This summary is based on reports from exporters for the period December 21-27, 2018. Wheat:  Net sales of 593,000 metric tons for 2018/2019 were up 15 percent from the previous week and 3 percent from the prior 4-week average.   Corn:  Net sales of 503,100 MT for 2018/2019 were down 70 percent from the previous week and 65 percent from the prior 4-week average. Soybeans:  Net sales of 1,051,500 MT for 2018/2019 were down 56 percent from the previous week and 39 percent from the prior 4-week average.   Daily Sales Not Announced:  Please note that for week ending December 27, 2018, export sales of 808,000 MT of soybeans for delivery to China during the 2018/2019 marketing year would have been report...