Nutritional evaluation of five species of forage brassica

Abstract

Despite a growing interest in ruminant nutrition and the ongoing, widespread use of forage brassicas, there is an absence of data that adequately describe the nutritional value of modern brassica cultivars in New Zealand. Five forage brassica species represented by 30 cultivars were evaluated in single-site, single-year trials in the South Island. Replicated plots of leafy turnips, bulb turnips, rape, swede and kale were harvested at 53, 91, 98, 181 and 186 days after sowing, respectively. Whole plants were dissected to leaf, stem and bulb and/or petiole and components assessed for nutritive value. Quality differed significantly among and within brassica species. Within species, percent dry matter (DM) was consistent for rape, bulb turnip and leaf turnip, but differed significantly among cultivars in kale and swede. Mean metabolisable energy (MJ ME) content at harvest was highest for swede (13.8 MJ ME/ kg DM) and lowest for kale (11.2 MJ ME/kg DM). For all brassica species, fibre content as estimated by neutral detergent fibre (NDF) content was below optimum for ruminal function, highlighting the importance of high NDF supplementary feeds for brassica-fed stock. For some but not all brassica species, nutritive value may be modified by cultivar selection however factors not investigated in this study including sowing rate and time from sowing to harvest remain important potential moderators of forage brassica quality. Keywords: Forage brassica, leafy turnip, bulb turnip, forage rape, swede, kale, nutritive value, feed testing