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© 2006 Plant Management Network.
Accepted for publication 6 March 2006. Published 31 May 2006.

Winter Harvests for Annual Forage Medics in the Southern Great Plains

James P. Muir, Texas A&M University Agricultural Experiment and Extension Center, 1229 North US Hwy 281, Stephenville 76401; William R. Ocumpaugh, Texas A&M University Agricultural Research Station, Beeville 78102; and Twain J. Butler, The Noble Foundation, 2510 Sam Noble Parkway, Ardmore, OK 73401

Corresponding author: James Muir. j-muir@tamu.edu

Muir, J. P., Ocumpaugh, W. R., and Butler, T. J. 2006. Winter harvests for annual forage medics in the southern Great Plains. Online. Forage and Grazinglands doi:10.1094/FG-2006-0531-01-RS.

Abstract

The tradeoff between late winter harvest of cool-season annual forage legumes and subsequent forage yields and seed production may shed light on how management affects stand persistence. Eight annual medics (Medicago sp.), along with crimson clover (Trifolium incarnatum L.) and hairy vetch (Vicia villosa Roth.) for comparison, were seeded in the autumn of 1998 at Stephenville, TX, and allowed to self-reseed in the autumn of 1999. Six forage harvest schedules were imposed on subplots: uncut; single-cut in January, February, or March; two-cut (February + March); and three-cut (January + February + March). ‘Dixie’ crimson clover had the greatest early-season forage production in contrast to hairy vetch that peaked later in the season. Because of winter freeze damage and spring insect predation, the annual medics produced less forage than the hairy vetch or crimson clover, and are not well adapted to the southern Great Plains. However, these annual medics still managed to set seed in quantities likely to ensure stand viability for future years. Date of initial harvest and subsequent repetition of harvests did not affect seed yield; therefore it appears that these winter annual legumes can be harvested until initiation of flowering without negative consequences on soil seed reserves and subsequent stand persistence.

Tradeoff in Grazing Versus Self-Reseeding of Annual Winter Legumes

To sustain self-reseeding, annual forage legume stands must be managed for seed set that will result in seedlings for stand regeneration in subsequent years (1). Even when soil fertility is poor and herbage production low, some species in this class can generate more seeds and, as a result, more seedlings the next season, than a stand can sustain (11). Pasture managers, however, need to know just how early and how often forage can be removed (grazing or hay) without excessively lowering seed production.

Previous research indicates that annual forage legumes (vetches, clovers, and medics) can produce over 1000 kg of seed per year (2) that lays dormant (impermeable to moisture) during summers with up to 40% seed viability the subsequent season (14). Summer survival decreases precipitously, however, in climates with humid summers (11) with some species such as crimson clover more prone to premature germination than others (3).

Annual medics may have potential for the southern Great Plains, even when heavily utilized. Intensity of defoliation, defined by harvest height, has only slight effects on seed yields of some annual, self-reseeding legumes in the southern Great Plains (9) and other locations (8). As a group, these species are capable of stand regeneration, as defined by seed production, under some defoliation. The effects of defoliation initiation and duration, however, are not as well understood and could affect subsequent forage yields, forage nutritive value, and seed production. Therefore, the objective of this trial was to measure the effect of defoliation frequency of eight annual medics on forage and seed yield in the dry climate of the southern Great Plains, and compare them to crimson clover and hairy vetch, both already utilized in the region.

Procedures for Assessing Harvest Frequency

The experiment was carried out during the fall/winter growing seasons of 1998-1999 and 1999-2000 at Stephenville, TX (32°15’N, 98°12’W, altitude 395 m). The soil was a Windthorst fine sandy loam (pH 6.2; P, 10 mg/kg; K, 256 mg/kg; Ca, 1509 mg/kg; and Mg, 481 mg/kg), and no fertilizers were applied throughout the trial in an effort to emulate local soil conditions. The site was tilled in November 1998 prior to seeding and sprayed with clethodim ([E)-2-[1[(3chloro-2-propenyl)oxy]imino]propyl]-5-[2-(ethylthio)propyl]-3-hydroxy-2-cyclohexen-1-one) at 85 g a.i./ha and ammonium salt of imazethapyr (±2-[4,5-dihydro-4-methyl-4-(1-methylethyl)-5-oxo-1H-imidazol-2-yl]-5-ethyl-3-pyridinecarboxylic acid) at 28.4 g a.i./ha in December of both 1998 and 1999 to control grasses and broadleaf weeds. Larval alfalfa weevil (Hypera postica Gyllenhal) and twelve-spotted cucumber weevil (Diabrotica unclecimpunctata Mann) were observed from February to March of both years but no effort was made to control these since tolerance to their herbivory is considered part of the natural adaptation to local growing conditions.

Rainfall in the first season of the trial (1998-1999) was greater (443 mm October-May) than the second season (1999-2000; 332 mm October-May) both of which were less than the 30-year average (510 mm October-May; Fig. 1). First-season rainfall (October-May) was 87% and second-season rainfall was 75% of the 30-year average. No irrigation was applied throughout the trial period.

Fig. 1. Monthly precipitation and 30-year average trend line from October to May at Stephenville, TX.

In October 1999, seeds of ten cool season annual legumes (Table 1) were mechanically scarified in a Forsberg drum lined with sandpaper (Forsberg Inc., Thief River Falls, MN) and medics were inoculated with a general medic Rhizobium (Sinorhizobium meliloti) while the vetch and clover were inoculated with their respective specific Rhizobia according to the manufacturers’ specifications (Urbana Labs, Urbana IL; MicroBio, Saskatoon, SK). Using a Hege small plot seeder (Hege Inc., Colwich, KS), seeds were drilled at recommended seeding rates and soil depths (Table 1) (1) into cultivated 9.0- by 1.5-m plots packed to improve soil-seed contact. Plots were allowed to reseed themselves in the autumns of 2000 and 2001 without additional seed. Since these entries have seedpods and seed heads that do not dehisce, seed fell directly under plants in the various plots and subplots.

Table 1. Cool-season annual legume common names, cultivar or origin, and scientific name, grown at Stephenville, TX.

Subplots of 1.5 by 1.5 m were hand-clipped at 5 cm on January 15 only (one-cut January), February 15 only (one-cut February), March 15 (one-cut March), February 15/March 15 (two-cut), and January 15/February 15/March 15 (three-cut), to measure forage and seed yields. No harvests were considered beyond March since seed production, essential for stand regeneration among these annuals, would have been impeded. An uncut subplot was left to measure seed production from plants that were never defoliated. Forage was collected from the inner 1.0 by 1.0 m of each subplot at each harvest and this material was used to determine forage yield by drying all the material in a forced-air oven at 55°C until weight loss ceased. Total above-ground DM production was estimated for the year by totaling all yields from each subplot.

Representative forage sub-samples from each harvest were weighted for relative contribution to total yield, batched by year, ground through a sheer mill equipped with 1-mm screen and analyzed for ADF, ADF-lignin, and N concentrations. The fiber component was measured by methods defined by Van Soest and Robertson (12). Total N concentrations were measured with a modification of the aluminum block digestion procedure of Gallaher et al. (5). Sample weight was 1.0 g, digest used was 5 g of 33:1:1 K₂SO₄:CuSO₄:TiO₂ and digestion was conducted for 2 h at 400°C using 17 ml of H₂SO₄. Nitrogen in the digestion fluid was determined by semiautomated colorimetry (6) using a Technicon Autoanalyzer II (Technicon Industrial Systems, Tarrytown, NY). Nitrogen concentration was multiplied by 6.25 and reported as crude protein (13). Concentrations of these plant components are reported as season-long weighted averages.

Mature seeds were collected from a 0.3 by 0.3 m area outside the 1.0 m² portion of each subplot used for measuring forage yield. This was done to avoid having artificial seed dispersal influencing subsequent forage yields although seeds were returned to their 0.09 m² of origin in the respective subplots. These collections occurred in April through May, depending on the maturity of each entry. Seed yields from uncut subplots therefore represent the seed production potential of the entries where no forage harvest removal occurred. Collected seedpods were then dried, broken down in a Forsberg drum lined with sandpaper, and the seeds removed by hand to estimate seed yield per square meter.

Plots were arranged in a randomized complete block design (four replications) and harvest schedules were super-imposed on these in a strip-plot arrangement. Year (one and two), entry, and harvest regime were used as independent variables in the model and analyzed for interactions as well as simple effects. Year was included in the model despite the fact that plots were seeded the first year and self-reseeded the second year because seedling numbers were not considered limiting either year (10). Results were submitted to an analysis of variance with a Least Significant Difference multiple range test (P < 0.05) used to separate means in the case of significant (P ≤ 0.05) effects.

Forage Yields

All two-way interactions for forage yield were significant, with year by species or year by harvest interactions (data not shown) attributable mainly to differences in yearly rainfall and freezing temperatures (Fig. 2). All entries germinated and had adequate seedling stands but the barrel medic plants were heavily damaged by freezing temperatures during December and January while all medics were severely defoliated by larval alfalfa weevil infestations that began both years in February-March.

Fig. 2. Monthly mean temperatures in 1998-2000 and 30-year average trend line from October to May at Stephenville, TX.

Harvest schedule by species interactions on forage production (Table 2) can be related to differences in species precocity. Crimson clover had the greatest January-only and February-only production, matching peek growth with the greatest rainfall months of December and January. In contrast, the only month that hairy vetch matched crimson clover forage yield was March, by which time crimson clover had set seed. In all cases, however, yields were low compared to those reported elsewhere in similar climates (8,9) primarily because harvests did not extend beyond March when the annual medics initiated flowering.

Table 2. Harvest schedule by species (interaction P = 0.001) forage yield of ten annual cool season legumes at Stephenville, TX (pooled over two years).

 * Values within columns followed by different upper case letters and within rows followed by different lower case letters differ according to least significant difference multiple range separations (P < 0.05).

Forage Nutritive Value

Entry CP concentrations ranged from 23.1 to 29.2 g/kg forage (pooled across harvest schedules), the later being hairy vetch which had 13% greater CP concentration than the next greatest, ‘Armadillo’ burr medic (Medicago polymorpha L.; Table 3). All of these values reflect late winter harvests that consisted primarily of vegetative growth, and are superior to those reported elsewhere (9) when plants were harvested beyond March. As plants matured, from January-only highs to March-only lows, CP concentration decreased 13% (Table 3; pooled across species).

Table 3. Crude protein (CP), and acid detergent fiber (ADF) concentrations (P = 0.001 for both) in the forage of annual cool season legumes harvested at five different schedules (pooled over two years and nine species) and of the nine annual cool season legumes at Stephenville, TX (pooled over two years and five harvest schedules).

 * Values within columns followed by different upper case letters differ according to least significant effect multiple range separations (P < 0.05).

Hairy vetch and ‘Parabinga’ barrel medic (M. truncatula Gaertn.) had the greatest ADF concentrations (Table 3; pooled across harvest schedules), 29% greater than the average value for the burr medics. In contrast to CP concentrations, January-only ADF was 20% lower than March-only ADF (Table 3; pooled across harvest schedules). Even March-only ADF concentrations were lower than those reported for the same species harvested later (from January-April) (9), reflecting a predictable relationship between maturity and ADF (4,7).

Acid detergent fiber lignin varied among entries depending on harvest schedule (Table 4). Hairy vetch was consistently high in ADF lignin, over 4.0% even in the January-only harvest. Crimson clover and ‘Devine’ little medic (M. minima L.) ADF lignin concentrations were at the other end of the spectrum, close to 3.0% in January-only harvests and below 4.0% in the March-only harvests. Other entries exceeded 4.0% concentrations in the March-only harvest, the medics as much because of leaf loss to insect herbivory as to maturity.

Table 4. Harvest schedule by species (interaction P = 0.001) forage acid detergent fiber lignin of nine annual cool season legumes at Stephenville, TX (pooled over two years).

 * Values within columns followed by different upper case letters and within rows followed by lower case letters differ according to least significant difference multiple range separations (P < 0.05).

Seed Yields

Plants that did not have forage removed (uncut) did not produce more seed than those harvested in late winter, regardless of year or species (data not shown), averaging 532 kg seed/ha/year. This value is comparable to those reported elsewhere for some of these species if adjustments are made for rainfall and insect damage (8,9,11). These same factors contributed to a year by entry interaction with four entries producing more seed in the first year (33% greater rainfall) than in the second year (Table 5). A prolonged freezing period in December 1998 killed most barrel medic seedlings the first year so that the two barrel medics produced greater quantities of seed the second year than the first year. Crimson clover, BEBLK black medic (M. lupulina L.), and little medic had the least variable production across years, indicating a more adaptable seed-production strategy in these entries.

Table 5. Year by species (interaction P = 0.001) seed yield of ten annual cool season legumes at Stephenville, TX (pooled over six harvest schedules).

 * Values within columns followed by different upper case letters and within rows followed by different lower case letters differ according to least significant difference multiple range separation (P < 0.05).

Early season forage production did not necessarily correlate with subsequent seed production. Black medic and little burr medic, for example, produced very little forage up to March (Table 2) since most of their forage appeared in subsequent warmer months (9,10); however, both yielded the greatest quantity of seed the first season and were inferior only to crimson clover the second year (Table 5). In contrast, crimson clover out-yielded the other entries in both forage and seed, indicating it is better suited in north-central Texas than the medics.

Summary and Conclusions

Initiating forage harvests in January through March did not negatively affect subsequent seed yields in any of the ten annual winter legumes included in this study. Nutritive value of the forage harvested at this time was very high, although actual production did not exceed 2000 kg except for crimson clover, which tends to grow in mid-winter in the southern Great Plains. Because of winter freeze damage and spring insect predation, the annual medics produced less forage than the hairy vetch or crimson clover, and are not well adapted to the southern Great Plains. However, these annual medics still managed to set seed in quantities likely to ensure stand viability for future years. Date of initial harvest and subsequent repetition of harvests did not affect seed yield; therefore it appears that these winter annual legumes can be harvested until initiation of flowering without negative consequences on soil seed reserves and subsequent stand persistence.

Literature Cited

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10. Muir, J. P., Ocumpaugh, W. R., and Read, J. C. 2003. Spring forage yield and nutritive value of Texas Black medic accessions. Agon. J. 95:908-912.

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12. Van Soest, P. J., and Robertson, J. B. 1980. Systems of analysis for evaluating fibrous feeds. Pages 49-60: Methods Feeds Proc. Intl. Workshop on Standardization Analysis, Ottawa, Can. 12-14. Mar. 1979. W. J. Pigden, C. C. Blach, and M. Graham, eds. Unipub, New York.

13. Van Soest, P. J. 1994. Nutritional Ecology of the Ruminant, 2nd Ed. Cornell Univ. Press, Ithaca, NY.

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