Seasonal Variation in Chemical Composition of Forage Brassicas. II. Mineral Imbalances and Antiquality Constituents

Abstract

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 determlne 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.

Nitrate accumulation in forage brassicas

“…Therefore, in paddocks with high soil N and therefore a high potential NO 3 -N concentration, growing bulb forage brassicas instead of leafy types may be a useful strategy to limit NO 3 -N accumulation. This contrasts with the results of Guillard & Allinson (1989), who found either that there were no differences in NO 3 -N concentration between brassica crops or that turnips had higher NO 3 -N concentration than swede, rape or kale crops, which depended on component and sowing date. The reasons for these contrasting results are unclear.…”

“…1) and also they make up a much greater proportion of the total plant biomass (Table 1). Previous reports agree with these results (Pelletier et al 1976;Guillard & Allinson 1989;Guillard et al 1995). In our experiment, the NO 3 -N in the bulb 'stem (R 2 00.96) was a better estimate than the leaf (R 2 00.52) of whole plant NO 3 -N (Fig.…”

“…Email: doc_afletcher@yahoo.com Concentrations of NO 3 in forage greater than 20.0 mg NO 3 /g (equivalent to 4.5 mg NO 3 -N/g) lead to poor performance of grazing animals and, in extreme cases, death (Nichol 2007). In addition, season, sowing date, grazing date and crop type also affect NO 3 -N content (Pelletier et al 1976;Guillard & Allinson 1989;Guillard et al 1995). There is little research investigating the response of NO 3 -N accumulation in forage brassicas to time of fertilizer N application.…”

“…Thus, we hypothesized that early fertilizer N applications would result in lower NO 3 -N content than later fertilizer N applications. The highest concentrations of NO 3 -N tend to be in stems, petioles and leaf midribs of forage brassicas, with bulbs having lower NO 3 -N content and leaf lamina having the lowest (Guillard & Allinson 1989;Guillard et al 1995). However, it is not clear which of these plant components accumulates additional NO 3 -N resulting from excessive N application.…”

Developing a critical nitrogen dilution curve for forage brassicas

“… Relationship between N concentration and NO 3 –N content (a) and between NNI and NO 3 –N contents as a proportion of total N (b) for a range of forage brassicas from Experiment 2 (diamonds) and the published data of Guillard and Allinson (1989b) (circles), McDonald et al. (1981) (squares), Pelletier et al.…”