harvest yield was an exception, having positive correlations with second and third harvest heights.
Second period regrowth (i.e., regrowth after second cut) was positively correlated with all yield measures.
Due to a lack of negative additive genetic correlations, we conclude that intensive selection for increased
regrowth and plant height should not have an adverse effect on biomass yield.
References
Falconer, D.S. and T.F.C Mackay. 1996. Introduction to quantitative genetics. Longman, Essex, England.
Holland, J. 2003. Example SAS programs for estimating genotypic and phenotypic correlations and their
standard errors. http://www4.ncsu.edu/~jholland/correlation/correlation.html (16 June 2003).
Levings, C.S., and J.W. Dudley. 1963. Evaluation of certain mating designs for estimation of genetic
variance in autotetraploid alfalfa. Crop Sci. 3:532-535.
Riday, H., and E.C. Brummer. 2003a. Heterosis in a broad range of alfalfa. Crop Sci. [in review].
Riday, H., and E.C. Brummer. 2003b. Morphological Variation of Medicago sativa subsp. falcata
genotypes and their hybrid Progeny. Euphytica [submitted].
Riday, H., and E.C. Brummer. 2002a. Forage yield heterosis in alfalfa. Crop Sci. 42:716-723.
Riday, H., and E.C. Brummer. 2002b. Heterosis of agronomic traits in alfalfa. Crop Sci. 42:1081-1087.