Plant Breeding 117, 251—254 (1998) © 1998 Blackwell Wissenschafts-Verlag, Berlin ISSN 0179-9541 Forage yield potential of alfalfa plants of varying crown size from old stands C. A. KIMBENG' and E. T. BINGHAM^ ' Agriculture Victoria, Private Bag 105, Hamilton, Victoria, Australia; ^Department of Agronomy, 1575 Linden Drive, University of Wisconsin, Madison, Wl 53706, USA With 2 tabies Received March 11, 1997/Accepted Juiy 7, 1997 Communicated by M. D. Hayward Abstract Alfalfa (lucerne) plants can persist from one growing season to the next because of their perennial crowns. Plants with large crowns often are selected for breeding purposes. The objective of this study was to determine the forage yield potential of plants with a full range of crown sizes sampled from old stands. This was accomplished by field evaluation of clones and Si progenies derived from plants of different crown sizes obtained from a 1 m^ of 5-year-old stands of cultivars 'Agate' and 'Magnum III'. Significant differences (P < 0.05) in dry forage yield (DFY) were found among entries in both tests. However, there was no consistent trend in DEY with respect to the crown size of the parental plants. Although the crown of space-planted clones dug after 16 months showed a strong association (r > 0.7, P < 0.01) with DFY, the crown size of these clones was poorly correlated with the crown size of their parental plants. These results suggest that selections based on crown size from older stands may not lead to an improvement in either crown size or forage yield. Crown health will remain an impor- tant selection criterion when plants are sampled from old stands; crown size, however, may not be as important. Key words: Medicago sativa old stands crown size — dry forage yield — The plant crown is an extremely important structure in the morphology and physiology of alfalfa (Medicago sativa L.). The alfalfa crown originates from contractile growth during plant estabhshment and contains the root-stem vascular con- tinuum (Teuber and Brick 1988). The crown is central to the development of root and shoot characteristics and whole-plant regrowth. Alfalfa and many other forage legumes can persist from one growing season to the next because of their perennial crowns (Groove and Carlson 1972, Montpetit and Coulman 1992). Thus, the survival of a plant depends on the ability of the crown to remain healthy and regrow after the plant has been exposed to various forms of biotic or abiotic stress factors. Despite its importance, relatively few reports exist that look at crown characteristics in alfalfa, perhaps because of the time and labour involved with crown observation (Pederson et al. 1984a). Forage yield in alfalfa has been correlated to the number of shoots produced per unit area (Foord 1985). Therefore, the crown which gives rise to these shoots can play a major role in determining yield. In a study involving alfalfa plant intro- ductions and cultivars, the highest positive correlation among crown characteristics was observed between crown width and the number of crown shoots (Marquez-Ortiz et al. 1996). Busch and Davis (1969) evaluated crosses between creeping-rooted and noncreeping alfalfa and observed that crown and root weight were positively associated with most ofthe aerial charac- teristics. According to Chloupek (1982), indirect selection for root system size in alfalfa was effective in increasing forage yield. Root system size was determined using electrical capaci- tance, but Kendall et al. (1982) did not obtain consistent esti- mates of root dry weight using this method. The present study differs from the above studies in that plants were removed from a 5-year-old stand and their yield potential evaluated in clonal and Si progeny tests. The objective was to detennine the relationship between crown size and forage yield potential of alfalfa plants from old stands. Materials and Methods In the autumn of 1993, all plants in a 1 m^ plot of two alfalfa cultivars, 'Agate' and 'Magnum IIF, were removed from a 5-year-old field plot at the University of Wisconsin Agricultural Research Station, Arling- ton, Wl, USA. 'Agate' tends to have a broader crown and is more autumn dormant than 'Magnum III', but both cultivars have exhibited a similar level of winter hardiness and persistence in our trials. A total of 19 plants in 'Agate' and 17 in 'Magnum IIP were dug using a shovel in adjacent field areas. Plants were washed free of soil, trimmed to a uniform topgrowth of 5 cm and taproot length of 12 cm, air-dried for a few minutes, and weighed to obtain crown size. Crown size is defined here as the weight of the 12 cm crown and root segment, together with 5 cm of stubble stem. The trimmed plants (crowns) were transplanted to pots containing 3:1:1 of soil:sand:peat mixture in the greenhouse. These plants became the parents from which vegetative cuttings (clones) and S, progenies were derived for field evaluation. Cuttings were pruned to a uniform length, and transplanted into the field at the University of Wisconsin Agricultural Research Station, Arlington, Wl, on 6 June 1994. The transplants were spaced 50 cm within rows and 1 m between rows. Thefieldlayout was a randomized complete block design with four replicates. A plot consisted of a single row, 2 m long with five cuttings per plant, giving an experimental total of 20 cuttings per plant. Excess cuttings were used to replace weak transplants and to plant a border around the experiment. One initial 'Agate' plant did not survive, and cuttings of another 'Agate' plant did not root well. Consequently, 17 plants from each cultivar were represented in this clonal study. Plots were harvested at about the 10% bloom stage on 16 August 1994, 14 June, 18 August and 12 September 1995. DFY was measured for each plant in a plot. On 4 October 1995, all the plants from plots in the first replicate were dug with a shovel. The crown weight of these plants were recorded as previously described to obtain a measure of crown size after 16 months of growth as spaced-plants. In the S| study, sufficient seeds (at least 150 seeds) for field testing were obtained only from 16 plants in 'Agate' and 10 plants in 'Magnum Iir. The 'Magnum III' parents that failed to yield enough S, seeds were observed closely and found to be either partially or completely pollen sterile. Forage yield was evaluated at the University of Wisconsin Agri- cultural Research Station, Arlington, Wl, in 1-m long single row plots. U. S. Copyright Clearance Center Code Statement: 0179-9541/98/1703-0251 $ 14.00/0