The Concentration of Yeast Assimilable Nitrogen In Wine Grape Juice Can Be Manipulated Using Deficit Irrigation, Crop Load Reduction and Nitrogen Fertilization.

The concentration of yeast assimilable nitrogen (YANC) in wine grape juice is a critical determinant of the rate and efficiency of fermentation and, ultimately, wine quality.  The implications for YANC of deficit irrigation, crop load reduction and nitrogen fertilization were examined in two trials using Merlot (Vitis vinifera L.) vines established on a sandy soil in the Okanagan Valley, Canada.  In trial 1, irrigation treatments (replicated six times) were applied once every three days from May to October to meet either 100% or 50% of evaporative demand, as determined by electronic atmometer; 50% of evaporative demand was attained by reducing the frequency, intensity or duration of irrigation in three separate treatments.  Subplots compared the effect of thinning crop loads to one cluster/shoot versus leaving crops un-thinned.  In trial 2, irrigation treatments (replicated six times) were applied once daily from May to October to meet either 100% or 50% of evaporative demand.  Subplots tested the effect of N applied as urea at four rates (0, 5, 10 or 20 g N/vine/year), split equally between bud break and full bloom.  Grape juice YANC was determined at harvest in 2007 through 2010 (trial 1) or 2006 through 2010 (trial 2).  In trial 1, YANC was highest under an irrigation regime that met 50% of evaporative demand by delivering water from one of two alternating drippers every three days, although treatment differences were not always statistically significant.  In all but one measurement year, crop load reduction also increased YANC.  In trial 2, deficit irrigation increased YANC in two of the five measurement years.  Only the highest rate of N addition was usually sufficient to increase YANC.  Clearly, deficit irrigation can increase YANC in wine grape juice, but its effectiveness can vary by year and depends on irrigation regime.  Fertilization with N or crop load reduction can be used to augment YANC above the 140 mg/L threshold considered sufficient to avoid ‘stuck’ fermentation.