Submitted to: Crop Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/7/2005
Publication Date: 5/30/2006
Citation: Burner, D.M., Mackown, C.T. 2006. Nitrogen effects on herbage nitrogen use and nutritive value in a meadow and loblolly pine alley. Crop Science. 46(3):1149-1155. Interpretive Summary: Silvopastures are agroforestry systems that aim for sustainable production of three main commodities: wood fiber, forage grass, and livestock. The adoption of this technology has been limited in the United States partly because of poor understanding of the competitive interactions between these commodities. For example, grass and tree growth can be mutually affected by competition for light, soil water, and fertilizer nitrogen at various stages in their growth cycle. Optimal fertilizer rates are important to maximize profits and avoid pollution problems that can accompany excess application. We applied six rates of nitrogen fertilizer to grass in a meadow and loblolly pine alley to determine whether there were differences in the use of nitrogen for growth and grass quality. Compared to the pine alley, grass in the meadow was able to use nitrogen for growth much more efficiently, probably because the meadow had favorable levels of light and soil water. Higher fertilizer nitrogen rates favored tall fescue grass production, but these rates in the pine alley increased nitrate to levels that might be toxic to grazing animals. Thus, only low rates of nitrogen fertilizer should be applied to grass in the pine alley. Optimal nitrogen rates will be site-specific according to soil fertility, tree planting design, and crop and tree species and management objectives. These results should interest growers and professionals working with landowners as they provide insight into fertilizer management practices for silvopastures.
Technical Abstract: Herbage response to N is poorly understood in alley cropping systems. Our objective was to determine site specific effects of N on herbage N use and nutritive value in separate experiments conducted in a meadow and a loblolly pine (Pinus taeda L.) alley (995 trees/ha) near Booneville, Arkansas. Tall fescue (Festuca arundinacea Schreb.) was the predominant herbage species. Fertilizer N was broadcast as split-applications at six rates (100 kg/ha increments from 0 to 500 kg/ha/yr). The proportion of total herbage as tall fescue was favored at rates > 200 kg N per ha, but these rates increased concentrations of herbage nitrate-N in the pine alley to potentially harmful levels for grazing ruminants (>2300 micrograms nitrate-N per g). Cumulative herbage N use efficiency (CNUE) was 23 and 10 kg dry weight per kg supplied N for the meadow and pine alley, respectively. Cumulative N acquisition efficiency (CNAE), not cumulative N conversion efficiency (CNCE), appeared to be the primary driver of low CNUE in the pine alley. The apparent increase in crude protein (CP) in pine alley vs. meadow herbage appeared to be a mechanistic response to decreased specific leaf weight. Only maintenance levels (< 100 kg/ha) of N fertilization should be applied to pine alley herbage given the low CNUE, and risk of nitrate-N toxicity to ruminant livestock at higher N rates.