Monsanto Co. and Alnylam Pharmaceuticals Inc. have formed an alliance to advance biological technologies for agricultural use. Monsanto will pay $29.2 million upfront to Alnylam, a Cambridge company that aims to develop drugs with its RNAi gene-silencing technology. It has no products on the market yet. Monsanto gets exclusive rights to use Alnylam’s technology and intellectual property in agriculture. Biologicals complement or replace chemical products in agriculture and represent a growing segment of a $1.7 billion annual market, Alnylam said. Alnylam is eligible to receive milestone payments, additional funding, and royalty payments over 10 years.