Effect of Brush on Forage Production and Composition

With no mesquite cover, peak herbaceous biomass on bottomland silt loam sites averaged 3300 vs. 2560 kg/ha [2943 vs. 2284 lb/ac] on clay-loam sites (P = 0.001). A linear decline of 14 kg/ha [12.5 lb/ac] in herbaceous biomass occurred for each percent increase in mesquite cover (P = 0.001). The slope of this decline was similar between soils (P = 0.135). Herbaceous biomass with increasing mesquite cover varied between years (P = 0.001) as did the slope of decline (P = 0.001).

Warm-season herbaceous biomass decreased linearly with increasing mesquite cover averaging a 73% reduction at 100% mesquite cover (P = 0.001) compared to 0% mesquite cover. Cool-season herbaceous biomass was similar between soils with no mesquite, 1070 kg/ha [955 lb/ac] for silt loam bottomland soils vs. 930 kg/ha [830 lb/ac]for clay loam soils, but averaged 340 kg/ha more on silt loam than on clay loam soils at 100% mesquite cover (P = 0.004). Increasing mesquite cover reduces livestock forage productivity and intensifies drought effects by increasing annual herbaceous biomass variability. From a forage production perspective there is little advantage to having mesquite present.

Figure 1. Total herbaceous production with increasing mesquite cover on silt-loam bottomland and clay-loam soils in according to the precipitation received each year from 1995 to 2001 on the Waggoner Experimental Ranch. [lb/ac × 1.121= kg/ha]

Increasing mesquite cover from 0 to 100% reduced herbaceous biomass by one half or more for C4 tall grasses, C4 mid grasses, and forbs, but reduced C4 short grasses less sharply, while C3 annual grasses doubled on average. Both cool and warm season forb functional groups declined with increasing mesquite cover. Herbaceous species richness was greater on silt loam bottoms than clay-loam flats and decreased little except at the highest cover levels of mesquite cover.

Figure 2. Biomass (kg/ha) of each herbaceous functional group in response to different levels of mesquite cover averaged over the years 1995-2001 on bottomland and clay-loam soils on the Waggoner Experimental Ranch. C4 tall grasses were only present on the bottomland soils. [lb/ac × 1.121= kg/ha]

All warm season (C4) grass species declined with increasing mesquite cover while cool season grass species (C3) were less affected, apparently because much of their growth occurs while mesquite is inactive. Species richness declines markedly only at the highest levels of mesquite aerial cover. The reduction in C4 grasses and forbs with increasing mesquite cover has negative implications for richness of non-game and game wildlife species, and pastoralists whose livelihoods depend on livestock.

Publications

Teague, W.R., R. J. Ansley, W.E. Pinchak, S.L. Dowhower, S.A. Gerrard, J. A. Waggoner. 2008. Interannual herbaceous biomass response to increasing honey mesquite cover on two soils. Rangeland Ecology and Management 61:496-508.
Ansley, R.J., W.E. Pinchak, W.R. Teague, B.A. Kramp, D.L. Jones, and P.W. Jacoby. 2004. Long-term grass yields following chemical control of honey mesquite. Journal of Range Management 57:49-57.

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