By Michelle Nichols UNITED NATIONS (Reuters) – A senior U.N. official had “constructive discussions” in Moscow with Russian First Deputy Prime Minister Andrei Belousov on facilitating Russian grain and fertilizer exports to global markets, U.N. spokesman Stephane Dujarric said on Tuesday. The U.N. official, Rebecca Grynspan, is now in Washington for more talks, he said. Grynspan is head of the U.N. Conference on Trade and Development (UNCTAD) and coordinator of the U.N. Global Crisis Response Group on Food, Energy and Finance that aims to help countries face economic shocks from the Ukraine wa…