How winter annual forage legumes persist in diverse soil moisture environments of a semi-arid region

How Winter Annual Forage Legumes Persist In Diverse Soil Moisture Environments of a Semi-arid Region

L. M. Lauriault, Forage Agronomist, and R. E. Kirksey, Superintendent, Agricultural Science Center at Tucumcari, New Mexico State University, 6502 Quay Road AM.5, Tucumcari, NM 88401; and D. M. VanLeeuwen, Agricultural Biometrician, Department of Agricultural and Extension Education, Agricultural Biometrics Service, Box 30003 MSC 3501, Las Cruces, NM 88003-8003

Abstract

Varied microenvironments complicate use of legumes in marginal lands. Seven annual cool-season legume species, sown for natural reseeding, were rated for bloom and germination dates, fall vigor, and spring percentage stand at Tucumcari, NM. Soil moisture effects were: furrow-irrigated once to promote germination, irrigated monthly April to October, irrigated monthly year-round, and poorly drained, saline/sodic soil. The study was a strip-split-split-split plot over four years having four replications nested within each soil moisture strip plot. Trifolium nigrescens did not establish in any soil moisture situation. Only T. alexandrinum established > 20% stand in poorly drained soil; however, neither it nor T. vesiculosum naturalized in any strip plot. Naturalized stand percentage declined differently across years for species and soil moisture treatments. T. incarnatum reseeding failed in 1999; the others, except T. vesiculosum and T. alexandrinum, reseeded throughout the test, Vicia sativa being most consistent across soil moisture treatments, but at lower levels of stand establishment than other species. T. alexandrinum might serve for annually replanted cover in poorly drained, saline/sodic soils. T. hirtum and V. villosa also might be useful.

Introduction

While it is anticipated that few species will be better adapted to a region than those already commonly grown (1,16), continued screening is still needed to identify the potential of previously untested species in some environments (16). Marginal areas and disturbed sites that should be established in permanent cover are of notable concern. Semiarid regions, such as the Southern High Plains, are often characterized by low winter precipitation (9), high pH, saline (16), and/or sodic conditions, and variations in soil moisture availability. Keeling et al. (8) stated that soils in the Southern Great Plains are highly erodible in late winter and early spring due to low precipitation and high winds.

Annual species can be effectively used to improve resource use efficiency, maintain ground cover, and even increase productivity where precipitation limits the use of perennial forage crops (4). Annual legumes generally establish quickly (7) and are more competitive against weeds than perennials (17). Additionally, they might have added value as winter cover in annual cropping systems (7,8,14) or as early seral species for re-establishment of disturbed sites. While only one or two legume species might be able to maximize production on a given site (1), several species might provide satisfactory production or other benefits across a variety of soil conditions.

Schlegel and Havlin (17) found that earlier maturing legumes left more water available for successive crops and Volesky et al. (18) mentioned that some annual cool-season legumes perform well when interseeded into dormant perennial warm-season grasses because their growth stops before the grass initiates growth in spring and they serve as a nitrogen source for the grass. A desirable trait of many annual legumes is the ability to sustain stands through natural reseeding (2,3). Hairy vetch is well adapted to the Southern High Plains when annually sown (10); however, its performance, or that of other cool-season annual legumes as a naturalized species (14) has not been extensively evaluated.

The objective of this research was to characterize the likelihood of stand persistence through naturalization of selected annual cool-season forage legumes under different soil moisture treatments typical of marginal lands in the semiarid Southern High Plains of the USA.

Experimental Design, Methodology, and Analysis

Studies were conducted from 1997 to 2001, at the New Mexico State University Agricultural Science Center at Tucumcari, NM (35.20°N, 103.69°W; UTM: 13619577 E, 3895834 N; elevation 1247 m). The experimental design was a strip-split-split-split plot in which the same species of cool-season annual forage legumes, as the subplot, were compared under different soil moisture treatments as strip plots, each having four replicates, as follows: furrow-irrigated once immediately after planting to promote germination of the initial seeding (dryland), irrigated approximately monthly beginning in mid- to late April using surface water (growing season irrigation); irrigated approximately monthly year-round using ground water during winter (year-round irrigation); and poorly drained, saline/sodic soil irrigated only as needed to maintain a moist soil surface, but generally less than once per cutting.

The soil for the dryland, growing season and year-round irrigation strip plots was an area of mixed Canez fine sandy loam (fine-loamy, mixed, thermic Ustollic Haplargids) and Quay fine sandy loam (fine-silty, mixed, superactive, thermic Ustic Haplocalcids) with initial soil test levels of 48 mg/kg P (NaHCO₃ extractant), 192 mg/kg K (ammonium acetate extractant), 8.2 pH, 0.4 mmhos/cm soluble salts, and 0.9% Na base saturation. The poorly drained, saline/sodic soil was Canez fine sandy loam, calcareous variant, which had soil test levels of 34 mg/kg P (NaHCO₃ extractant), 236 mg/kg K (ammonium acetate extractant), 8.2 pH, 2.4 mmhos/cm soluble salts, and 15.4% Na base saturation, which were considerably higher than the soils used for the other strip plots and made it borderline saline/sodic (6), and the water table was approximately 1 m below the surface (L. M. Lauriault, unpublished data, 1997 and 1998). The site was downslope from other irrigated land and an unlined irrigation canal and was usually kept wet by subsurface drainage from those areas, generally negating any irrigation. A drainage ditch constructed around the area was used when necessary to allow sufficient soil drying to support equipment. Although this soil/moisture effect was not imposed as the irrigation treatments were, it does reflect a common situation in semiarid regions, whether or not irrigation is used. Consequently, it is equal in classification to the irrigation treatments in regard to its effects on plants grown in it.

Seven cool-season annual legumes were tested as subplots, including: arrowleaf clover (Trifolium vesiculosum Savi. cvs Meechee and Yuchi), ball clover (T. nigrescens Viv. cvs Segrest and VNS), berseem clover (T. alexandrinum L. cvs Bigbee and Joe Burton), crimson clover (T. incarnatum L. cvs Chief, Dixie, and Tibbee), rose clover (T. hirtum All. cvs Hykon and Overton R18), common vetch (Vicia sativa L. cvs Cahaba white, Nova II, Vanguard, and Vantage), and hairy vetch (cvs Americus, AU Earlycover, and VNS). Variety within species was the sub-subplot.

Seedbeds were conventionally tilled and formed into beds on 0.9 m centers for furrow irrigation. Plots, 4.6 m × 1.8 m, were sown 18 and 19 September 1997, using a disk drill (20-cm drill spacing) fitted with a seed-metering cone. The seeding rate was 18.0 kg/ha for arrowleaf clover; 4.5 kg/ha for ball clover; 22.5 kg/ha for berseem, crimson, and rose clovers; and 39.3 kg/ha for the vetches. Seed of each species was inoculated with the appropriate strain of Rhizobium prior to planting. Seed placement was approximately 1.3 to 1.9 cm below the soil surface to overcome common establishment problems due to moisture loss from the surface of sandy soils (8).

During the four days after planting, 28.7 mm of precipitation fell, which promoted germination (8). All plots were irrigated on 3 October 1997, after which precipitation events occurred approximately every two weeks until mid-November keeping the surface 5 cm moist and helping to overcome the common problem of establishing smaller-seeded species like those used in the present study on sandy soils (8). The 3 October 1997 irrigation was the only time the dryland test was irrigated. Irrigations were delivered through gated pipe and were of sufficient duration to completely wet the center of the beds for their full length. Historical irrigation flow rate data at this location, collected as described by Ziska et al. (19), was used to estimate that approximately 20 cm of water was applied with each irrigation. It is not likely that all of the applied water infiltrated the soil because furrow irrigation efficiency can be as low as 50% (15). Additionally, it is likely that some deep percolation occurred, some of which eventually accumulated at the poorly drained, saline/sodic site. Experimentation was discontinued after the 2001 growing season because irrigation water was unavailable due to drought in the watershed. No fertilizers were applied for the test duration. No forage was removed to allow natural reseeding (18), but plots of arrowleaf clover were clipped with a sickle mower on 12 August 1998, to promote seed-to-soil contact.

In April 1998, stand density as a percentage of drilled row occupied by established plants was visually observed. Percentage ground cover of the sown species was visually rated in April 1999 to 2001. Julian date (JD) of 50% bloom (12) was determined for each plot in 1998, as was JD of uniform germination that would produce a uniform stand from reseeding. Bloom and germination ratings were taken at four-day intervals until 50% of plants had bloomed on all plots for JD of 50% bloom or germination had plateaued. Data for JD to 50% bloom and uniform germination were not collected from plots in the poorly drained, saline/sodic soil because the area was too wet to enter. On 30 November 1998, vigor of reseeded plants were rated based on a 0 to 5 scale, where 0 represented no plants and 5 indicated the largest plants on that date. All ratings were based on a whole plot estimate by the same observer throughout the study.

Weather data were collected from a National Weather Service station located within 1 km of the study area. The climate in the region is continental, characterized by cool, dry winters and warm, moist summers (9). Approximately 83% of the precipitation occurs as intermittent, relatively intense rainfall events from April through October. July and August typically have the highest precipitation (9). Three of four winter growing seasons were warmer than average (Table 1). Precipitation was slightly above average in 1997-1998 and 2000-2001 and well above average in 1998-1999, but 1999-2000 was well below average (Table 2).

Bloom and germination date, and fall vigor were analyzed as a strip-split-split plot (11) with soil moisture treatment as the strip plot, and species and variety within species as the sub, and sub-subplots, respectively. A similar analysis for stand percentage used a strip-split-split-split plot over time where year was the sub-sub-subplot. All data were subjected to SAS PROC MIXED ANOVA (SAS Institute Inc., Cary, NC) to test the main effects of soil moisture treatment, species, variety within species, and year (percentage stand only) and all possible interactions. Replicates were nested within soil moisture treatments. Rep × soil moisture treatment, rep × soil moisture treatment × species, rep × soil moisture treatment × variety within species and residual mean squares were considered random and used as denominators for tests of significance (11). All differences reported are significant at P ≤ 0.05. When an interaction was significant, the PDIFF option was used to slice the data by species and protected least significant differences were used to determine where differences occurred among species and soil moisture treatments. Differences due to variety within species were generally small in magnitude compared to species differences, and will not be discussed.

Stand Persistence of Annual Cool-season Legumes

Ball clover was not included in the analysis (13) due to poor stand percentage prior to winter 1997 that did not increase in spring 1998 (data not shown). While plants of ball clover were present in 1998, achieving 50% bloom on JD 113, they were sparse (<13% stand across soil moisture treatments) and not sufficient to produce a uniform stand in any soil moisture treatment for reseeding in fall 1998 (data not shown). Very small seed size, less than optimum seed-to-soil contact, and the planting depth all may have been factors in poor establishment of ball clover (8). Although, whenever autumn drought did not occur, Pedersen and Ball (13) established stands producing measurable yields of ball clover by placing seed 1.0 to 1.3 cm below the soil surface in soils having a similar texture to those used in the present study. Salt intolerance also is a trait of many clover species (16). That some seeds of ball clover did germinate and produce plants may indicate the possibility of genetic improvement to broaden that species’ use in marginal environments of semiarid regions.

Julian Date of 50% Bloom

While differences existed among soil moisture treatments for JD of 50% bloom in 1998 (Table 3), they were not considered biologically significant (5); however, moisture stress does often hasten maturity. Among species, arrowleaf and berseem clover bloomed much later than the other species while crimson clover and common vetch bloomed much earlier (Table 4). The timing and order of bloom for most species were similar to those observed by others (2,7,12).

Table 3. Bloom date, germination date, and fall vigor during 1998 of winter annual forage legumes sown in 1997 into differing soil moisture treatments at Tucumcari, NM.^s

Soil moisture treatment	Bloom date^t	Germination date^u	Vigor,^v 11/30/98
Dryland^w	116	237	2.8
Growing season irrigation	117	232	2.9
Year-round irrigation	118	237	2.3
Poorly drained, saline/sodic	No obs.^x	No obs.	0.6
LSD, 0.05^y	1	NS^z	0.4

^s Species include arrowleaf, ball, berseem, crimson, and rose clovers and common and hairy vetches. Data are the means of seven species, each represented by two to four varieties, and four reps.

^t Julian date when 50% bloom occurred.

^u Julian date when uniform germination occurred.

^v 0 = no plants; 5 = largest plants on the date of rating.

^w Dryland, growing season irrigation, year-round irrigation, and poorly drained, saline/sodic signify: irrigated once after planting in 1997; irrigated after planting and approximately monthly from mid-April to mid-October, 1998 through 2001; irrigated monthly year-round; and soil surface remained moist throughout the growing season and shortly thereafter, respectively.

^x No observations. The poorly drained, saline/sodic soil was too moist from mid-April through October for entry to take ratings. Data for this soil moisture treatment was excluded from statistical analysis for these variables.

^y Least significant difference (P < 0.05).

^z Not significant.

Table 4. Bloom date, germination date, and fall vigor during 1998 of winter annual forage legumes sown in 1997 into differing soil moisture treatments at Tucumcari, NM.^v

Species	Bloom date^w	Germination date^x	Vigor,^y 11/30/98
Arrowleaf clover	144	279	1.3
Berseem clover	140	226	1.1
Crimson clover	103	216	3.9
Rose clover	112	249	3.1
Common vetch	106	222	3.7
Hairy vetch	116	223	3.6
LSD, 0.05^z	1	3	0.7

^v Soil moisture treatments include dryland (irrigated once after planting in 1997), growing season irrigation (irrigated after planting and approximately monthly from mid-April to mid-October, 1998 through 2001), and year-round irrigation. Data are the means of four reps and two to four varieties.

^w Julian date when 50% bloom occurred.

^x Julian date when uniform germination occurred; germination of ball clover in 1998 was not sufficient to produce a uniform stand.

^y 0 = no plants; 5 = largest plants on the date of rating.

^z Least significant difference (P < 0.05).

Seed production is critical to building a soil seed-bank for sustaining stands of annual species (1,4). Earlier maturing species (Table 4) can take advantage of winter precipitation and cooler temperatures during spring in temperate or subtropical Mediterranean climates (2,12). In temperate or subtropical continental climates having warm, dry winters (9), however, early maturity might occur during periods of low precipitation, reducing seed yield and quality. Later maturing species or cultivars might have more available soil moisture, but also are exposed to higher temperatures possibly leading to reduced seed yield and quality (4). Establishment and maintenance of a soil seed-bank, thus, becomes more critical (1,4).

Julian Date of Uniform Germination

There was no difference among soil moisture treatments for date of uniform germination (Table 3). Germination in 1998 resembled a broadcast seeding rather than drilled rows, which was the technique used for the initial planting and indicates that the stand was not regenerated to a great extent from hard seed that had been sown in 1997.

Differences did exist among species in date of uniform germination (Table 4). The differences among berseem, rose, and arrowleaf clovers (Table 4) are consistent with the findings of Evers (4). Beuselinck et al. (1), Evers (4), and Fairbrother and Pederson (5) all stated that stand persistence of small-seeded cool-season annual legumes, such as those in the present study, was dependent on hardseededness that establishes a soil seed-bank to overcome times of less than optimum environmental conditions. Keeling et al. (8) attributed stand failure occurring 30 to 50% of the time to low soil moisture for germination or seedling desiccation after germination. Additionally, Evers (4) mentioned that poor or slow germination is often a cause of poor stands in cool-season annual legumes and that identifying clovers that did not germinate if moisture became available when temperatures were warm was critical to maintaining stands by naturalization in the southeastern USA (1).

Hence, loss to the seed-bank can be even greater in non-Mediterranean environments, such as the Southern High Plains (9) because summer precipitation promotes germination at higher temperatures (4), but the dry seasons of autumn, winter, and spring can cause stand failure due to drought or reduced seed production (4,13). Fairbrother and Pederson (5) stated that germination regulation is the most important factor determining persistence of naturalized annual clovers. Volesky et al. (18) found that rose clover produced high levels of hard seed that were slower to soften than crimson and subterranean clover. Viable rose clover seed has been found after 23 years in dung (18). Long-term seed viability helps to build a soil seed-bank allowing the rose clover to reseed even after years of low seed production (4,18).

Fall Vigor

Vigor of the few plants of all species from reseeding in the poorly drained, saline/sodic soil was considerably lower than those growing under year-round irrigation, which was lower than both dryland and growing season irrigation (Table 3). The water used for irrigation during the winter for the year-round irrigation treatment was from a well located in close proximity to the poorly drained, saline/sodic site, having 291 ppm Na, a sodium absorption ratio of 6.23, and an electrical conductivity of 1.68 in spring 1998 [(6); L. M. Lauriault, unpublished data, 1998]. This also might have been a factor in poor vigor of the year-round irrigation treatment (Table 3).

Differences in vigor existed among species (Table 4). Volesky et al. (18) mentioned that rose clover was known for poor seedling vigor and fall growth, especially during dry weather. Precipitation during the late summer and autumn likely promoted germination of all species but arrowleaf and berseem clovers (Table 4). There was a soil moisture treatment × species interaction because fall vigor of the clovers as a group was less under year-round irrigation, but the vetches were consistent across soil moisture treatments (data not shown).

Percentage Stand

The year × soil moisture treatment × species interaction existed among species (Fig. 1). For the most part, the interaction appears to be due to differences in the rate of decline over time (20) among species across soil moisture treatments. Arrowleaf clover and common vetch did not produce 80% stand from the initial seeding in any soil moisture treatment (7) (Fig. 1) while all other species did, except in the poorly drained, saline/sodic soil. In that soil moisture treatment, stand percentage of all species was lower and none reseeded sufficiently in 1998 (Fig. 1). Drilling in rows might have enhanced establishment of the initial seeding in the poorly drained, saline/sodic soil. It was observed that, as the soil surface dried, soil ridges turned white with salts (6). Depressions formed by packer wheels on seed drills might be beneficial for establishing salt-sensitive species, like many clovers (16), by collecting water to leach salts away from the seed zone and by forming ridges away from the seed zone where salts may crystallize at the surface (6). Initial stands might have been greater in the other soil moisture treatments (Fig. 1) if seeding had been broadcast rather than drilled. However, establishment usually is better using drills with packer wheels compared to broadcasting on the soil surface. Volesky et al. (18) stated that even if establishment is successful from the initial seeding, soil properties could lead to reseeding failures.

Fig. 1. The difference over years in spring percentage stand of selected legumes grown under different soil moisture treatments at Tucumcari, NM. Data are the means of four reps and two to four varieties. Bars indicate the LSD (P < 0.05) for within year comparisons. Absence of an LSD within a year indicates no significant difference among soil moisture treatments. Dryland, growing season irrigation, year-round irrigation, and poorly drained, saline/sodic signify: irrigated once after planting in 1997, irrigated after planting and approximately monthly from mid-April to mid-October, 1998 through 2001, irrigated monthly year-round, and infrequently irrigated because the soil surface remained moist throughout the growing season and shortly thereafter, respectively.

Arrowleaf clover in the present study had lower stand percentage with year-round irrigation compared to dryland and growing season irrigation (Fig. 1), again, probably due to the salt content of the year-round irrigation water [(6, 16); L. M. Lauriault, unpublished data, 1998-2001]. Arrowleaf clover is generally a good seed producer (16) and poor stand persistence across soil moisture treatments might have been due to poor seed-to-soil contact in 1998 (Fig. 1). Arrowleaf clover was the latest to bloom (Table 4) and stems remained erect with seed burrs retained. Arrowleaf clover plots were clipped 12 August 1998, to promote seed-to-soil contact, but that might not have been effective; however, abnormally high August 1998 precipitation should have promoted seed-to-soil contact (Table 2). Edwards (3) stated that annual reseeding of arrowleaf clover was dependent on soil disturbance. Uniform germination of arrowleaf clover that should have produced a stand was observed on 6 October 1998 (Table 4). Consequently, other factors also may have had a role in poor stands in spring 1999 that may be related to soil properties, including alkalinity (13,18), leading to poor fall vigor (Table 4).

Berseem clover, which is well adapted to alkaline soils (13), established excellent initial stands in all soil moisture treatments, except the poorly drained, saline/sodic soil where it still had the highest stand of all species tested (Fig. 1). It also was observed to have the greatest fall growth and abundant spring growth [(13,16); L. M. Lauriault, unpublished data, 1997 and 1998]. Consequently, this species might have value as fall and spring forage or as short-term ground cover for later re-establishment of disturbed lands with native warm-season grasses (18). Still, berseem clover, which is known to be a poor reseeder (16), did not reseed in the present study (Table 4) so there also would be an annual establishment cost (2,3) if it were used.

Crimson clover produced good initial stands and reseeding was more consistent across soil moisture treatments, except the poorly drained, saline/sodic soil (Fig. 1). Boquet and Dabney (2) reported successful reseeding by crimson clover in the first and second year after seed production. Quality seed production and early fall germination are likely the cause of good fall vigor (Table 4), which enhances winter survival and the likelihood of successive seed production.

Common vetch, although producing poor stand percentage (Fig. 1), also was consistent in reseeding and for a longer duration under growing season irrigation than crimson clover. Rose clover and hairy vetch both continued to reseed throughout the study period (Fig. 1). Performance across soil moisture treatments, though nearly consistent between the two species, was not nearly as consistent as within other species (Fig. 1).

The decline in percentage stand in 2000 for both rose clover and hairy vetch grown under dryland conditions (Fig. 1) is likely due to low precipitation throughout the 1999-2000 growing season (Table 2) (8). The resurgence of stand percentage in the dryland treatment for these two species in 2001 (Fig. 1) is likely due to a high percentage of hard seed in the soil bank (1,4,18) and more favorable precipitation, particularly in late winter and very early spring 2001 (Table 2), prior to the initiation of active growth. While it is unknown whether they generated from hard seed or a spring seed crop, rose clover plants were observed in the dryland treatment strip plot of the present study in autumn 2006 although other species had encroached.

While hairy vetch did not produce much fall growth, it did produce a significant amount of growth in the spring [(7,18); L. M. Lauriault, unpublished data, 1997 and 1998] so it might be valuable for spring forage, a green manure crop, or for reclamation of disturbed lands in semiarid continental regions. Rose clover did not produce measurable forage in any season [(7); L. M. Lauriault, unpublished data, 1997 and 1998], but might have value for providing nitrogen to low-input warm-season grass systems or as a green manure crop for warm-season annual crops such as cotton (Gossypium hirsutum L.) or sorghum [Sorghum bicolor (L.) Moench] for grain or forage.

Conclusions

Genetic improvement of ball clover may develop cultivars that are better adapted to marginal conditions in semiarid regions. Berseem and crimson clover might have value for fall and spring forage in the Southern High Plains of the USA. None of the species tested appears to be adapted to poorly drained, saline/sodic soil, but berseem clover holds the most promise for that purpose. It would, however, likely have to be reseeded each year. Stand persistence of hairy vetch and rose clover were of longer duration and across a broader range of soil moisture treatments, except poorly drained, saline/sodic soil, than the others. These species might have value for protecting marginal lands or for reclamation after road construction, mining or fire throughout the Southern High Plains and southern Rocky Mountains. Winter irrigation of cool-season annual legumes is not particularly beneficial if low quality water is used.

Acknowledgments

We gratefully acknowledge the technical and field assistance of George Arguello, Eutimio Garcia, and Leslie Robbins; secretarial assistance of Terri Warren, Doris Hight, and Patty Cooksey; and the staff at the NMSU Library Document Delivery Service.

A contribution of the New Mexico Agric. Exp. Stn., New Mexico State Univ., Las Cruces.

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