Fall cutting management affects yield and persistence of alfalfa in Atlantic Canada

The existing recommendation to avoid harvesting alfalfa during a critical fall rest period, based on calendar dates, is under review in Canada and adjacent areas of the United States. The effect on yield and persistence of four fall cutting management treatments (based on cumulative growing degree-days between harvests) was investigated for two cultivars of alfalfa (Medicago sativa L. 'Apica' and 'Oneida VR') at five sites in Atlantic Canada. The average seasonal DM yield for the five sites in the first production year was 1.6 to 2.3 t ha(-1) greater with an additional harvest as compared with the two-harvest system (Truro, Nappan, Fredericton, and Charlottetown) and the one-harvest system in St. John's. Seasonal DM yield benefits were reduced over time; by the third production year, there was no increase in seasonal DM yield with an additional harvest. The seasonal DM yield increased with increasing the interval between the final harvest and the previous one. Taking an additional harvest resulted in higher winter plant mortality between the first and second production years in Truro and Nappan, and the third and fourth production years in St. John's. At the other two sites (Charlottetown and Fredericton), and for the first three production years in St. John's, winter plant mortality was unchanged with an additional harvest but regrowth potential in the following year was reduced. At the two sites that experienced a severe winter (Nappan and Truro), the cultivar Oneida VR was more adversely affected by the additional harvest than Apica. Our results indicate that in Atlantic Canada, taking an additional harvest increases the risk of winterkill. If an additional harvest is to be taken, increasing the interval between the final harvest and the previous one to at least 500 growing degree-days will decrease the risk of winterkill and might have a positive effect on next year's regrowth.