Agronomy Journal 95:908-912 (2003)
© 2003 American Society of Agronomy
FORAGES
Spring Forage Yield and Nutritive Value of Texas Black Medic Accessions
James P. Muir*,
William R. Ocumpaugh and
James C. Read
Texas A&M Univ. Agric. Exp. Stn., 1229 North U.S. Hwy. 291, Stephenville, TX 76401
* Corresponding author (j-muir{at}tamu.edu)
Received for publication March 29, 2002.
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ABSTRACT
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Black medic (Medicago lupulina L.) has become naturalized in south-central USA where, in semiarid climates, it acts as a cool-season annual. Cultivation for wildlife and livestock forage and seed has not been fully realized as cultivars adapted to warm and dry climates have not been developed. This study evaluated forage yields and nutritive value of black medic accessions collected in Texas. March, April, and May forage yield, acid detergent fiber (ADF), acid detergent lignin (ADL), N, and P concentrations of 11 black medic accessions collected in Texas were compared with cultivar George at Stephenville, TX, during establishment and first self-reseeding years. Entries exhibited no frost damage and insignificant insect damage, but some Texas accessions were more vigorous in February and flowered earlier than other Texas accessions and George. Some Texas accessions were also more productive (P < 0.01) than others as well as George. Forage yields among entries ranged from 0.6 to 1.5 Mg ha-1 yr-1 when averaged over years and harvest months. Harvest month affected all entry yields equally and peaked at 2.3 Mg forage ha-1 yr-1 during May of the first season but declined in the second, self-reseeding year despite greater rainfall. March ADF, ADL, N, and P concentrations tended to be lower than those of later harvests while the higher-yielding accessions also tended to have greater ADF and ADL concentrations. The results indicate that forage yield and nutritive value of naturalized black medic germplasm from Texas is variable and that a blend from the southern Great Plains should be developed.
Abbreviations: ADF, acid detergent fiber • ADL, acid detergent lignin • DM, dry matter • LSD, least significant difference
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INTRODUCTION
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BLACK MEDIC, a cool-season, weak perennial of Eurasian origin, has become naturalized throughout much of the southeastern USA where it volunteers as a self-reseeding annual (Diggs et al., 1999; Ball et al., 1996). Reported forage yields in the northern USA vary from 1 Mg ha-1 yr-1 (de Haan et al., 1997) to 3.6 Mg ha-1 yr-1 (Zhu et al., 1996), with production usually concentrated in late spring (Ball et al., 1996). Despite reports of excellent adaptation in cooler, dry climates (Rumbaugh and Johnson, 1986), agronomic performance of black medic in the dryer portions of south-central USA has not been well documented, and no commercial cultivars are presently available from this region. Black medic's ability to self-reseed, its high quality forage, and its wide adaptation to growing conditions make it an ideal candidate for wildlife plantings and other low-intensity pasture systems.
One objective of this trial was to compare the adaptation and productivity of locally collected Texas accessions with the only commercial black medic presently available in Texas, cultivar George from Montana (Sims et al., 1985). The Texas accessions were collected by Texas Agricultural Experiment Station personnel on the basis of vigor and productivity, with the expectation that they should possess greater adaptation to local edapho-climatic, disease, and insect pressures. Selection was based on limited populations (under 100 plants for each location) in distinct climates and soils across the state and involved seed collection from the most vigorous plants in naturalized stands. The ability to self-reseed, likely dependent on early flowering where seed set must occur during spring rains, is paramount if persistence is to be achieved in sustainable pasture systems. The second objective was to compare the relative productivity and quality of entries in March, April, and May. Entries that produce significant quantities of forage through the colder winter months (indicated by yields at a March harvest) would be particularly useful because most native forages are dormant during those months.
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MATERIALS AND METHODS
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The trial included 11 Texas accessions and George (Table 1) and was located at Stephenville, in the Cross Timbers physiographic region of north-central Texas (32°13' N, 98°12' W; 399 m elevation). Soil at the site was a Windthorst fine sandy loam (fine, mixed, thermic Udic Paleustalf) with an initial pH of 6.7, 11 mg P kg-1 soil, 96 mg K kg-1 soil, 970 mg Ca kg-1 soil, and 138 mg Mg kg-1 soil (A&M Extractant). The soil was not amended with any nutrients during the trial, despite low levels of P, in an effort to emulate native range conditions.
Rainfall at Stephenville in the first season of the trial was low, totaling 332 mm from October 1999 to May 2000. Irrigation was applied in months that had less than 50% of the long-term average rainfall, giving a total of 449 mm of moisture (rainfall and irrigation) for the 1999–2000 winter growing season. Rainfall in the second season (2000–2001) was greater (654 mm) and well distributed so that no irrigation was applied.
The study area was tilled in late November 1999 and kept free of weeds with the November application of clethodim {(E)-2[1-[[(3-chloro-2 propenyl)oxy]imino]propyl]-5-[2-(ethylthio)propyl]-3-hydroxy-2-cyclohexene-1-one} at 74 g a.i. ha-1 and ammonium salt of imazethapyr {(±)-2-[4,5-dihydro-4-methyl-4-(1-methyl)-5-oxo-1 H-imidazol-2-yl]-5-ethyl-3-pyridine carboxylic acid at 148 g a.i. ha-1 both years.
Eleven of the black medic entries in the study were collected from around Texas (Table 2) based on vigor and abundant seed production and were compared with George, a cultivar developed in Montana. All seeds were scarified and inoculated with Medicago spp.–specific Rhizobium sp. (Urbana Lab., St. Joseph, MO), hand-broadcast onto 1.5- by 3-m plots at 12 kg seed ha-1, and firmly packed with a roller in November 1999. The plots were arranged in a completely randomized block design with three replications. The trial depended solely on self-reseeding for re-establishment in the autumn of 2000, and no tillage took place that second year.
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Table 2. Establishment and branch length (from crown base) of 12 black medics in February 2000 at Stephenville, TX.
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Plots were visually evaluated for vigor (0 = low vigor; 3 = high vigor) and length or height of branch development in February of 2000. A 1- by 1-m area in each plot was harvested at a 4-cm height in March, April, and May of 2000 and 2001. A different area was used for each harvest so that each month's harvest represented accumulated dry matter (DM) yield up to that point. Each of these sample areas was not harvested before or following the assigned harvest month and year. These samples were dried for 72 h at 55°C in a forced-air oven and weighed to estimate forage DM yield. Each sample was then ground through a sheer mill fitted with a 1-mm screen. Total N and P concentrations were measured in the forage by using a modification of the aluminum block digestion procedure of Gallaher et al. (1975). 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. Phosphorus and N in the digestate were determined by semiautomated colorimetry (Hambleton, 1977) using a Technicon Autoanalyzer II (Technicon Industrial Syst., Tarrytown, NY). Acid detergent fiber and ADL were determined utilizing the method originally described by Van Soest and Robertson (1980).
Measured variables were submitted to analysis of variance by entry for vigor and height in the first season. Year, entry, and harvest were used in the model for yield, ADF, ADL, N, and P concentrations. A least significant difference (LSD) was used to separate means among entries (P < 0.05) wherever appropriate.
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RESULTS AND DISCUSSION
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Season-of-establishment plant vigor was lowest (P < 0.05) in the STBLK and LABLK accessions although the later was undifferentiated from George (Table 2). LABLK (20 cm) and BEBLK (18.5 cm) had the longest (P < 0.05) branches (Table 2). March flowering in both the establishment and reseeding years was noted for LABLK, DEBLK, GRBLK1, GRBLK3, and STBLK while NTBLK blend and GRBLK2 also initiated flowering in March of the reseeding year (data not shown). All other entries initiated flowering by April. No frost damage was observed on any entry throughout the trial despite temperatures that reached down to -9°C and stayed below 0°C for more than 48 h. These results contrast with those of Brandsaeter et al. (2002), who found black medic ‘Virgo Pajberg’ to be susceptible to damage in freezing chambers below -6°C. Also in sharp contrast to an adjacent burr medic (Medicago polymorpha L.) trial where insect herbivory nearly destroyed plants (Muir et al., 2001), only light insect damage by larval alfalfa weevil (Hypera postica Gyllenhal) was observed and only during the second season. Damage in the second year may have been a result of greater autumn growth that year due to earlier, natural germination.
Complete canopy cover (data not reported) was achieved by April in all plots both years. Yield differences (P < 0.001) among entries were the same across both year and harvest month. Accessions GRBLK1, GRBLK2, GRBLK3 as well as BEBLK, which averaged nearly 1.5 Mg ha-1 yr-1 forage production during the trial, had greater forage DM yield than the other entries except COBLK and NTBLK (Fig. 1). These yields are lower than those reported by Harvey et al. (1950) where an unidentified black medic yielded almost 2.8 Mg ha-1 yr-1 in south-central Texas. Despite lower cumulative rainfall and irrigation levels the first year, yields were greater in the first year compared with the second year during the April and May harvests (year x harvest month interaction, P < 0.001), indicating that self-reseeding was not as effective the second year as broadcast seeding was the first year (Table 3). Seed broadcast the first season was scarified, so most seed in the plots in the reseeding year were produced the previous season. During both years, however, yields increased as the season progressed, and May of 2000 had the greatest average production, with more than 2.3 Mg ha-1 forage (Table 3).
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Fig. 1. Forage yields of 12 black medic entries at Stephenville, TX, averaged over 2 yr and March, April, and May harvests [P < 0.001; means with the same letter are undifferentiated according to LSD (LSD0.05 = 336) means separation at  = 0.05].
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Table 3. Forage yields of black medic in March, April, and May of 2000 and 2001 at Stephenville, TX, averaged for 12 entries. Yields are estimates of cumulative yearly forage production up to month of harvest and were clipped from previously unharvested plants.
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Acid detergent fiber values in March (Table 4) averaged less than 200 g kg-1 in both years and increased to an average 252 g kg-1 forage in May of both years (year x harvest month x entry interaction, P = 0.02). Differences in entry ADF values between years at the same harvest dates reflected differences in plant development resulting from weather differences between years. However, the values in general indicate high quality forage.
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Table 4. Acid detergent fiber (ADF) concentration in the forage of 12 black medic entries harvested during 2000 and 2001 in March, April, and May (year x harvest month x entry interaction at P = 0.04) at Stephenville, TX.
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The same general patterns were apparent for ADL values (Table 5; year x harvest month x entry interaction, P = 0.03) except that lignin concentration rose more sharply than ADF from April to May (30% increase on average for ADL compared with 17% rise for ADF). Forage ADL concentration was highest in the May harvest of 2001, despite lower yields that year, indicating that factors other than plant development affected accumulation of this plant component. One possible explanation was that seeds, whether immature or mature, may have caused a sharp increase in ADL when plants reached this reproductive stage.
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Table 5. Acid detergent lignin (ADL) concentration in the forage of 12 black medic entries harvested during 2000 and 2001 in March, April, and May (year x harvest month x entry interaction at P = 0.03) at Stephenville, TX.
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Forage ADF and ADL concentrations increased with maturity and generally were greater in the more productive entries. However, at peak production in May of both years, even the higher-yielding entries averaged only 260 g ADF kg-1 forage, lower than the average ADF values reported for annual Medicago spp. in other trials (Zhu et al., 1996; Shrestha et al., 1998). Plant N concentrations were similar to those reported for medics in other trials (Zhu et al., 1996; Shrestha et al., 1998) although the high ADL values, in many entries averaging over 50 g kg-1 forage, may indicate that a considerable portion of this N is unavailable to herbivores.
There was a year x harvest month x entry interaction (P = 0.05) for forage N concentration (Table 6). Average concentrations peaked in April both years but were generally higher the second year except in May. In April 2000, forage N concentrations reached 39 g kg-1, with nine entries undifferentiated, while in April 2001, HUBLK had the highest concentration at 43.6 g kg-1. The highest-yielding entries tended to have lower concentrations of N, indicating a possible dilution of root-absorbed and atmospherically fixed N in those entries. However, even the entries with the lowest N concentrations still had levels over 31 g N kg-1. BEBLK, for example, averaged 33.3 g N kg-1 forage throughout the trial and yielded an average 1470 kg ha-1 yr-1 forage, almost 50 kg N ha-1 yr-1.
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Table 6. Nitrogen concentration in the forage of 12 black medic entries harvested in March, April, and May of 2000 and 2001 (year x harvest month x entry at P = 0.05) at Stephenville, TX.
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Phosphorus concentrations in the forage reflected a year x harvest month x entry interaction (P = 0.002). In general, concentrations in 2000 peaked in May while values in 2001 were greatest in April (Table 7), contrary to what has been observed in cool-season annual grasses where P concentrations in younger plant material tended to be higher (Vander Horst et al., 1998) or as high (Chen et al., 1996) as more mature plant material. These concentrations were within the range reported for other cool-season legumes although some values, such as 4.4 g P kg-1 for George in May 2000, were greater than the 2.5 to 3.3 reported in Pierzynski and Logan's (1993) review of the literature.
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Table 7. Phosphorus concentration in the forage of 12 black medic entries harvested in March, April, and May of 2000 and 2001 (year x harvest month x variety at P = 0.002) at Stephenville, TX.
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CONCLUSIONS
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Forage yields as well as plant vigor and branch length indicate that there were accessions of Texan origin that grew more (less dormant) during the winter months than the commercial cultivar George. In particular, GRBLK1, GRBLK2 ,and GRBLK3 as well as BEBLK outyielded George, regardless of harvest month and year. The average 2.3 Mg ha-1 forage for a single May harvest in 2000 might be improved if management requirements and multiple-month harvests are imposed on the more productive accessions. The wide range of forage fiber and mineral concentration observed in this trial can also be utilized in selecting and managing for a higher quality black medic for eventual release in drier regions of southern USA. A blend of those entries that combine winter productivity, early seed production, high forage yields, and the best nutritive value should be further tested in a wide range of drier, southern USA environments for possible future release. Low yields, however, bring into question a reasonable return on establishment costs although subsequent self-reseeding and the benefits derived from highly nutritious forage should add to the crop value. Improved fertility (especially P and K, which were low at the study site) or the selection of better soils as well as other management factors should be addressed before the release of a commercial cultivar.
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ACKNOWLEDGMENTS
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This material is based in part on work supported by the Texas Advanced Technology Program under grant no. 517-48-1999. The authors also thank Mike Schubert, Jeff Rahmes, and others for their efforts in collecting this germplasm in Texas.
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REFERENCES
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ABSTRACT
INTRODUCTION
