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CROP ECOLOGY, MANAGEMENT & QUALITY

Nitrogen Effects on Herbage Nitrogen Use and Nutritive Value in a Meadow and Loblolly Pine Alley

^a USDA-ARS, Dale Bumpers Small Farms Research Center, 6883 South State Highway 23, Booneville, AR 72927
^b USDA-ARS Grazinglands Research Laboratory, 7207 W. Cheyenne St., El Reno, OK 73036

^* Corresponding author (dburner{at}spa.ars.usda.gov)

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^–1) near Booneville, AR. Tall fescue (Festuca arundinacea Schreb.) was the predominant herbage species. Fertilizer N was broadcast as split-applications at six rates (100 kg ha^–1 increments from 0 to 500 kg ha^–1 yr^–1). The proportion of total herbage as tall fescue was favored at rates 200 kg N ha^–1, but these rates increased concentrations of herbage NO₃^––N in the pine alley to potentially harmful levels for grazing ruminants (>2300 µg NO₃^––N g^–1). Cumulative herbage N use efficiency (CNUE) was 23 and 10 kg dry weight per kilogram 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^–1) of N fertilization should be applied to pine alley herbage given the low CNUE, and risk of NO₃^––N toxicity to ruminant livestock at higher N rates.

Abbreviations: CER, CO₂ exchange rate • CHY, cumulative herbage yield • CMN, cumulative mineralized soil N • CNAE, cumulative herbage N acquisition efficiency • CNCE, cumulative herbage N conversion efficiency • CNS, cumulative N supply • CNUE, cumulative herbage N use efficiency • CNY, cumulative N yield • CP, crude protein • HSD, honest significant difference • IVDMD, in vitro dry matter digestibility • NUE, N use efficiency • PAR, photosynthetically active radiation • SLW, specific leaf weight