Selecting for Winter Hardiness in Non-Dormant Alfalfa.

 

Mindy A. Ryerson and E. Charles Brummer

Department of Agronomy, Iowa State University, Ames, IA 50011

 

Alfalfa in the Midwestern United States typically becomes dormant in autumn to allow the plant to survive the stresses of winter. Beginning in the late summer the plant responds to decreasing sunlight and cooling temperatures by slowing its growth as it goes into dormancy. A non-dormant plant does not have this period of slowed growth, which is preferable from a fall yield standpoint to those that exhibit dormancy. A strong correlation exists between dormancy as measured by fall height and winter hardiness. Non-dormant alfalfa characteristically exhibits a large amount of fall growth; however they are much less winter hardy. The objective of this study was to select among four non-dormant cultivars for winter hardiness. Four non-dormant cultivars (5939, GT13RT, CUF101, and Magna 8) were seeded into 1m by 4m plots at 15 kg ha-¹ in April 1997 at Ames, IA. The following spring surviving plants were selected for spring vigor and these plants were removed from the field and intercrossed within each variety. The resulting seed was then planted in the field the summer 1998. In spring 1999, selections for spring vigor were again made and the plants were intercrossed to form cycle 2 populations.  The seed from these crosses was then planted with the C0 seed in trials at both Ames and Nashua, IA in August 1999. Height was measured in October 1999 to assess fall dormancy (Table 1). The C0 generation had a higher average height than the C2 generation for all four varieties at Ames, but only CUF101 and 5939 differed at Nashua. In April 2000 spring vigor was assessed visually.  Because the 1999-2000 winter was exceptionally mild, few plants of either cycle died, so selection was made by evaluating spring vigor. The C2 plants for all varieties had much higher spring vigor values at Ames. The C2 of GT13RT and Magna 8 were not higher than C0 at Nashua. The values observed thus far suggest that selection for winter survival is possible in non-dormant cultivars and that decreased fall growth is likely to occur concurrently. We are continuing this study to evaluate changes in these populations more extensively.

 

Table 1. Fall height and spring vigor of cycle 0 and Cycle 2 populations selected for winter survival from four non-dormant cultivars.

 

	Fall Height				Spring Vigor
	Ames		Nashua		Ames		Nashua
Cultivar	C0	C2	C0	C2	C0	C2	C0	C2
	cm				score †
CUF101	16.76	11.98**	6.22	4.44*	2.86	5.02****	2.88	4.04*
GT13RT	13.12	9.14*	5.34	4.71	3.66	5.58****	3.48	4.30
Magna 8	13.12	9.50*	5.08	5.48	3.44	5.56****	3.76	4.54
5939	18.02	11.86***	5.33	3.69*	2.94	5.20****	2.38	3.56*

† Score: 1=very weak to 9=very vigorous.

*, **, ***, **** - C0 and C2 are significantly different at P=0.05, 0.01, and 0.001 respectively.