Seasonal Nitrogen and Carbohydrate Partitioning in Forage Brassicas

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 summer and autumn-grown brassicas in CT. Kale (Brassica oleracea var. 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 [34 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 pant, 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.