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Seasonal Nitrogen and Carbohydrate Partitioning in Forage Brassicas

Dep. of Plant Science, U-67, Univ. of Connecticut, 1376 Storrs Rd., Storrs, CT 06269-4067

Kimberly A. Cassida, Mary Hagemann Wiedenhoeft, Robert L. Hough, Barbara A. Barton and Derek W. Allinson

Dep. of Crop and Soil Sci., Michigan State Univ., East Lansing, MI, 48824
Dep. of Plant, Soil, and Environ. Sci., Univ. of Maine, Orono, ME 04469
Harmony Hill Farm, Blakes Hill Rd., Northwood, NH 03261
Purina Mills, Inc., Dairy Research, St. Louis, MO 63166.
Dep. of Plant Science, U-67, Univ. of Connecticut, 1376 Storrs Rd., Storrs, CT 06269-4067

^* Corresponding author.

Nitrate toxicosis is more prevalent when ruminants consume forages high in NO₃ and low in readily fermentable carbohydrates. Environmental conditions vary with season and may differentially affect herbage N and total nonstructural carbohydrate (TNC) composition. Experiments were conducted to determine the concentrations and partitioning of NO₃–N, total N (TN), TNC, and inorganic N in summerand autumn-grown brassicas in CT. Kale (Brassica oleracea van acephala DC.), rape (B. napus L.), turnip (B. rapa L.), and tyfon [B. rapa L. x B. pekinensis (Lour.) Rupr.] were established in June and August of 1988 and 1990 on a Paxton fine sandy loam soil (coarse-loamy, mixed, mesic Typic Dystrochrept). The experimental design in each season was a split-plot, with species as main plots and sampling dates [30, 60, 90, and 120 days after seeding (DAS)] as subplots. Concentrations of NO₃–N and TN decreased with time in both seasons, but the rate of decline for both constituents was generally greater during the summer than autumn. Concentrations of TNC in the whole plant and most plant parts increased with time and peaked at 90 or 120 DAS in both seasons. Inverse relationships were found between TN and TNC (except in laminae), and between NO₃–N and TNC (except in roots). Concentrations of NO₃–N were relatively low in the whole plant, central midrib-petioles, stems, and roots when the concentrations of TN were <29 g kg^–1, but increased linearly when TN was greater than this. Concentrations of NO₃–N increased linearly in laminae only when TN concentrations were >54 g kg^–1. Whole-plant inorganic N concentrations, as a percentage of TN, were <10% at 60 DAS. The potential risk for NO₃ toxicosis in ruminants is higher if animals graze brassicas before 60 DAS than if they graze brassicas after 60 DAS.

Received for publication May 9, 1994.

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