Evaluation of pre-harvest desiccation strategies in red clover (Trifolium pratense L.) and white clover (Trifolium repens L.) seed crops

Abstract

Desiccation with diquat about one week before seed harvest has been common practise in Norwegian clover seed production. However, after withdrawal of diquat in 2020, clover seed growers no longer have desiccators available. In 2019 and 2020, six field trials in red clover and two field trials in white clover were carried out to evaluate alternative chemical products at different rates and at two different spraying dates, either early at 50% mature seed heads and / or late at 65% mature seed heads. Products included, either for one or two years, was Spotlight Plus (carfentrazonethyl), Beloukha (pelargonic acid), Glypper (glyphosate), Gozai (Pyraflufen-ethyl), Harmonix LeafActive (acetic acid), Harmonix FoliaPlus (pelargonic acid), Flurostar (fluroxypyr) and Saltex (sodium chloride) and liquid urea-based fertilizers. In addition, swathing was examined as an alternative in two red clover trials in 2020. While none of the tested chemicals were superior to diquat, the most promising alternatives were Harmonix FoliaPlus and Harmonix LeafActive in red clover or Harmonix FoliaPlus in white clover. Although usually less effective than these products, Beloukha also had an acceptable desiccation effect, especially when sprayed early and late. Swathing before harvest, using finger bar cutters, was an effective drying method under favourable weather conditions.