Irrigated Annual Ryegrass Responses to Nitrogen and Phosphorus on Calcareous Soil Hagen Lippke,* Vincent A. Haby, and Tony L. Provin ABSTRACT Annual ryegrass (Lolium multiflorum Lam.) is increasingly being planted under irrigation as forage for stocker cattle in south central USA, but fertilizer recommendations for maximum production are not well defined. Labile N and P are generally deficient in this region’s calcareous soils; K availability is usually considered adequate for plant growth. Growth responses of annual ryegrass to a factorial array of five levels of N and four levels of P were studied for 3 yr on a Knippa clay soil (fine, mixed, superactive, thermic Vertic Calciustolls). Responses to N and P were generally curvilinear, but they also had a strong linear component in Year 1. Increments of growth responses to levels of fer- tilizer above 269 kg applied N or 39 kg applied P ha 21 were relatively small. Response surface models projected maximum annual dry matter yields that averaged 9.23 t ha 21 and were associated with predicted requirements for applied N and P that averaged 488 and 61 kg ha 21 , respectively. Economically optimal levels of applied N were predicted to range from 250 to 315 kg ha 21 ; the predicted economically optimal range for applied P was 31 to 41 kg ha 21 . Nitrogen concentration in ryegrass forage increased as applied N increased. The data suggest that fertilizing vegetative ryegrass to maintain N in leaf tissue $32 g kg 21 provides economically optimal growth for both the ryegrass crop and the young cattle grazing it. A NNUAL Italian ryegrass is becoming an important species for grazing stocker cattle in the south cen- tral USA, particularly on irrigated land. Data for yield responses to applied fertilizer for annual ryegrass grown under irrigation on alkaline soils of this region are not available. Fertilizer recommendations from soil testing laboratories for irrigated ryegrass production in this re- gion have been based on research data from nonirri- gated ryegrass fertilizer trials at other locations (Robinson et al., 1987; Westfall et al., 1971). Annual ryegrass yields in southeastern USA have generally increased with total annual N application rates as high as 450 kg ha 21 , which is usually the highest rate applied (Allen et al., 1974; Dunivan, 1975; Robinson et al., 1987). Hovermale (1993) obtained maximum rye- grass yields with 380 kg N ha 21 ; however, the maximum yield was only 6.7 t ha 21 . Robinson et al. (1987) com- pared N rates up to 560 kg ha 21 by 112-kg ha 21 incre- ments applied as ammonium nitrate. Significant yield increases at N rates to 336 kg ha 21 were observed in 1 yr and to 448 kg ha 21 in 2 yr. Three-year mean yields were higher at 448 kg N ha 21 than at 336 kg N ha 21 and 90% of the maximum yield, or 10.7 t ha 21 , occurred at an aver- age of 390 kg N ha 21 during the 3 yr. The potential effect of fertilizer N on forage CP level is also a concern, because CP is inherently a major fac- tor in growth rate of young grazing ruminants. The CP concentration in ryegrass needed for high performance is increased when grazed, because efficiency of utiliza- tion of CP is particularly low in fresh forages (Beever et al., 1974). Stocker cattle have demonstrated growth rates .1.2 kg d 21 (Lippke et al., 2000; Worrell et al., 1990). Calculations from FORAGVAL (Lippke and Herd, 1990) suggest that CP in grazed forage must be at least 20% (32 g N kg 21 forage) to support high levels of growth. The majority of data on the response of annual rye- grass to P has been derived from research on acid soils, usually in conjunction with limestone treatments applied to elevate soil pH to the slightly acidic levels that are more suitable for increased grass production. Haby and Robinson (1997) summarized currently available re- search on ryegrass response to residual and applied P, noting that response to applied P on P-deficient soils is highly dependent on soil chemical properties. Hillard et al. (1992) reported a gradual increase in ryegrass for- age yield from 3.2 to 5.1 t ha 21 as the rate of applied P was increased from zero to 480 kg ha 21 in two split ap- plications to a Lilbert loamy fine sand (Arenic Plinthic Paleudult) with an average pH of 4.6 and 1 M KCl exchangeable Al concentration averaging 15 mg kg 21 . In the same season, ryegrass yield was increased to an average of 8.4 t ha 21 where 3.8 t of limestone (64% effective CaCO 3 equivalence) ha 21 had been applied and incorporated 2 yr earlier, raising pH to 6.3 and lower- ing exchangeable Al to less than 1 mg kg 21 . Ryegrass response to applied P ranged from 8.3 to 8.6 t ha 21 as extractable soil P (Hons et al., 1990) increased from 3 to 55 mg kg 21 (Hillard et al., 1992). Robinson and Eilers (1996) obtained 90% of maximum ryegrass DM yield, or 9.8 t ha 21 , with 45 to 50 kg P ha 21 on a Tangi silt loam (Typic Fragiudult) at pH 6. Higher rates of P had little influence on yield but significantly increased P removed by the crop. The clay and clay loam soils of south central and southwestern USA exhibit pH . 7.5 and may contain lime concretions in deeper horizons. Phosphorus activity is lower in soils that have high Ca 21 activity, large amounts of Ca-saturated clay, and highly reactive CaCO 3 (Tisdale et al. (1985).) In such soils it is necessary to add larger quantities of P fertilizers to maintain a given level of P activity in the soil solution (Tisdale et al., 1985). H. Lippke, Texas Agric. Exp. Stn., 1619 Garner Field Rd., Uvalde, TX 78801; V.A. Haby, Texas Agric. Exp. Stn., P.O. Box 200, Overton, TX 75684; and T.L. Provin, Dep. of Soil and Crop Science, Texas A&M Univ., College Station, TX 77843. Contribution of the Texas Agric Exp. Stn. and partially supported by a grant from the Potash & Phospho- rus Institute. Received 17 Aug. 2005. *Corresponding author (h-lippke@ tamu.edu). Published in Agron. J. 98:1333–1339 (2006). Fertilizer Management doi:10.2134/agronj2005.0236 ª American Society of Agronomy 677 S. Segoe Rd., Madison, WI 53711 USA Abbreviations: CP, crude protein; DM, dry matter; N A , applied nitro- gen; N S , initial soil nitrate nitrogen; P A , applied phosphorus; P S , initial available soil phosphorus; Yr, year. Reproduced from Agronomy Journal. Published by American Society of Agronomy. All copyrights reserved. 1333 Published online September 5, 2006