'Marshall' ryegrass, a top variety in nine southern states, was developed by mass selection. For 29 years, seed produced each spring on a small field of annual ryegrass at Holly Springs, Mississippi was used to plant the field the following fall. Gradually, the better plants replaced the weaker ones and the enhanced germplasm named Marshall was released as an improved cultivar.
To be effective, mass selection requires a variable, sexual, seed producing population of cross-pollinated plants. Genetic male-sterile genes can be used in self-pollinated species such as the soybean to create a cross-pollinated population (Brim and Stuber 1973). Enhancing such populations requires effective screens for the characters sought, effective methods for intermating the selected plants, and recurring cycles of these procedures to build up the frequency of the genes controlling the traits being selected.
`Grimm' alfalfa, the first winter-hardy alfalfa cultivar bred in the USA is a good example of the use of mass selection to enhance germplasm (Tysdal and Westover 1937). In 1858, Wendelin Grimm planted on his newly acquired land in Chaska, Minnesota, the 15 pounds of alfalfa seed he had brought from south Germany where the winters were mild. Only a few plants survived the first severe winter but his bees intermated the survivors. Grimm harvested and planted the seed produced and completed the first cycle of mass selection. Repeated cycles of winter screening and bee intermating produced 'Grimm' alfalfa, the first cultivar adapted to the northern USA.
Using mass selection to enhance traits desired in a good population avoids introducing undesirable traits that often come with exotic germplasm. Wayne Hanna, ARS geneticist at Tifton, Georgia, used Pennisetum glaucum var. monodii to supply dominant genes for immunity to rust, Puccinia substriata Ell. and Barth. var. indica Ramacha and Cumm. and leaf spot, Pyricularia grisea (Cke) Sarc (Hanna et al. 1985). But crosses required to transfer the disease resistant genes also brought with them undesirable monodii genes for seed shattering, short-day sensitivity, sterile cytoplasm and susceptibility to Helminthosporium leaf spot. Five backcrosses and 9 selfed generations were required to produce the resistant 'Tift 85A' and 'Tift 85B' used to produce 'Tifleaf 2' pearl millet.
Mass selection can retain in a population desirable characters not selected for or against. Increasing yield in Pensacola bahiagrass by mass selection did not alter its digestibility.
Forages are better suited to mass selection enhancement than most crops. Many forage species are highly variable and uniformity is usually not required. Many are perennials and most important characters such as whole plant yield are multigenic in their inheritance. Seed yield is usually less important, characters can be selected visually, and cycles can often be shorter.
Mass selection has usually not been recommended as a breeding method to increase yield because heritability for spaced plant yields has usually been low. But heritability can be improved by keeping the environment uniform for all plants in a population from the time the seeds are planted until the spaced plants are selected. This has been a major objective as we have tried to improve mass selection as a plant breeding method to increase forage yields.
In 1960, we decided to try to use mass selection to improve the yield of Pensacola bahiagrass, a seed-propagated, perennial, pasture grass widely grown in the deep South. At that time, few plant breeders would have advised its use as a plant breeding method.
From 1960 to 1988, we made a number of modifications and restrictions in conventional mass selection that have made it possible to produce one cycle per year with the same progress that we made in the beginning when we used three years per cycle. The plant breeding method resulting from these modifications of mass selection we call 'recurrent restricted phenotypic selection (RRPS)' (Burton 1982).
RRPS in a closed-population of diploid, cross-pollinated Pensacola bahiagrass each year has effectively intermated the 200 largest visually-grid-selected plants from 1000 spaced plants, the largest seedlings in 20,000 RRPS intermated seedlings and has increased forage yield 16% per cycle through cycle 14 in spaced-plant-tests and 5% per cycle through cycle 9 in 3-year seeded-plot-tests.
The enhanced germplasm developed by RRPS can provide breeders seed for a new cultivar equal to that cycle in performance. 'Tifton 9' Pensacola bahiagrass officially released as an improved cultivar in 1988 is an increase of cycle 9 of RRPS applied to broadbased population A of Pensacola bahiagrass, a blend of seed from 39 Georgia farms. In a 3-year replicated small plot test clipped to simulate grazing 'Tifton 9' produced 47% more forage than the Pensacola bahia control. It was also leafier and much more vigorous in the seedling stage. In 1980, a selfed progeny of two-clone Pensacola bahia F1 hybrid 15-21 x 17-29 that had topped a 3-year 9 x 9 lattice square 2-clone hybrid test was used to start the narrow based population B. RRPS was applied to population B as to population A. In the 1987 spaced-plant-population-progress test, the closed-population B cycles 1 and 7 yielded as well as closed-population A cycles 6 and 12. Thus, the selected narrow-based population B responded to RRPS as well as the broad based population, A and started at a much higher yield level.
Experience with mass selection and its improved RRPS indicates that germplasm is malleable and can be enhanced with recurrent cycles that intermate superior plants selected with an effective screen.