The data below demonstrate that in the studies, grain yields were optimized with 60 lb N/acre. For all cover cropped plots yield was maximized with 0 fertilizer N. This suggests that cover cropping provides an equivalent of 60 lb N/acre of fertilizer N.
Effect of covercropping on grain yield1
Fertilizer N No Purple Lana Bell
Rate Covercrop Vetch Vetch Bean
lb N/acre grain yield at 13% moisture (cwt/acre)
0 2 67.6 82.8 80.2 83.3
30 73.2 78.1 76.7 79.7
60 3 78.2 67.2 73.5 77.6
90 70.4 63.1 52.7 59.2
         
1Data collected from Skinner Ranch, Butte Co., 1989
2Grain yield was maximized with no fertilizer N on all cover-
cropped plots.      
3Grain yields were maximized with 60 lb/acre of fertilizer N on
plots with no covercrops.
After: Pettygrove G.S. and J.F. Williams. Nitrogen-fixing Covercrops for California Rice Production available through the UC Cooperative Extension Rice Project

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