Wednesday, November 15, 2006

Optimizing Sativa and Falcata Alfalfa Subspecies Ratios for Leafcutter Bee Pollination to Maximize Hybrid Seed Ratios.

Heathcliffe Riday, US Dairy Forage Research Center, 1925 Linden Drive West, Madison, WI 53706

Developing seed production schemes which capture and maintain desired alfalfa (Medicago sativa) heterosis for utilization by growers has been problematic.  The semi-hybrid seed production scheme is one proposed means of capturing heterosis in alfalfa cultivars.  This scheme proposes mixing equal quantities of seed of the two heterotic breeding populations in the syn three seed production field.  Pollination of this field should theoretically yield 50% within and 50% between (i.e. hybrid) population seed.  Previous research has shown that alfalfa subsp. sativa (purple flowered) by subsp. falcata (yellow flowered) hybrids produce biomass yield heterosis.  Using these two subspecies, semi-hybrid seed production and cultivar development techniques could be implemented.   However, previous research with leafcutter bees (Megachile rotundata), the most commonly used alfalfa pollinator, demonstrated a pollinator preference for alfalfa subsp. sativa over falcata.  This study measured hybrid versus non-hybrid seed produced using three different field plant ratios of sativa and falcata: 10:90, 25:75, and 50:50.  Results of this study indicate that despite pollinator preference, 40% hybrid seed production may be possible compared to 50% hybrid seed expected with no pollinator preference.  This study further suggests that a slight excess of sativa versus falcata plants may produce the highest hybrid to non-hybrid seed ratios