Effects of irrigating forage turnips, Brassica rapa var. rapa cv. Barkant, during different periods of vegetative growth. 2. Nutritive characteristics of leaves and roots

The effects of irrigating spring-sown forage turnips, Brassica rapa var. rapa cv. Barkant, during four discrete but consecutive periods of vegetative growth on the nutritive value of both leaves and roots of turnip was measured three times in 4 weeks in two field experiments conducted in north-west Tasmania during the 1999–2000 and 2000–01 spring and summer seasons. The harvests commenced ~10 weeks after sowing and near the maximum growth rate of the turnip. The harvests coincided with times when turnip would normally be grazed by dairy cows in mid lactation. The aim was to quantify both the effects of applying irrigation during four discrete but consecutive periods of vegetative growth and advancing maturity on metabolisable energy, neutral detergent fibre, crude protein and non-structural carbohydrate in both the leaves and roots. Irrigation applied during the four periods of vegetative growth produced relatively small and sometimes inconsistent effects on the nutritive characteristics of turnip leaf and root as the crop matured. However, their cumulative effect was to reduce both crude protein and the crude protein : non-structural carbohydrate ratio despite increasing total nitrogen uptake, and to increase leaf water-soluble carbohydrate and starch above rainfed turnip. These cumulative effects became more pronounced as the crop maturity increased from 27 to 75 days after the onset of root expansion. The cumulative effects on metabolisable energy and neutral detergent fibre were small and of no agricultural importance. Nutritive values were sufficient to meet the requirement of lactating dairy cows in mid lactation except for fibre and root crude protein.