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A major land use, especially in drier areas of subtropical regions is for forage production. Nutrient availability, as well as moisture, limit production; however, little is known concerning optimum fertilization practices and how nutrient use interacts with rainfall to affect forage yields. A study was conducted on a sandy upland soil under subtropical conditions to determine the effects of nitrogen (N) and phosphorus (P) fertilization on yield, rainfall use efficiency, nutrient uptake, and apparent fertilizer recovery on improved pastures. Established buffelgrass showed dramatic yield responses to N application, while newly planted Pretoria 90 bluestem showed yield responses to N only after residual nutrients had been depleted. Yield responses to N application were mostly quadratic, showing decreasing benefit from N with increasing N application rate. Smaller yield responses to P occurred in the first 2 years on Pretoria 90 and at higher N application rates on buffelgrass. Rainfall use efficiency (yield per unit of rainfall received) for buffelgrass increased greatly with increasing N application rate. Rainfall use efficiency of newly planted Pretoria 90 increased during each of the first 3 years as the stand developed, but was not greatly affected by N fertilization level. Nutrient contents in both grasses generally increased with increasing nutrient application. Both N and P removal by buffelgrass increased primarily with increasing N application. Apparent N and P fertilizer use effkiency by both grasses was affected very little by rate of application of either nutrient but increased with increasing annual rainfall. Forage production is a major land use in subtropical regions, and numerous management techniques are employed to enhance yield and quality. Improved pastures substantially increase production potential. Forage production is largely relegated to the more arid areas, therefore, moisture availability is often a major constraint on yield potentials. Warm temperatures accelerate nutrient cycling in the ecosystem and increase nutrient requirements. Improved pastures have been established on much of the mixedbrush rangeland in the South Texas resource area known as the Rio Grande Plain. Buffelgrass (Cenchrus ciliaris L.) is a droughttolerant, subtropical perennial bunchgrass and occupies more than 90% of the area seeded to grasses in Texas south of San Antonio (Holt and Bashaw 1976). Buffelgrass has high yields (Woodward 1980b); however, its range is limited by lack of cold tolerance and low survival on poorly drained soils. ‘Pretoria 90’ bluestem (Dichanthium annulatum (Forsk.) Stapf.) is also a subtropical perennial bunchgrass that is well adapted to subtropical areas (Conrad 1976, McBee 1959, Woodward 1980a); however, poor seed production has prevented Pretoria 90 from becoming used extensively. Fertilization should benefit forage production in subtropical areas; however, little has been done to determine specific requirements or magnitude of responses. Nutritional quality of unfertilized buffelgrass in native rangeland pasture in South Texas varies Authors are assistant professor. former assistant professor (currently director, Department of Alfalfa Breeding, Pioneer Hi-Bred International. Inc., Johnston. Iowa). and former area specialist (currently a private consultant, Mercedes, Texas). Texas A&M University Agricultural Research and Extension Center. Weslaco 78596. This study is a contribution of the Texas Agricultural Experiment Station, Texas A&M University. College Station 77843 and was partially supported by Metz and Kappler. Edinburg: American Agri-Services. Elsa; and Tide Products. Edinburg. Manuscript accepted July 23. 1984. 242 seasonally but is typically low (Gonzalez and Everett 1982). Improvements in forage quality with fertilization have been shown in the Northern Great Plains (Hansen et al. 1978, Black and Wight 1979, Lutwick and Smith 1979) and could also be expected in more southern regions. Reports of fertilizer uptake efficiency by forages varies widely from 8-14s (Lutwick and Smith 1979) and 25-43s (Hansen et al. 1978) in the northern regions, to 40-50s on a clay soil (Kissel et al. 1979) and up to 100% on coarser textured soils (Prine and Burton, 1965) in more southern areas. Fertilization has also been shown to increase soil water extraction by forages and to improve water use efficiency (Wight and Black 1978, Williams et al. 1979). This study was conducted to evaluate the effects of nitrogen and phosphorus fertilization on yield, rainfall use efficiency, nutrient uptake, and apparent fertilizer recovery of 2 forage grasses under subtropical conditions. Materials and Methods A field study was established in the spring of 1979 to evaluate forage responses under subtropical conditions to various fertilization treatments. The study was located in northwest Hidalgo County on a Hebbronville soil (coarse-loamy, mixed, hyperthermic Aridic Haplustalf), a gently sloping, rapidly permeable, sandy upland soil. The area is semiarid, receiving approximately 500 mm rainfall annually with peaks occurring in May and August, and subtropical with an annual growing season of 315 days. The area selected had a well-established stand of common buffelgrass which had not received fertilizer in recent years. Part of the area was disked to remove the buffelgrass and to provide a good seedbed. Pretoria 90 bluestem seedlings, which were started in potting media in a greenhouse, were transplanted in the disked area on 25 May. Plants were spaced at 0.5 m in rows 1 m apart. Approximately 1 liter of water was applied to each plant four days after planting and within 2 weeks over 5 cm of rainfall ensured survival of the seedlings. Plots 3.1 by 10.2 were established on each area for annual fertilizer treatments consisting of 0, 112, and 224 kg N/ha and 0,15 and 29 kg P/ha combined in a factorial arrangement. The 9 treatments were replicated 4 times in a randomized complete block design for each grass. All fertilizers were applied broadcast, with P applied in I application in the spring each year as triple super phosphate (O-44-0), and N annually applied as ammonium nitrate (33-O-O) in 3 split applications once in the spring and after each of the first 2 harvests. Soil samples were taken in the spring of each year prior to fertilization from 1979 through 1983 to a depth of 30 cm. In 1979, soil samples were taken at random throughout the area for each grass, and in subsequent years soil samples were taken from each plot. These soil samples were analyzed for NOs‘-N by specific ion electrode in a water extract from 1979 to 198 I, and by Kjeldahl distillation (Bremner 1965) in 1982 and 1983. Available P was extracted with ammonium acetate at pH 4.2 and then the chlorostannous-reduced molybdophosphoric blue color method was used in a hydrochloric acid system (Jackson 1970) to determine P concentration. JOURNAL OF RANGE MANAGEMENT 38(3), May 1985

Forage Responses of Buffelgrass and 'Pretoria 90' Bluestem to Nitrogen and Phosphorus Fertilization in a Subtropical Climate