South Korea\'s exports of agricultural machinery and parts jumped more than 20 percent in the first half of the year on strong demand in the U.S. and China, the government said Sunday.    The country sold US$412 million worth of farm machines and parts in overseas markets during the January-June period, up 20.6 percent from a year earlier, according to the Ministry for Food, Agriculture, Forestry and Fisheries.    The ministry attributed the growth in exports to improved post-sales services and aggressive marketing in key markets, such as the U.S.    Out of all outbound shipments, 38.6 percent of products were sold in the U.S. China came in next with a portion of 8.5 percent, followed by Japan with a 3.6 percent share, the ministry said.    For the full year, exports of farm machinery are expected to surge 23 percent to reach $750 million, according to the ministry. Last year, South Korea shipped $608 million worth of farm machines and parts.    The country seeks to boost its exports of farm machines to $1 billion by 2015.