Interactive Effects of Trees and Nitrogen Supply on the Agronomic Characteristics of Warm‐Climate Grasses

Abstract

Plant adaptations to cope with shade may vary according to the degree of shade tolerance and nutrient availability for each species. Studies of different understory species and their responses to combined shade and N effects are important to identify ways to optimize the quantity and quality of forage production. Our objective was to measure the dry matter yield (DMY) and nutritive value of six C4 grasses grown in two systems (full sunlight [FS] vs. a naturally shaded system composed of Eucalyptus dunnii Maiden trees) with two N levels (0 vs. 300 kg N ha−1yr−1) using the same target sward conditions (i.e., 50% depletion of the canopy height set by 95% light interception). Over 3 yr, the decreases in DMY under shade compared with FS ranged from 7% [Urochloa brizantha (Hochst. ex A. Rich) R.D. Webster] to 56% (Cynodon spp.) in fertilized treatments and between 11% (Paspalum notatum Fluegge) to 46% (Cynodon spp.) in treatments without N. The N effect was more important to the nutritive value of the forage than the shade effect, particularly for crude protein (CP, +46 g kg−1 with N supply), acid detergent fiber (ADF, –46 g kg−1) and leaf proportion (+11.2%). However, most of the species displayed higher leaf digestibility under shade due to decreases in ADF. Even with intense shading (light approximately 48% of unshaded), the digestible DMY and CP yield under trees were, on average, 70 and 71% of the 6.8 and 1.4 t ha−1 recorded in FS, respectively.Core Ideas The study identifies C4 species for the optimization of forage production under trees. Nitrogen and shade have combined effects on forage production and nutritive value. Limitations and benefits of integrated crop–livestock systems with trees. Shade effects on nutritive value occur at the leaf level and of the total biomass harvested.