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a Dép. de Phytologie, Univ. Laval, Sainte-Foy, QC, Canada, G1K 7P4
b Agriculture et Agroalimentaire Canada, Sainte-Foy, QC, Canada, G1V 2J3
c Agriculture et Agroalimentaire Canada, Normandin, QC, Canada, G8M 4K3
* Corresponding author (castonguayy{at}agr.gc.ca).
The regrowth interval between the last summer harvest and the fall harvest is a major determinant of alfalfa (Medicago sativa L.) persistence and spring regrowth. This study assessed the impact of the timing of a fall harvest on persistence, dry matter yield (DMY), and root carbohydrate and N reserves of field-grown alfalfa. Two cultivars (AC Caribou and WL 225) were harvested either only twice during the summer or three times with the third harvest taken 400, 500, or 600 growing degree days (GDD) after the second summer harvest. Plant density was not affected by a fall harvest across three production years (1997 to 1999). The DMY of the first harvest in the second production year (1998) was significantly reduced by prior fall harvests taken at 400 or 500 GDD. However, the seasonal DMY in 1998 was higher with the 500 or 600 GDD harvest management than with the two harvest system. Root dry weight (DW) was generally reduced by a third harvest in the fall, especially when taken at 400 GDD. Amounts of root carbohydrate reserves (raffinose family oligosaccharides [RFO], sucrose, and starch), as well as the amounts of total amino acids and soluble proteins were significantly reduced by harvesting at 400 or 500 GDD. Untimely fall harvest reduced alfalfa spring regrowth by impeding the accumulation of both carbohydrate and N reserves. Seasonal DMY remained advantageous as long as an interval of 500 or 600 GDD was kept between the last summer and the fall harvests.
Abbreviations: DM, dry matter • DMY, dry matter yield • DW, dry weight • GDD, growing degree days • FW, fresh weight • HPLC, high performance liquid chromatography • MCW, methanol–chloroform water • RFO, raffinose family oligosaccharides • TNC, total nonstructural carbohydrate • VSP, vegetative storage protein
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