The weaker-than-expected launch of Monsanto's latest generation of genetically modified soybeans has prompted Plant Health Care to warn that sales of a key seed treatment will miss targets.

Shares in Plant Health Care, which while based in the US is listed in London, slumped by more than one-third to their lowest since 2006.

The company said that sales through Monsanto of its Harpin product, which improves crop resistance to nematode worms, might fall more than 50% below target this year after "lower-than-expected" takings in the spring sowing season.

The warning, which prompted Plant Health Care to lower its hopes for full-year profits, follows reports of unrest among farmers at prices for Monsanto's Roundup Ready 2 soybeans, which boast higher yields as well as resistance to herbicides, so allowing farmers to spray off weeds.

'Rethink needed'

At Evolution, analyst Philip Sparks said that the "slow start" for Roundup Ready 2 would prompt him to cut to 50m acres, from 80m acres, his forecast for the amount of next-generation seed coated with Harpin that Monsanto will sell in 2015.

"Harpin and Roundup Ready 2 are valuable products, but Monsanto may need to rethink how they are marketed to farmers," Mr Sparks said.

He cut to 170p his target for Plant Health Care shares, and reduced to "neutral" from "buy" his rating on the stock.

'Pricing issues' 

A person close to Plant Health Care, noting "factors related to pricing issues", highlighted Monsanto's strategic revamp in the face of tumbling sales of its Roundup herbicide, and some "teething problems" in seeds.

However, Plant Health Care dismissed the setback as a "one-time event" and forecast that Monsanto would by the year end run through stocks of Harpin it had built up.

"Deliveries of our Harpin product will realign with expectations beginning with the 2011 marketing year," Plant Health Care said.

The company's shares closed 64p lower at 110p in London, their lowest since September 2006.