Seasonal Variation in Chemical Composition of Forage Brassicas. II. Mineral Imbalances and Antiquality Constituents
- K. Guillard and
- D. W. Allinson
Although the nutritional value of Brassica crops is apparently high, antiquality constituents and elevated concentrations of certain minerals in herbage may negatively affect animal performance. This paper reports on seasonal variation of Ca/P, K/(Ca + Mg), and (K + Ca)/Mg ratios, and concentrations of thiocyanate (SCN−) and nitrate (NO−3-N) in various Brassica crops. Turnip (B. rapa L.), rape (B. napus L.), swede (B. napus L.), and Tyfon [B. rapa L. × B. pekinensis (Lour.) Rupr.) were no-till seeded in Connecticut on a Paxton fine sandy loam soil (coarse-loamy, mixed, mesic Typic Dystrochrept) in summer and autumn of 1983 and 1984. Kale (B. oleracea L. var. acephala DC.) and the species listed above were seeded in the summer of 1987 and autumn of 1986 and 1987, to determine NO−3-N partitioning. Foliage Ca/P ratios were above 1:1 to 2:1, whereas root ratios were close to or slightly below this range. Ratios of Ca/P were significantly greater in summer than in autumn. The (K + Ca)/Mg ratio in 1983 exceeded the maximum desirable limit of 5.2 in autumn foliage and roots. Only turnip root ratios exceeded 5.2 in 1984. Concentrations of SCN− ranged from 160 to 840 mg kg−1 in foliage and from 274 to 845 mg kg−1 in roots. Foliage concentrations of NO−3-N ranged from 21 to 1011 mg kg−1 in summer and from 2293 to 4750 mg kg−1 in autumn. Concentrations of NO−3-N were lower in roots and showed the same seasonal pattern as foliage. Accumulation of NO−3-N was highest in stems and petioles, lowest in leaf laminae, and intermediate in roots. Nitrate toxicity risk may be greater for ruminants grazed on autumn-grown Brassica than those grazed on summer-grown Brassica. Supplementation of P and iodine, controlled intake of Brassica crops, or addition of roughages should reduce concerns about mineral imbalances or antigrowth constituents associated with these forages.
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