Use of best management practices to increase nitrogen use efficiency and protect environmental quality in a broccoli-corn rotation of Central Mexico

Underground water reserves in Central Mexico have been declining and NO3--N concentrations of some irrigation wells have been found to exceed 10 ppm. There is the need to develop best management practices (BMPs) that can incr ease N and water use efficiency by vegetables such as broccoli (Brassica ol eracea L. Italica Group) in this region. Three field experiments were condu cted during 1996 through 1998 at the Experimental Station of INIFAP in Cela ya, Gto., Mexico to develop BMPs for the region. To determine the N suffici ency levels in the most recently fully expanded leaf (MRFEL), dry midrib an d press sap were analyzed for total N and NO3--N. Maximum marketable yields of 24.5 to 27.0 ton ha(-1) were attained with rates of 400 to 425 kg N ha( -1). Sufficiency levels for N in the MRFEL were in the ran-e of 5.5 to 6.59 o at the be-inning of the season, and then it was reduced to 5.5 to 6.0% at heading and then to 4.0-5.0% during the preharvest stage. The critical NO3 --N concentration in the midrib ranged from 15,000 to 20,000 mg kg(-1) at t he beginning of the season to 3,500-5,000 mg kg(-1) for the pre-harvest sta ge of development. The adequate levels of NO3--N in press sap ranged from 1 ,600-2,000 mg L-1 at the beginning of the season to 500-800 mg L-1 during p re-harvest growth stage. Broccoli fertilized with 400 ka N ha(-1) provided residual effects in the following crop of maize (Zea mays L.), equivalent t o 60% of that produced with N fertilizer. This study shows that drip irriga tion, spoon feeding of N, monitoring of N status during the growing season and crop rotations with grains has the potential to increase the N fertiliz er use efficiency for vegetables-grain cropping systems and contribute to p rotecting environmental quality in Central Mexico.