C. Dhont^1*, P. Nadeau², Y. Castonguay², G. Bélanger² and F.-P. Chalifour¹

1Université Laval, Ste-Foy, QC, Canada, G1K 7P4 and ²Agriculture

and Agri-Food Canada, Ste-Foy, QC, Canada, G1V 2J3

Fall cutting management is a determinant factor of alfalfa persistence in northern climates. In eastern Canada, the existing recommendation is to not harvest alfalfa during a critical rest period of 4 to 6 weeks from early September to mid October. The negative effect of a fall harvest on alfalfa persistence and the following spring regrowth has been historically attributed to a reduction in the levels of organic reserves, especially non-structural carbohydrates (TNC). Although high TNC accumulation used to be associated with a superior persistence and a more vigorous regrowth, recent reports pointed out the role of specific carbon (C) and nitrogen (N) reserve components in winterhardiness and in the vigor of spring regrowth. Soluble sugars such as sucrose, raffinose and stachyose were found to be closely related to the acquisition of cold tolerance and are thought to have important cryoprotective functions. On the other hand, specific proteins and amino acids were shown to constitute N pools that markedly influence shoot regrowth and tolerance to defoliation.

This study was undertaken to assess the impact of contrasting fall harvesting treatments (with or without fall harvest) on regrowth in relation to the quantitative and qualitative evolutions in C and N reserves in alfalfa taproots during fall and winter. The experiment was conducted under simulated winter conditions in an unheated greenhouse, with two alfalfa cultivars(AC Caribou and WL 225) differing in winter adaptation. Plants were sampled in November, January and March and assessed for their regrowth potential and C and N reserves. Fall harvest markedly reduced shoot regrowth in both cultivars. Cryoprotective sugars (sucrose, raffinose and stachyose) accumulated to higher levels in the hardier cultivar (AC Caribou). The two harvest treatments did not affect the accumulation of TNC. However, there was a tendency for superior accumulation of cryoprotective sugars in plants harvested in fall. The fall harvest also significantly reduced amounts of total N and amino acids such as arginine and histidine. There were only minor differences in concentrations of total soluble proteins between harvest treaments. In both cultivars, we noted a marked accumulation of some proteins during fall and winter, especially a 19 kD MW. This accumulation however is affected by the cultivars and harvest treatments.

Our results confirm the importance of N reserves for spring regrowth of alfalfa. This information will help to optimize fall harvest management of alfalfa and to develop new selection approaches to improve its persitence in northern climates.