Published in Agron J 91:902-910 (1999)
© 1999 American Society of Agronomy
677 S. Segoe Rd., Madison, WI 53711 USA
Agronomy Journal 91:902-910 (1999)
© 1999 American Society of Agronomy
FORAGES
Response of Overseeded Alfalfa and Bermudagrass to Alfalfa Row Spacing and Nitrogen Rate
Vincent A. Habya,
J.V. Davisa and
A.T. Leonarda
a Texas A&M Univ. Agric. Res. & Ext. Ctr., P.O. Box 200, Overton, TX 75684-0200, Texas Agric. Exp. Stn., Texas A&M Univ. System USA
v-haby{at}tamu.edu
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ABSTRACT
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Alfalfa (Medicago sativa L.) is a rare forage crop on Coastal Plain soils. Acid soils, wet conditions, and the prevalence of perennial, warm-season grasses limit alfalfa production. Development of grazing-tolerant varieties raised interest in growing alfalfa on the Coastal Plain. This three-year dryland field study was conducted to evaluate coincident production of `Alfagraze' alfalfa and `Coastal' bermudagrass [Cynodon dactylon (L.) Pers.] as a sustainable forage system. Limestone (effective calcium carbonate equivalence ECCE 72%) at a rate of 6.1 t ha-1 was incorporated by roto-tilling 15 cm deep in an established sod of Coastal bermudagrass on a Darco loamy fine sand (loamy, siliceous, thermic Grossarenic Paleudults) in late winter 1990, with an additional 3 t ha-1 surface-applied in June 1991. Alfalfa was seeded in October 1990 at 23, 46, 69, and 92 cm between rows in main plots of a split-plot design. Nitrogen rates from 0 to 112 kg ha-1 in increments of 28 kg ha-1 were applied to subplots for every bermudagrass regrowth cycle. Other plant nutrients (including P, K, Mg, S, B, Zn, and Cu) were applied at rates considered adequate for alfalfa on a low-fertility soil. Yield of alfalfa at the 23-cm row spacing in 1991 was 8.8 t ha-1 and declined to 6.7 t ha-1 at
, while yield of bermudagrass increased from 3.2 to 5.7 t ha-1, respectively, at these row spacings. In 1992, alfalfa yield increased an additional 2.2 t ha-1 at each row spacing, with a compensating decline in bermudagrass production. Alfalfa yielded 11 t ha-1 at all row spacings in 1993, despite a midseason drought, while bermudagrass yield was <450 kg ha-1. Row spacing had no effect on total forage production in any year. Higher N rates increased bermudagrass yield the first two years. Applied N increased alfalfa yield at certain harvests, but had no effect on total annual production. Crude protein in alfalfa declined or remained similar as row spacing was widened. Soil pH was lowered by increasing N rates and by narrower alfalfa row spacings. Results indicate that alfalfa competes well with Coastal bermudagrass, even in drought conditions.
Abbreviations: CEC, cation exchange capacity • ECCE, effective calcium carbonate equivalence
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INTRODUCTION
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THE COASTAL PLAIN
of the southern and southeastern United States is dominated by leached, acid soils. The majority of these soils are Ultisols (Buol et al., 1973). Bermudagrass grown for hay or grazing is well adapted to Coastal Plain soils (Burton and Hanna, 1985). At low N availability, yield and quality of hybrid bermudagrasses [Cynodon dactylon (L.) Pers.] are reduced, but these grasses produce excellent quality and yields in response to high rates of N (Wilkinson and Langdale, 1974). High rates of applied N can increase soil acidity (Pierre, 1928), particularly on low-buffer-capacity, acid, sandy Ultisols. Due to its intolerance to acidity and to wet soils, and the poor hay drying conditions in this region, alfalfa (Medicago sativa L.) has not been a common forage on the Coastal Plain.
Results from alfalfa research on Coastal Plain soils vary. Burton (1976) reported 8.7 to 9.9 t ha-1 five-year average yields of alfalfa–`Coastal' bermudagrass mixtures. Morris et al. (1992) reported loss of alfalfa stand after three years on a limed Tangi silt loam (Typic Fragiudults) in southeastern Louisiana. Brown and Byrd (1990) presented an excellent review of the compatibility of alfalfa grown with warm-season perennial grasses. Their research with bermudagrass demonstrated that neither N fertilization at 100 kg ha-1 on a Davidson soil (Rhodic Paleudult) nor row spacing of alfalfa at 15 and 30 cm on a Cecil soil (Typic Hapludult) affected yield or botanical composition of mixtures of alfalfa and hybrid bermudagrasses. In more recent work on row spacings of alfalfa in mixed stands with hybrid bermudagrass on Norfolk sandy loam (Typic Kandiudult) and Cecil sandy clay loam soils (Typic Kanhapludult), Stringer et al. (1994) reported that interseeded alfalfa increased total yields compared with bermudagrass alone that received season-total N rates of 224 kg ha-1 or less. They indicated that increasing row spacings of alfalfa to 60 cm decreased yields of interseeded mixtures and that increasing N rates sometimes compensated slightly for wide rows. Stringer et al. (1994) showed that bermudagrass percentage in the mixed stand increased linearly as alfalfa row spacing was increased from 20 to 60 cm.
Our research on mixed stands of alfalfa and hybrid bermudagrass developed from the need to improve the quality of forage in bermudagrass pastures on sandy, low-buffer-capacity soils where high rates of applied N rapidly increase soil acidity. Our objective was to evaluate production of Alfagraze alfalfa with Coastal bermudagrass as a sustainable high-quality forage system for the Coastal Plain. The release of the grazing-tolerant cultivar `Alfagraze' by Bouton et al. (1991) was timely because results of this research could lead to dual-forage grazing systems studies.
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Materials and methods
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A Darco loamy fine sand near Overton, TX (95° W, 33° N), was cleared of a mixed stand of pine and hardwood forest in 1987 and sprigged to Coastal bermudagrass in late February 1988. The surface 15-cm depth had a pHw of 5.7 and tested very low to low in P, K, Mg, S, and B.
Limestone Treatment and Fertilization
Calcitic limestone with an ECCE of 72% was applied at 6.05 t ha-1 to the Darco soil on 24 Oct. 1989 and was incorporated into the surface 15 cm by roto-tilling in late winter of 1990 to raise pHw (2:1 v:v H2O:soil) to approximately 7.0. Bermudagrass rapidly reestablished a covered canopy on the site during the following growing season. Additional ECCE 72% limestone at 3 t ha-1 was surface-applied to the site on 14 June 1991. Soil samples collected to the 15-cm depth from individual plots in July 1992 showed that pH rapidly declined in proportion to increasing N rates. Additional limestone was applied in late summer 1992 at rates of 0, 1.12, 2.24, 3.36, and 4.48 t ha-1, respectively, to offset the increasing acidity created by the first seven applications of N at rates, which totaled 0, 196, 392, 588, and 784 kg ha-1.
Plant nutrients other than N applied as fertilizer during this study are shown in Table 1
. Variable N rates applied for each bermudagrass regrowth amounted to annual N applications of 0, 112, 224, 336, and 448 kg ha-1 in 1991 and 1993 and to 0, 140, 280, 420, and 560 in 1992. Analysis of alfalfa from selected plots the seedling year indicated that the Mg content of the top growth was rapidly declining. Additional Mg was applied the second growing season.
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Table 1 Plant nutrients, other than N, applied as fertilizer to the Darco soil (loamy, siliceous, thermic Grossarenic Paleudults) near Overton, TX, each year
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Alfalfa Establishment and Experimental Treatments
In October 1990, bermudagrass was cut to a stubble height approximating 8 cm after cool weather had slowed its growth. Alfalfa was no-till seeded using a drill with double-disk openers on 5 Oct. 1990 in east–west oriented rows spaced 23, 46, 69, and 92 cm apart at rates equivalent to 22.4, 11.2, 7.5, and 5.6 kg of seed ha-1, respectively. The 6.1- by 15.2-m main-plot alfalfa row spacings in this split-plot experimental design were divided into five 3- by 6-m subplots. Randomized subplots were individually treated with 0, 28, 56, 84, and 112 kg of N ha-1 as NH4NO3 applied by hand-spreading for each regrowth of bermudagrass. All main-plot treatments were randomized in four replications. Alfalfa was harvested five times in 1991 and six times in 1992 and 1993. During the seedling year, the first alfalfa harvest was made in late spring. In succeeding years, alfalfa was ready for first harvest in early spring. Bermudagrass was harvested with alfalfa, except for the first cutting in 1992 and 1993, when cool temperatures delayed growth initiation of bermudagrass, which is a C4 photorespiration-type plant. Four harvests of bermudagrass were made in 1991, five in 1992, and (because of late-season drought), only three in 1993. (The harvest dates for alfalfa and bermudagrass are given in Tables 2 and 3
, respectively.) Yields were determined by clipping a 1.5-m-wide strip in each plot with a self-propelled, forage-plot harvester at approximately 10% bloom on alfalfa. Length of the harvested strip approximated 5 m and was always measured to determine the fraction of a hectare harvested. Collected subsamples were placed into labeled individual paper bags that were immediately sealed in individual plastic bags to prevent moisture loss. Subsamples were weighed, sorted as alfalfa and bermudagrass components, dried at 60°C in a forced-draft oven, and reweighed to determine dry forage yield.
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Table 2 The analysis of variance for the effects of alfalfa row spacing and N rate on dry matter yield of Alfagraze alfalfa by year and harvest
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Table 3 The analysis of variance for the effects of alfalfa row spacing and N rate on dry matter yield of Coastal bermudagrass by year and harvest
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Sample Chemical Analysis
Dried plant samples were ground to pass a 0.85-mm screen, digested in LiSO4–H2SO4–Se–CuSO4 according to the method of Nelson and Sommers (1980) and analyzed for K, Ca, Mg, Zn, Fe, and Mn using atomic absorption spectroscopy. Boron and Cu were analyzed after ashing plant samples at 550°C and dissolving the ash in dilute HCl. Boron was determined colorimetrically by azomethine-H color development at 420 nm (Wolf, 1974). Copper was analyzed by atomic absorption spectroscopy. Nitrogen in plant samples was determined by near-infrared reflectance spectroscopy calibrated to N analyzed by wet chemistry. Plant P was analyzed colorimetrically using the vanadomolybdophosphoric yellow color method (Jackson, 1958).
Pretreatment subsamples of soil representative of the research area were collected in fall 1989 to the 15-cm depth, dried at 60°C, screened to 0.85 mm, and extracted for P, K, Mg, Ca, and Na by the method of Hons et al. (1990) for 1 h at 240 cycles min-1 on a rotary shaker. Phosphorus was determined colorimetrically by chlorostannous-reduced molybdophosphoric blue color (Jackson, 1958). Potassium, Ca, and Mg were determined by atomic absorption spectroscopy, and Na was analyzed by flame emission spectroscopy. Cation exchange capacity was estimated by repeated saturation of the soil with NH4 and analysis of exchangeable cations (Thomas, 1982). Aluminum, soluble in 0.01 M CaCl2 (Hoyt and Nyborg, 1972), was determined using graphite furnace atomic absorption spectroscopy. Exchangeable H was analyzed by the method of Yuan (1959). In July 1992, six subsamples of soil were collected from the 0- to 15-cm and 15- to 30-cm depths in each plot, composited, dried at 60°C for 48 h, and ground to pass a 0.85-mm screen in preparation for analysis for pHw. Statistical analyses were conducted using PROC GLM and PROC Reg in SAS (1991).
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Results and discussion
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Soil Characteristics and Rainfall
Chemical characteristics of the 15-cm depth of the Darco loamy fine sand prior to initiation of this research included a pH of 5.7, and 3, 21, 19, 337, and 83 mg kg-1 P, K, Ca, Mg, and Na, respectively. Soil CEC, Al, and H were 3.24, 0.03, and 0.07 cmolc kg-1, respectively. A pH of 5.7 is typical of recently deforested soils of this region. The generally low fertility levels and cation exchange capacity are indicative of an unfertilized grossarenic soil, i.e., sand depth greater than 100 cm over an argillic horizon. Rainfall received during the growing season for the three years of this study is shown in Fig. 1
. Early-season precipitation in the seedling year was abundant for establishment of alfalfa. Less rain was received in March, April, and May of the second and third growing seasons. Extremely low amounts of rainfall were received after June 1993, with no measurable precipitation occurring in July.
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Fig. 1 Mean maximum and minimum air temperatures and rainfall by month for 1991, 1992, and 1993, along with the long-term average rainfall. Months are numbered consecutively, with 1, 13, and 25 representing January in respective years
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Dry Matter Yield
Effect of Row Spacing (Averaged Over N Rates)
Analysis of variance for the effects of row spacings and N rates on dry matter yield of alfalfa and bermudagrass is shown in Tables 2 and 3. Seedling-year alfalfa yielded 8.8 t ha-1 when planted in rows spaced 23 cm apart (Fig. 2)
. Widening the row spacing of alfalfa decreased yield of alfalfa and increased bermudagrass yield. Alfalfa was a greater proportion of total yield in 1992. Averaged over all row spacings, alfalfa comprised 97.5% of the total forage production in 1993. On this recently deforested soil, alfalfa planted in rows spaced 23 cm apart almost eliminated the Coastal bermudagrass. Alfalfa row spacing had no significant effect on total forage production in any year.
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Fig. 2 Response of Alfagraze alfalfa and Coastal bermudagrass to alfalfa row spacing in 1991 through 1993. Where present, error bars indicate the standard deviation from the mean and represent statistically significant yield differences at
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Alfalfa yield declined with increasing width between rows in four harvests in 1991 (Table 4)
and in the first harvest of 1992 (Table 5)
. Row spacing had little effect on alfalfa production in 1993 (Table 6)
. Alfalfa dry matter production was highest in early harvests and declined as the season progressed. Bermudagrass growth essentially ceased in the summer of 1993, due to drought and competition from alfalfa for stored soil water. Sandy soils such as Darco can hold an estimated 100 mm of plant available water in the 1.2-m depth (Rawls and Brakensiek, 1983).
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Table 4 Main effect of alfalfa row spacing, harvest date, and N rate on yields of Alfagraze alfalfa and Coastal bermudagrass during the seedling year, 1991. Nitrogen applied for each regrowth of bermudagrass
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Table 5 Main effect of alfalfa row spacing, harvest date, and N rate on yields of Alfagraze alfalfa and Coastal bermudagrass during the second year, 1992. Nitrogen applied for each regrowth of bermudagrass
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Table 6 Main effect of alfalfa row spacing, harvest date, and N rate on yields of Alfagraze alfalfa and Coastal bermudagrass during the third year, 1993. Nitrogen applied for each regrowth of bermudagrass
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Widening the row spacing of alfalfa significantly increased Coastal bermudagrass dry matter production in 1991 and 1992 (Tables 4 and 5) and in the third bermudagrass harvest of 1993 (Table 6). Before the first harvest of the seedling year, temperatures were favorable for bermudagrass growth and reasonable grass yields were measured. In 1992 and 1993, one harvest of alfalfa was made before the bermudagrass initiated growth.
Effect of N Rate (Averaged Over Row Spacings)
Increasing rates of N had little effect on total annual yield of alfalfa in the three years of this study (Fig. 3)
. As occurred with row spacing, alfalfa became a greater portion of total forage yield each season. Nitrogen application significantly increased bermudagrass dry matter production in each of the three years, but at a declining rate each year. At the 84 kg ha-1 rate of applied N, bermudagrass produced 13.0 kg of dry forage for each 1 kg of N applied in 1991, 4.2 kg in 1992, and only 1.4 kg in 1993. The decreasing efficiency of applied N for bermudagrass production reflects the competitive effects of alfalfa in this forage system after the seedling year.
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Fig. 3 Response of Alfagraze alfalfa and Coastal bermudagrass to N rates applied for each bermudagrass regrowth in 1991 through 1993. Where present, error bars indicate the standard deviation from the mean and represent statistically significant yield differences at
. Annual total N rates applied in 1991 and 1993 were 0, 112, 224, 336, and 448 kg ha-1 and in 1992 were 0, 140, 280, 420, and 560 kg ha-1
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Applied N increased alfalfa dry matter production at specific harvests in 1991 (Table 4), 1992 (Table 5), and 1993 (Table 6). Total yield of alfalfa increased from 0.6 t ha-1 in 1991 to 1.1 t ha-1 in 1993 due to applied N. However, 450 kg N applied per hectare to produce an additional 1.2 t of forage per hectare is not an economically viable input. Bermudagrass yield increased due to increasing rates of applied N, but response to increasing N rate declined in later harvests each season.
Alfalfa yield response to increasing N rates at specific harvests may be related to climatic stresses. The 3 Dec. 1991 harvest occurred after cold temperatures affected growth (Table 4). The May, August, and September harvests occurred following low-rainfall periods in 1992 (Table 5). Alfalfa yields were also increased by N in the July 1993 harvest following a very dry July, and in November during cool temperatures (Table 6). The effect of these extremes in temperature and drought may have slowed the activity of rhizobia that fix atmospheric N2 near the soil surface. A decline in N2 fixation may have contributed to the alfalfa response to applied N.
These results concur with conclusions made by Burton (1976) and Brown and Byrd (1990) and later verified by Stringer et al. (1994) that alfalfa can be grown simultaneously with hybrid bermudagrasses. By the third year in our study, however, alfalfa was the dominant forage even at the 92-cm alfalfa row spacings. Because of its sandy texture and low available water holding capacity, Darco is considered a droughty soil. Adequate rainfall in spring of 1991 and excellent drainage in this soil allowed alfalfa to become well established at all row spacings. The first and highest-yielding of four harvests was made on 12 June with succeeding harvests made at approximately 4-wk intervals (except for the 19 August harvest, when low rainfall in July increased the harvest interval to almost 6 wk). Alfalfa yield declined at each succeeding harvest, due to less favorable moisture conditions.
The annual increase in alfalfa and the decline in bermudagrass yield verify results reported by Stringer et al. (1994) and indicate the competitiveness of alfalfa on this initially low-fertility soil. After the seedling year, even with N rates above 100 kg ha-1 for each regrowth, the bermudagrass was not competitive with alfalfa. At narrow row spacings, the shading effect of alfalfa (mentioned by Stringer et al., 1994) and the competitiveness of alfalfa for soil water reduced bermudagrass growth. However, bermudagrass yield was also suppressed by alfalfa in rows spaced 92 cm apart in the second and third years of this study, while total forage yield was not reduced. At this wide row spacing, shading by the alfalfa still occurred, but especially nearer to harvest time when the middles between alfalfa rows were covered. At wider alfalfa row spacings, the competitiveness of alfalfa for soil water may be more of a factor contributing to reduced bermudagrass yield than is shading. At this location, post-seedling-year alfalfa initiates growth in late February and is well into its second regrowth by the time bermudagrass initiates vegetative growth in late April and early May. In this concurrent cropping system, well-established alfalfa with a measured tap root length beyond 1.2 m competes with hybrid bermudagrass for sunlight and soil water. With limited rainfall after June and the increasing competitiveness of the alfalfa as the stand became better established, bermudagrass, which had its greatest root mass in the top 80 cm of soil and with few fine roots below that level, declined as a percentage of the total forage produced. Observation of forage root systems exposed in the wall of a soil pit showed that bermudagrass roots seldom invaded the rooting area directly below the alfalfa crown. However, alfalfa roots grew laterally into the soil occupied by the mass of bermudagrass roots (Fig. 4) . After each cutting, alfalfa initiated vegetative regrowth and therefore competition for soil water several days earlier than did the bermudagrass. In a highly fertile soil, such as one that has been fertilized and grazed for 20 years, Coastal bermudagrass appeared to be more competitive with established alfalfa (Rouquette and Haby, 1996) due to increased concentration of nutrients below the surface depth, particularly when N fertilizer is applied for the grass.
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Fig. 4 The extensive tap root system of alfalfa, compared with the fibrous roots of bermudagrass, which primarily remained directly below the grass. The depth scale encompasses  84 cm
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The interactive effects of row spacing and N rate were significant for alfalfa only in the 20 May harvest of 1993 (Table 2) and for bermudagrass in the 11 August and 15 September harvests of 1992 (Table 3). These interactive responses were of minor importance, with an R2 of 0.39. For bermudagrass at the 15 September harvest, increasing distance between alfalfa rows linearly increased bermudagrass dry matter yield a maximum of 146 kg ha-1. Bermudagrass yield was maximized at 585 kg ha-1 by application of 84 kg of N ha-1 at the 69-cm row spacing.
Crude Protein and N Uptake
Widening the alfalfa row spacing to 
46 cm decreased average crude protein levels in alfalfa and bermudagrass in 1991 (Table 7) . Average crude protein concentration in both forages was higher in the second year than in the seedling year, but row spacing had no effect on crude protein in either forage in 1992. Applied N increased crude protein in the bermudagrass in 1991 and 1992. The mean crude protein level in first-cutting alfalfa in 1991 was 167 g kg-1 (data not shown). In succeeding harvests, crude protein increased to 193, 208, and 234 g kg-1 in Harvests 2, 3, and 4, respectively. In a December harvest of stockpiled alfalfa, mean crude protein content was 226 g kg-1. In 1992, the mean crude protein level of the first harvest was 283 g kg-1 and declined to 263, 231, and 155 g kg-1 in Harvests 2, 3, and 4, respectively. In the final two harvests, crude protein remained constant at 226 and 223 g kg-1.
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Table 7 Main effect of alfalfa row spacing and N rate on crude protein concentrations in Alfagraze alfalfa and Coastal bermudagrass grown concurrently
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Crude protein in Coastal bermudagrass in 1991 increased from 97 g kg-1 in the first harvest to 136, 136, and 147 g kg-1 in the last three harvests. The second season, crude protein in bermudagrass in the first two harvests remained high and constant at 210 and 211 g kg-1 before declining to 118 and 133 g kg-1 in the last two harvests.
Because alfalfa yield and crude protein were lowered by widening the spacing between rows of alfalfa, total N yield in alfalfa decreased as expected in 1991 and 1992 due to wider row spacing (Table 8)
. Rates of N to 450 kg ha-1 in 1991 and to 560 kg ha-1 in 1992 had no effect on N uptake by alfalfa. Bermudagrass N uptake increased in 1991 due to greater yield at wider alfalfa row spacings. In that year, total N rates to 224 kg ha-1 and higher (56 kg N ha-1 per application) increased total N uptake by bermudagrass (Table 8).
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Table 8 Main effect of alfalfa row spacing and N rate on total N uptake by Alfagraze alfalfa and Coastal bermudagrass grown concurrently
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Plant Nutrient Content Other Than N
Row spacing had no significant effect on K, Ca, Mg, S, Fe, Mn, Zn, Cu, or B concentrations in alfalfa in 1991 or 1992 (data not shown). Widening the row spacing of alfalfa from 23 to 56 cm, however, increased P in bermudagrass from 3.7 to 4.0 g kg-1. Increasing the rate of applied N from 0 to 112 kg ha-1 for each regrowth of grass decreased bermudagrass P concentration from 4.7 to 3.7 g kg-1. Similar responses to applied N were noted in bermudagrass in 1992. Increases in applied N raised plant levels of Ca, Mg, and Cu in 1991 but not in 1992.
Concentrations of P, K, and Mn in alfalfa were high in Harvest 3 in 1991 and 1992 (Table 9)
, based on data published by Kelling and Matocha (1990). Iron levels varied from adequate to high, B and Zn were adequate, and Cu varied near the critical level over these harvests. Calcium and Mg were consistently in the deficient category according to published levels (Kelling and Matocha, 1990). Nutrient deficiency symptoms were not observed in alfalfa in this study. Since the soil at this research site was limed to pHw approximating 7.0 using calcitic limestone and was treated with 27 kg of Mg ha-1 the first year and even more the second year, adequate levels of Ca and Mg were expected to be available for alfalfa uptake. Considering the levels of Ca and Mg in alfalfa in this study, the 18 g kg-1 of Ca and 2.5 g kg-1 of Mg represented as critical for alfalfa by Kelling and Matocha (1990) appear to be at least moderate levels. Additional research on critical levels of Ca and Mg in alfalfa may be needed to resolve this discrepancy.
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Table 9 Mean plant nutrient concentrations in Alfagraze alfalfa and Coastal bermudagrass from Harvest 3 in 1991 and 1992
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Soil pH
Increasing the N rate from 0 to 112 kg ha-1 for each of seven bermudagrass growth periods lowered soil pH in the 23-cm alfalfa row-spacing plots from 7.28 to 6.12, respectively, in the 0- to 15-cm depth soil samples collected in July 1992 (Fig. 5)
. This pH decline after application of 784 kg of N ha-1 indicates that the ECCE 72 limestone was not sufficiently fine to buffer the increasing acidity produced at these high rates of N. Soil pH in the 92-cm row spacing plots was uniformly higher by 0.26 over the range of N treatments. This shows the effect that a denser stand of alfalfa has on lowering soil pH over a 1.5-year production period. The decline in pH with increasing alfalfa plant density may be related to increased N fixation and consequent mineralization and nitrification from plant residues. The increased uptake of cations (Table 9), particularly of Ca2+, with the subsequent release of H+ to the soil, could also have an effect on increasing soil acidity.
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Fig. 5 Effect of N rates applied for each of the first seven bermudagrass growth cycles and alfalfa row spacing on soil pH in the 0- to 15-cm depth. Soil samples were collected in July of the second growing season. (The equation uses N ratio per growth cycle multiplied by 7.)
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In the 15- to 30-cm depth, soil pH response to increasing N rates followed the quadratic function
(significant at the
level with an
). This equation revealed that the 28 kg N rate ha-1 applied as NH4NO3 had no effect on soil pH in the 15- to 30-cm depth, but higher N rates rapidly lowered pH in this depth.
Summary
Results from this research on alfalfa row spacing and N rates in a concurrent alfalfa and hybrid bermudagrass production system show the competitiveness of alfalfa compared with the grass on a fertilized Darco soil. Yield of alfalfa increased and bermudagrass production declined during the three years of this study. The tolerance of alfalfa to stress imposed by drought was observed. Alfalfa appeared to respond to applied N during times when it was stressed due to moisture deficit and cool temperature extremes. Detected Ca and Mg levels in alfalfa plant tissue appeared to be below published critical levels, yet no deficiency symptoms were apparent. Increased N rates and narrower alfalfa row spacings significantly lowered pH on this low-buffer-capacity Darco soil.
Although Alfagraze alfalfa was developed primarily to withstand moderate grazing pressure, we cannot (based on this research) speculate on the effects that grazing would have on this dual-forage system. In a grazing situation, grazing intensity and duration (trampling of growth buds) and livestock preferences for forage type could have a major effect on alfalfa sustainability.
The lack of sustainability of the Coastal bermudagrass in this concurrent cropping system appears to be related to climatic conditions that delay growth initiation in early spring and to its shallower rooting depth compared with alfalfa. Both conditions allow alfalfa to compete effectively with bermudagrass for stored soil water.SAS Institute 1991
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ACKNOWLEDGMENTS
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The authors thank the following for financial support of our research efforts to overcome the production limitations to alfalfa as an alternative forage on Coastal Plain soils: Texas-Louisiana Aglime and Fertilizer Association; U.S. Borax and Chemical Corp.; Farmland Industries, Inc.; IMC-Agrico Co.; The Sulphur Institute; AlliedSignal, Inc.; Western Ag-Minerals Company; Foundation for Agronomic Research; Texas Crushed Stone Co.; New Mexico Potash Sales; Texas Ag-Industries Association; and the Texas Agricultural Experiment Station Research Enhancement Program.
Received for publication April 27, 1998.
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REFERENCES
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ABSTRACT
INTRODUCTION
