Influence of Some Management Parameters on Glucosinolate Levels in Brassica Forage1

Abstract

AbstractBrassicaforage species may be used as alternative crops to provide supplemental herbage during the fall in the Northeast. However, these crops contain naturally occurring and potentially toxic constituents (glucosinolates and S‐methyl cysteine sulfoxide). Research was conducted to determine glucosinolate concentrations in 21 cultivars of five Brassicaspecies: turnip (Brassica rapaL.), rape (B. napusL.), kale (B. oleraceaL.), swede (B. napusL.), and Chinese cabbage type (B. cumpestris sensulatoL. ✕ B. rapaL.). Top (leaves plus stem), leaf (blade plus petiole), and root samples of each species were collected 60 or 90 days postseeding at Rock Springs, PA on a Hagerstown silt loam (fine, mixed, mesic Typic Hapludalf). Glucosinolates extracted from freeze‐dried, ground samples were bound to an anion exchange resin and hydrolyzed. The released glucose was quantified on an autoanalyzer. Of the five Brassicatypes, tops or leaves of kales had the lowest glucosinolate Concentration, 1.2 to 6.3 g kg−1; tops or leaves of rapes had the highest concentration, 2.9 to 11.9 g kg−1. Of the 21 cultivars, ‘Gruner’ and ‘Marrowstem’ kale, ‘Doon Major’ and ‘Calder’ swede, ‘Rangi’ rape, and ‘Sirius’ turnip had the lowest glucosinolate levels in tops and leaves; ‘Sipal,’ ‘Silona,’ and ‘Solo’ rape and ‘Green Globe’ and ‘York Globe’ turnip had the highest glucosinolate concentration. Roots of turnip and swede had glucosinolate concentrations as high or higher than those found in tops or leaves. All cultivars combined had significantly higher glucosinolate concentrations in 1980 than in 1979, and glucosinolate levels were significantly higher at 90 days than at 60 days postseeding each year. In a second field study, P and K fertilization at different rates only slightly increased glucosinolate concentration of ‘Fora’ rape, ‘Perko’ Chinese cabbage type, and Sirius turnip, whereas dry matter yields were dramatically increased (fourfold) by high N (132 kg N ha−1) and P (60 kg P ha−1). These data established that glucosinolate levels were sufficiently high (> 3 g kg −1) to produce inorganic isothiocyanate‐induced goiter (thyroid enlargement) in young growing sheep (Ovis aries) and cattle (Bos taurus). To minimize the potential for animal health problems, we conclude that Brassicaspp. should be fed in combination with other forages and that new varieties with reduced glucosinolate concentration should be developed.