Petchell, who was speaking at last month's Euroasper conference in Coventry, told delegates that the firm's breeding programme has now whittled more than 300 potential hybrids down to just 45 - many of which have outperformed the widely-used varieties Backlim and Gijnlim.
He explained: "We have been assessing these trials for six years now. It was clear from very early on that a number of these hybrids would not work, but after six years we were able to count the number of surviving plants, compare them to standard varieties and make a fair comparison."
"We are looking at new hybrids that are going to increase efficiencies and improve returns."
Petchell said that the variety Guelph Millenium, which grower John Chinn describes as Hargreaves' "greatest find", is the firm's "most successful hybrid to date."
Bred at the University of Guelph in Ontario, Canada, it is hardy and tolerant to cold winters but also performs well in hot weather and late seasons.
Petchell added that it has "the best head tightness of all of them - with an average score for head tightness higher than the Dutch controls in our trials for each full season".
He also said that, in terms of Class 1 spears and marketable yields, Guelph Millenium out-yields all of the competing green types in the North American market, as well as Backlim in the UK.
Its average marketable yield is 3.4 tonnes per hectare, which compares favourably with 2.5 t/ha for Backlim and 3.3 t/ha for Gijnlim.
Petchell told delegates that other promising new varieties include the German-bred Mondeo which, he said, is "a real competitor to Gijnlim".
Its seasonality is similar, but Mondeo has a marketable yield of 13.2 t/ha - compared with Gijnlim's 12.4t/ha.
Other new varieties include the German-bred Ariane and the New Zealand-bred Pacific 2000, which has a sweet flavour.
Petchell added that JP 33 and JP 58 are two varieties that are still some way from being released to the industry, but which are so far showing yield increases above the industry standard of 30 per cent and 50 per cent respectively.