Soviet agriculture and US-Soviet grain agreement

In August the Soviet government agreed to the second one-year extension of the US-USSR grain agreement, which has regulated grain trade between the two countries since 1976. Although a net exporter of grain for the best part of this century, the USSR has become increasingly dependent on imports since the beginning of the seventies. What are the reasons for this? What should the US be looking for when a new, long-term agreement is negotiated?