Influences of Selection Method and Nitrogen Environment on Breeding Alfalfa for Increased Forage Yield and Quality

Alfalfa ( Medicago sativa L.) can use both mineral N and N 2 when effective Rhizobium bacteria are present. Our group previously increased dry matter and N assimilation in unadapted alfalfa germplasm sources by using independent culling levels to choose individual plants with high forage dry weight and N concentration when grown sequentially under N 2 ‐ and NH 4 NO 3 ‐dependent conditions. The objectives of the present study were (i) to assess the potential of other selection methods for improving forage yield and protein content, (ii) to evaluate the need for screening under both N 2 ‐ and NH 4 NO 3 ‐dependent conditions, and (iii) to test whether the original breeding protocol could be applied successfully to agronomically adapted germplasm. First, data from ‘Moapa 69’ plants grown sequentially under N 2 ‐ and NH 4 NO 3 ‐dependent conditions were used for simulated selection of single and multiple traits in each N environment. Results predict that single trait selection will produce undesirable changes in the unselected trait and that independent culling levels will increase forage dry weight and N concentration in both N environments. The greatest progress was made with a base index selection system, but this system required an excessive number of N determinations. Second, small populations were developed from Moapa 69 using independent culling levels to select for high forage dry weight and increased forage N concentration in plants grown under N 2 , NH 4 NO 3 , or a sequence of N 2 ‐ and NH 4 NO 3 ‐dependent conditions. After two cycles of phenotypic recurrent selection, the resulting germplasm pools were evaluated for growth on both N 2 and NH 4 NO 3 . Sequential selection on N 2 and NH 4 NO 3 increased forage dry weight more than 24% on both N sources ( P < 0.10); selection only on NH 4 NO 3 increased forage dry weight significantly ( P ≤ 0.10) only under NH 4 NO 3 ‐dependent conditions; but selection only on N 2 did not improve forage dry weight in either N environment significantly ( P ≤ 0.10). Forage N concentration was not increased by selection in either N environment. Finally, large populations from Moapa 69 grown sequentially on N 2 and NH 4 NO 3 were synthesized through three cycles of phenotypic recurrent selection using independent culling levels for high forage dry weight and greater N concentration. The third cycle population produced 54% more forage dry weight (P < 0.001) with 9% higher forage concentration ( P < 0.001) and 67% more total crude protein ( P < 0.001) than the original Moapa 69 in results averaged across both N environments in the glasshouse. The conclusion from these studies is that selection for dry weight and N concentration must be done on both N 2 and NH 4 NO 3 dependent environments to develop alfalfa with increased forage yield and higher crude protein concentration in both N environments under glasshouse conditions.