WELLINGTON (REUTERS) - A businessman was sentenced to eight-and-a-half years in prison on Wednesday (March 23) for threatening to contaminate infant formula in New Zealand, the world's largest dairy exporter.

Jeremy Kerr pleaded guilty to two counts of blackmail after sending threatening letters to New Zealand's national farmers' group and dairy giant Fonterra in 2014.

The messages were accompanied by packages of infant formula laced with pesticide 1080 and demanded that use of the toxic pesticide be stopped. Kerr's business sold a competing pesticide.

The police said in October they had arrested a man after an 11-month investigation.

A court in Auckland, New Zealand's largest city, found that Kerr was motivated by financial gain.

"The impact on Fonterra, Federated Farmers and the potential impact on New Zealand's trade relationships with China and other countries was extremely serious," said Justice Geoffrey Venning in sentencing Kerr.

China is the biggest buyer of New Zealand dairy products. The head of a New Zealand exporters' group said last year there had been a fall in Chinese demand after the threat to infant formula, which is prized among China's middle class.