Legume Monocultures Research Articles

Nitrous oxide emissions increase exponentially with organic N rate from cover crops and applied poultry litter

Best management practices to reduce nitrous oxide (N2O) emissions following application of organic sources of N to soils are poorly developed. For example, while cover crops are promoted for their conservation benefits, their impact on N2O emissions varies considerably. Subsurface banding of animal manures is promoted to reduce nitrogen losses through ammonia volatilization, but the impact on soil N2O emissions is uncertain. To assess the interactive effects of cover crops and subsurface banded poultry litter (SSB PL), we measured annual N2O emissions for three years in a field trial of corn (Zea mays L.) in Beltsville, MD, following either winter fallow or mulched cover crops (cereal rye [Secale cereale L.], hairy vetch [Vicia villosa Roth.], or a mixture of both species), with four rates of SSB PL (9276 kg plant available N (PAN) ha-1) and selected contrasts with surface banded urea-ammonium nitrate (UAN, 150 kg N ha-1) or broadcast incorporated PL (67 kg PAN ha-1). N2O emissions increased exponentially with total N input from organic sources (SSB PL + cover crop residue). This relationship differed by cover crop treatment, with lowest emissions following cereal rye, and highest emissions following hairy vetch. The model intercept ranged from 0.3061.371 kg N2O-N ha-1 (cereal rye < bare ground = mixture < hairy vetch; p = 0.03, 0.24, 0.01), and the exponential coefficient ranged from 0.003000.00603 kg N2O-N kg-1 N (hairy vetch < cereal rye = bare ground = mixture; p = 0.02, p > 0.10). Corn grain yield was similar (mean, 13.8 Mg ha−1) for all cover crops when PL was applied at 135 kg PAN ha−1. SSB PL increased N2O emissions relative to tillage-incorporated PL at an equivalent rate following hairy vetch or cover crop mixture (by 76% or 60%, respectively; p < 0.001) while increasing corn yield by 32% or 16%, respectively. SSB PL decreased emissions relative to surface banded UAN at an equivalent rate following the cover crop mixture, but increased emissions following bare ground (by 34% and 45%, respectively; p = 0.002) while having no influence on corn grain yield, which averaged 13.7 Mg ha−1. Our results indicate that grass:legume cover crop mixtures with SSB PL can lower N2O emissions compared to legume monoculture with SSB PL while maintaining comparable N inputs and corn grain yields.

Soil-Plant Indices Help Explain Legume Response to Crop Rotation in a Semiarid Environment.

Crop productivity is typically affected by various soil–plant factors systematically as they influence plant photosynthesis, soil fertility, and root systems. However, little is known about how the productivity of legumes is related to crop rotation systems. The objectives of this study were to determine the effect of rotation systems on legume productivity and the relationships among legume productivity and soil–plant factors. Three annual legumes – chickpea (Cicer arietinum L.), pea (Pisum sativum L.), and lentil (Lens culinaris Medikus), were included in various diversified rotation systems and compared with legume monoculture in the 8-year rotation study. Soil N and water conditions, and canopy and root systems were evaluated at the end of 8-year rotation in the semiarid Canadian prairies. Results showed that diversified rotation systems improved leaf greenness by 4%, shoot biomass by 25%, nodule biomass by 44%, and seed yield by 95% for chickpea and pea, but such effects were not found for lentil. Pea monocultures increased root rot severity by threefold compared with diversified rotations, and chickpea monoculture increased shoot rot severity by 23%, root rot severity by 96% and nodule damage by 219%. However, all the legume monocultures improved soil N accumulation by an average 38% compared to diversified systems. Pea and chickpea displayed considerable sensitivity to plant biotic stresses, whereas lentil productivity had a larger dependence on initial soil N content. The 8-year study concludes that the rotational effect on legume productivity varies with legume species, the frequency of a legume appearing in the rotation, and the integration of relevant soil and plant indices.

Growing degree days and cover crop type explain weed biomass in winter cover crops

Cover crops are increasingly being adopted to provide multiple ecosystem services, including weed suppression. Understanding what drives weed biomass in cover crops can help growers make the appropriate management decisions to effectively limit weed pressure. In this paper, we use a unique dataset of 1764 measurements from seven cover crop research experiments in Pennsylvania (USA) to predict, for the first time, weed biomass in winter cover crops in the fall and spring. We assessed the following predictors: cover crop biomass in the fall and spring, fall and spring growing degree days between planting and cover crop termination, cover crop type (grass, brassica, legume monocultures, and mixtures), system management (organic, conventional), and tillage before cover crop seeding (no-till, tillage). We used random forests to develop the predictive models and identify the most important variables explaining weed biomass in cover crops. Growing degree days, cover crop type, and cover crop biomass were the most important predictor variables in both the fall (r2 = 0.65) and spring (r2 = 0.47). In the fall, weed biomass increased as accumulated growing degree days increased, which was mainly related to early planting dates. Fall weed biomass was greater in legume and brassica monocultures compared to grass monocultures and mixtures. Cover crop and weed biomass were positively correlated in the fall, as early planting of cover crops led to high cover crop biomass but also to high weed biomass. In contrast, high spring cover crop biomass suppressed weeds, especially as spring growing degree days increased. Grass and brassica monocultures and mixtures were more weed-suppressive than legumes. This study is the first to be able to predict weed biomass in winter cover crops using a random forest approach. Results show that weed suppression by winter cover crops can be enhanced with optimal cover crop species selection and seeding time.

Grass-Legume Mixtures for Improved Soil Health in Cultivated Agroecosystem

Planting grass-legume mixtures may be a good option to improve soil health in addition to increased forage productivity, improved forage nutritive value, and net farm profit in a hay production system. A field experiment was conducted from 2011 to 2014 at Lingle, Wyoming to evaluate soil microbial biomass under different seeding proportions of two forage grasses (meadow bromegrass, Bromus biebersteinii Roem. &amp; Schult.; and orchardgrass, Dactylis glomerata L.) and one legume (alfalfa, Medicago sativa L.). Nine treatments included monoculture grass, monoculture legume, one grass and one legume mixture, two grasses and one legume mixture, and a control (not seeded with grass or legume). Monoculture grass received either no nitrogen (N) or N fertilizer (150 kg N ha−1 year−1 as urea) whereas monoculture legume, grass-legume mixtures, and control plots received no N fertilizer. The study was laid out as a randomized complete block design with three replications. The plots were harvested 3–4 times each year after the establishment year. Soil samples were collected and analyzed for microbial biomass using phospholipid fatty acid (PLFA) analysis at the end of May in 2013 and 2014. Soil samples were also analyzed for mineralizable carbon (C) and N in 2013 and 2014. The total above-ground plant biomass was higher in 50–50% mixture of grass and alfalfa than monoculture alfalfa and monoculture grass (with and without N fertilizer) during the entire study period. The application of N fertilizer to the grass hay production system had little effect on improving mineralizable soil C, N, and soil microbial biomass. However, grass-legume mixture without N fertilizer had great effect on improvement of mineralizable soil C and N, and total, bacterial, and actinomycetes microbial biomass in soil. The 50–50% mixture of grass and alfalfa performed consistently well and can be considered to use in Wyoming conditions for improving soil health and forage productivity.

Seeding ratios and phosphate fertilizer on ecosystem carbon exchange of common vetch and oat

Few studies have investigated the effect of phosphate (P) fertilizer on net ecosystem CO2 exchange (NEE) and its components under mixed legume and grass pastures sown, which will limit our understanding about their effects on carbon (C) sequestration in soils. Our aim was to test the hypothesis that a combination of legumes with grass and P fertilizer increases NEE due to greater increases in gross primary productivity (GPP) compared to increases in ecosystem respiration (Re).We measured NEE, GPP and Re under 5 different seeding ratios of oat (Avena sativa L.) and common vetch (Vicia sativa L.) according to their thousand kernel weight and germination rate (i.e., seeding rate were: M1: 0:150, M2: 150:105, M3: 300:75, M4: 450:45 and M5: 600:0) with and P unfertilized on the Tibetan Plateau from 2011 to 2013. NEE was measured at 1-week intervals from June to September each year using a transparent chamber technique (Licor-6400). Our results showed that both P fertilizer and mixed ratio had significant effect on NEE, Re and GPP, but their interactive effects were not significant during the growing season. P fertilizer primarily increased NEE due to increased GPP for the monoculture and mixtures with a higher proportion of common vetch. NEE, Re and GPP were higher for the mixed pastures only compared with legume monoculture. From the perspective of forage yield and C sequestration, planting mixed legume-grass pastures with P fertilizer is a preferable way to balance the twin objectives of forage production and C sequestration in alpine regions.

Performance of legume-based annual forage crops in three semi-arid Mediterranean environments

Legume-based annual forages could be pivotal for the sustainable intensification of forage production in drought-prone Mediterranean cereal-livestock systems. This study aimed to optimise the composition of these crops for three climatically contrasting areas. Four legumes (field pea of semi-dwarf and tall type; Narbon vetch; common vetch) and two cereals (oat; triticale) were grown in three autumn-sown sites (Sassari, Italy; Sétif, Algeria; Marchouch, Morocco) for 2 years as pure stands and legume-cereal binary and four-component mixtures. We assessed dry matter yield, weed content and farmers’ acceptability of the crops, and legume content and Land Equivalent Ratio of the mixtures. Legumes’ competitive disadvantage ranged from very high in Sétif to nearly nil in Sassari. Pea- and common vetch-based mixtures out-performed Narbon vetch-based ones in terms of yield, legume content and farmers’ acceptability. The tall pea, featuring greatest competitive ability against cereals, maximised the yield and legume content of legume-cereal crops. Vetch-cereal mixtures exhibited lower weed content than the average of the components’ pure stands. Oat monoculture was top-yielding but modestly appreciated by farmers. Pea provided the only legume monoculture combining good yielding ability and high farmers’ appreciation. Greater species diversity as provided by complex mixtures did not display any production advantage over binary mixtures.

Can legumes provide greater benefits than millet as a spring cover crop in southern Queensland farming systems?

Cover crops grown during fallows can increase organic matter inputs, improve soil surface cover to reduce erosion risk, and enhance rainfall infiltration. An experiment compared a chemical fallow control with six different cover crops terminated at either 60 or 90 days after sowing. The commercial choice of millet (Echinochloa esculenta) was compared with two summer legumes (lablab (Lablab purpureus) and soybean (Glycine max)), and three winter legumes (field pea (Pisum sativum), faba bean (Vicia faba) and common vetch (Vicia sativa)). Cover crop biomass growth, atmospheric nitrogen (N) fixation, surface residue cover, and soil water and mineral N dynamics during the growth period and subsequent fallow were measured. Soil water and N availability and yield of wheat crops following the experimental treatments were simulated over a 100-year climate record using APSIM. Both experiments and simulations found the legumes inferior to millet as spring-sown cover crops, because they were slower to accumulate biomass, required later termination and provided groundcover that was less persistent, resulting in lower soil water at the end of the fallow. After 90 days of growth, the summer legumes, lablab and soybean, produced the most biomass and fixed more N (up to 25 kg N/ha) but also extracted the most soil water and mineral N. Legume N fixation was low because of high soil mineral N status (&gt;100 kg N/ha) and occurred only when this had been depleted. At the end of the subsequent fallow in April, soil water was 30–60 mm less and soil mineral N 80–100 kg/ha less after both millet and 90-day terminated summer legume cover crops than the chemical fallow control. Simulations predicted soil-water deficits following legume cover crops to be &gt;50 mm in the majority of years, but soil mineral N was predicted to be lower (median 80 kg N/ha) after millet cover crops. In conclusion, monoculture legume cover crops did not provide advantages over the current commercial standard of millet, owing to less effective provision of groundcover, low N fixation and possibly delayed release of N from residues. Further work could explore how legumes might be more effectively used as cover crops to provide N inputs and soil protection in subtropical farming systems.

Managing the trade off between nitrogen supply and retention with cover crop mixtures

The ability of cover crop mixtures to provide both nitrogen (N) retention and N supply services has been extensively studied in research station experiments, especially with grass-legume bicultures. Mixtures are often as effective as grass monocultures at N retention, but the N supply service can be compromised when non-legumes dilute the presence of legumes in a cover crop stand. To study the tradeoffs between N retention and supply when using cover crop mixtures, we measured N retention and supply in distributed on-farm experiments, developed multiple linear regression models to predict N retention and supply based on cover crop functional characteristics and environmental variables, and synthesized the regression models into a graphical analysis tool. The experiments took place on three organic farms and a research station in Pennsylvania, USA and tested 3-species and 4-species cover crop mixtures in comparison to commonly used grass and legume monocultures. Cover crop treatments were planted between a small grain crop harvested in mid-summer and a maize (Zea mays L.) crop planted the following spring. Potential nitrate (NO3−) leaching below 30cm, an indicator of the N retention service, declined as the presence of non-legume species in a cover crop increased (r2=0.72). Potential NO3− leaching increased as the August soil NO3−-N concentration increased and as the fall biomass N content of winter-killed species or canola (Brassica napus L. ‘Wichita’) increased. Relative maize yield, an indicator of the N supply service, decreased as fall and spring cover crop biomass carbon-to-nitrogen (C:N) ratios increased and increased as total spring biomass N content and soil carbon (C) concentration increased (r2=0.56). Synthesizing the regression models in a graphical analysis tool revealed a tradeoff between N supply and retention services for cover crop mixtures, where increasing the fractional non-legume seeding rate to reduce potential NO3− leaching also reduced relative maize yield. The tradeoff could be minimized by managing environmental conditions and cover crop composition so that potential NO3− leaching remains low even when the fractional non-legume seeding rate is low. The regression models suggest this could be achieved by maintaining low soil NO3−-N concentrations prior to cover crop planting in August, not including winter-killed legumes in the mixture, and using non-legume species that are the most efficient at N retention. Thus, with thoughtful management of cover crops and soils, farmers may be able to realize the potential of cover crop mixtures to provide high levels of both N retention and supply services.

Functional Diversity in Summer Annual Grass and Legume Intercrops in the Northeastern United States

A warm‐season annual intercropping experiment was conducted across the northeastern United States with four trials in 2013 and five trials in 2014 with four crop species selected based on differences in stature and N acquisition traits: (i) pearl millet [Pennisetum glaucum (L.) R. Br.], (ii) sorghum sudangrass [Sorghum bicolor (L.) Moench × S. sudanense (Piper) Stapf], (iii) cowpea [Vigna unguiculata (L.) Walp], and (iv) sunn hemp (Crotalaria juncea L.). Crops were seeded in monoculture and in three‐ and four‐species mixtures using a replacement design where monoculture seeding rates were divided by the number of species in the intercrop. Crop biomass was sampled at ∼45 and 90 d after planting. When averaged across the nine site‐years, biomass at the first and second sampling dates, respectively, of the monoculture treatments ranged from 1040 and 2500 kg ha−1 (cowpea) to 3000 and 9300 kg ha−1 (pearl millet). In general, biomass production of the legume monocultures was lower than the grass monocultures and intercrops at both sampling dates. All intercrops had land equivalent ratios (LERs) greater than one, indicating complementarity, likely a result of resource partitioning. Intercrops, particularly the four‐species mixture, exhibited greater stability in yields across environments. The pearl millet–sorghum sudangrass–sunn hemp intercrop had the greatest evenness, suggesting that species selected for annual intercrops should have similar monoculture growth rates to minimize asymmetric competition.

Competition between cocksfoot (Dactylis glomerata L.) and companion species: Evidence for allelopathy

Cocksfoot (Dactylis glomerata L.) is a widely grown temperate perennial grass for livestock production worldwide. Recent studies have observed that cocksfoot plants inhibit the growth and development of companion and volunteer species. However, there has been little research on how this inhibitory effect happens and whether it occurs widely or on a limited number of species only. The aim of the study was to quantify the inhibitory or allelopathic effect and its possible role in the effect of cocksfoot on the growth and seed germination of a range of grass, legume and volunteer species. Three experiments were conducted: Experiment 1 was a greenhouse experiment that used 4 grass and legume monocultures and 9 mixtures with 3 mixtures ratios; Experiment 2 was a lab experiment that used different concentrations of cocksfoot rhizosphere extracts to test the germination and seedling growth of 5 commonly sown pasture species; Experiment 3 was a field experiment to validate the inhibitory effects of cocksfoot as shown in Experiments 1 and 2 under field conditions. The results demonstrated that cocksfoot had strong inhibitory effects on all the species tested in this study although the magnitude varied between species. On average, the cocksfoot percentage in mixtures was increased by 120% for herbage accumulation, and 168% for tiller density relative to its designed proportions in mixtures under field conditions. Cocksfoot rhizosphere extracts considerably reduced the germination and seedling growth of all tested species except lucerne that had lower seedling vigour. It is concluded that cocksfoot roots may have produced metabolites that inhibit the seed germination and plant growth of companion or volunteer species. This could be a contributor to the superior persistence of the species and have significant practical implications on weed control. The biochemical compounds of cocksfoot root metabolites and their functions deserve further investigation.

Cascading effects of belowground predators on plant communities are density-dependent.

Soil food webs comprise a multitude of trophic interactions that can affect the composition and productivity of plant communities. Belowground predators feeding on microbial grazers like Collembola could decelerate nutrient mineralization by reducing microbial turnover in the soil, which in turn could negatively influence plant growth. However, empirical evidences for the ecological significance of belowground predators on nutrient cycling and plant communities are scarce. Here, we manipulated predator density (Hypoaspis aculeifer: predatory mite) with equal densities of three Collembola species as a prey in four functionally dissimilar plant communities in experimental microcosms: grass monoculture (Poa pratensis), herb monoculture (Rumex acetosa), legume monoculture (Trifolium pratense), and all three species as a mixed plant community. Density manipulation of predators allowed us to test for density-mediated effects of belowground predators on Collembola and lower trophic groups. We hypothesized that predator density will reduce Collembola population causing a decrease in nutrient mineralization and hence detrimentally affect plant growth. First, we found a density-dependent population change in predators, that is, an increase in low-density treatments, but a decrease in high-density treatments. Second, prey suppression was lower at high predator density, which caused a shift in the soil microbial community by increasing the fungal: bacterial biomass ratio, and an increase of nitrification rates, particularly in legume monocultures. Despite the increase in nutrient mineralization, legume monocultures performed worse at high predator density. Further, individual grass shoot biomass decreased in monocultures, while it increased in mixed plant communities with increasing predator density, which coincided with elevated soil N uptake by grasses. As a consequence, high predator density significantly increased plant complementarity effects indicating a decrease in interspecific plant competition. These results highlight that belowground predators can relax interspecific plant competition by increasing nutrient mineralization through their density-dependent cascading effects on detritivore and soil microbial communities.

Global Synthesis of Drought Effects on Food Legume Production.

Food legume crops play important roles in conservation farming systems and contribute to food security in the developing world. However, in many regions of the world, their production has been adversely affected by drought. Although water scarcity is a severe abiotic constraint of legume crops productivity, it remains unclear how the effects of drought co-vary with legume species, soil texture, agroclimatic region, and drought timing. To address these uncertainties, we collected literature data between 1980 and 2014 that reported monoculture legume yield responses to drought under field conditions, and analyzed this data set using meta-analysis techniques. Our results showed that the amount of water reduction was positively related with yield reduction, but the extent of the impact varied with legume species and the phenological state during which drought occurred. Overall, lentil (Lens culinaris), groundnut (Arachis hypogaea), and pigeon pea (Cajanus cajan) were found to experience lower drought-induced yield reduction compared to legumes such as cowpea (Vigna unguiculata) and green gram (Vigna radiate). Yield reduction was generally greater when legumes experienced drought during their reproductive stage compared to during their vegetative stage. Legumes grown in soil with medium texture also exhibited greater yield reduction compared to those planted on soil of either coarse or fine texture. In contrast, regions and their associated climatic factors did not significantly affect legume yield reduction. In the face of changing climate, our study provides useful information for agricultural planning and research directions for development of drought-resistant legume species to improve adaptation and resilience of agricultural systems in the drought-prone regions of the world.

Grass-legume mixtures impact soil N, species recruitment, and productivity in temperate steppe grassland

In this study, we aimed to understand effects and mechanisms of legume species and their relative abundance on soil N, species recruitment, and productivity in mixed grassland. We also assessed the utilization prospect of several legumes in natural grassland. In 2006, grass-legume combinations (GLCs) with different ratios (GLR) were established using Leymus chinensis and four legume species (Medicago ruthenica, Lespedeza daurica, Medicago falcata, and Medicago sativa). In 2009, plant and soil samples were taken to examine the differences in soil nitrogen (N) properties, biological N fixation (BNF), recruitment characteristics, and aboveground biomass of the community under different mixture patterns. Soil total N and available N concentration increased when legume component increased from GLR 1:0 (grass monoculture) to GLR 1:1 (grass:legume 1:1) but decreased as legume component increased further from GLR 1:3 (grass:legume 1:3) to GLR 0:1 (legume monoculture). GLR 1:1 had the highest BNF in most GLCs except for L. chinensis-M. falcata. For any GLR with legume, L. chinensis-M. sativa combination (L-MS) had higher soil total N, available N, water content, and BNF. The legume combinations with a GLR lower than 1:1 resulted in more species recruitments compared with the grass monoculture, and L-MS induced relatively more species recruitments than other GLCs, which were correlated with improved soil water and NO3 −-N status. The grassland productivity increased as the GLR decreased; in any GLRs with legume, higher productivity was found under L-MS combination, and current results showed that grassland productivity was positively linked to soil NO3 −-N availability influenced by grass-legume mixture. We concluded that the species and relative abundance of legumes had great impact on soil N status, species recruitment, and productivity in this temperate grassland ecosystem. In this study site, legume introduction can be considered as an alternative to N fertilization for increasing grassland productivity. M. sativa has the greatest economic and ecological potential species to be mixed into natural temperate steppe grassland, and grass-legume ratio 1:1 is the best combination in mixed communities.

Effects of monoculture and mixed culture of grass and legume forage species on soil microbial community structure under different levels of nitrogen fertilization

We used PLFA analysis to monitor the soil microbial community structure in a gramineous–legume forage field under different inorganic nitrogen (N) fertilization regimes in Southwest China. The gramineous–legume forage system included one grass (Paspalum wetsfeteini) and one legume (Medicago sativa) in three planting systems: P. wetsfeteini monoculture, M. sativa monoculture, and P. wetsfeteini and M. sativa mixed culture. The fertilization treatments included three N levels: 338 (low), 450 (moderate), and 675 (high) kg N ha−1 yr−1. The results showed that biomasses of total microbes, bacteria, fungi, and green alga were significantly greater and protozoan biomass tended to be greater under legume monoculture than that under grass monoculture; and fungal biomass was significantly greater under grass-legume mixed culture than under grass monoculture in wet season. However, principal component analysis (PCA) only revealed a tendency that the microbial PLFA composition under legume monoculture differed from that under grass monoculture in the wet season. In addition, the soil microbial community structures were not significantly different among the three planting systems in the dry season. The PCA results showed that the microbial PLFA composition under low N fertilization was apparently different from that under moderate and high N fertilization in the wet season. Particularly, the biomasses of total microbes, bacteria, and green algae were significantly greater under moderate N fertilization than under low N fertilization and the green algal biomass was significantly greater under high N fertilization than under low N fertilization in the wet season. Additionally, PCA also revealed that the microbial PLFA compositions were different under the low and moderate N fertilizations in the dry season. However, the biomass and diversity of microbial community had no significant difference among the three levels of N fertilization. The results suggest that legume cultivation increased the biomasses of soil microbial community.

Adverse Effects of Allelopathy from Legume Crops and Its Possible Avoidance

Plant releases many bioactive chemicals from its various parts such as leaves, stem, root and sometimes decomposed body through different mechanism into its surrounding environment. These bioactive chemicals are often termed as allelochemicals because they interact with the surrounding environment. This interaction is either positive or negative. Effects of allelochemicals to the agricultural and biological ecosystem are well documented. In leguminosae family many species are involved in releasing of allelochemicals. Many researchers found that this allelochemicals have both positive and detrimental effects on the successive legume crops. Legume monoculture is common in many parts of the world where they cause a numbers of ecological and economic problems such as decline in crop yield due to soil sickness, regeneration failure and replant problem. These negative effects of allelochemicals open a great concern on allelopathy research. This article reviews the adverse effects of allelochemicals, their extraction and isolation, mechanism inside the plant body. These all are done to find out the possible selection methods of succeeding crops to avoid the allelopathic effects in the next crop of a monoculture farm field. The main purpose of this article is to highlight the adverse allelopathic effects of legume crops to provide ways for sustainable development in agro-ecosystem.

Forage Legumes for Grazing and Conserving in Ruminant Production Systems

As a plant group, forage legumes present some unique advantages and disadvantages for ruminant production. When compared to grasses or cereals their main advantages are generally (i) low reliance on fertilizer nitrogen (N) inputs, (ii) high voluntary intake and animal production when feed supply is non-limiting and (iii) high protein content. The main disadvantages of forage legumes are generally (i) lower persistence than grass under grazing, (ii) high risk of livestock bloat and (iii) difficulty to conserve as silage or hay. In comparison to grass or legume monocultures, grass + legume mixtures have particular advantages such as more balanced feeding values, increased resource use efficiency and increased herbage production. However, maintaining the optimum legume contents (40-60% of herbage dry matter) to achieve these benefits remains a major challenge on farms. When compared to ruminant systems based on grass or cereals supplemented with fertilizer N, forage legume based ruminant systems tend to have less negative environmental impact on biodiversity, N losses to water and greenhouse gas emissions. Economically, the primary advantage of forage legumes over other forages is their ability to reduce fertilizer N costs and their main disadvantage is usually lower intensity of animal production per ha of land. Despite the numerous benefits of forage legumes for ruminant farming (to the farmer and wider society), their use is reported as being low or declining relative to other forages in many regions. This is most likely a result of their disadvantages being perceived to outweigh their advantages at farm level. This may change if the price ratio of fertilizer N to product (meat/milk) continues to increase as it has done in some regions in recent years.

SOME BIOMETRIC CHARACTERISTICS AND SEED PRODUCTIVITY OF ANNUAL DROUGHT–RESISTANT FORAGE SPECIES FOR FOOTHILL REGIONS OF CENTRAL NORTHERN BULGARIA

During the period 2011–2013 in Research Institute of Mountain Stockbreeding and Agriculture, Troyan (Bulgaria) the comparative testing of some annual drought–resistant cereal and legume pure crops (bitter vetch, chickpea, foxtail millet, true millet) was accomplished. It was found that the determining parameters the quantity of seed yields (weight of seeds in 1 inflorescence and weight of 1000 seeds) in cereals had at smaller values than legume crops. In respect of the yield of seeds can be deemed that they amount to minimum values characteristic of the corresponding forage crop. The reasons for this are the typical soils (light–gray forest pseudopodzolic with acid reaction, poorly stocked with essential nutrients, especially nitrogen) for the region, drought climate in the region in recent years and the absence of fertilization in this experiment. For this reasons the foothill regions of the Central Balkan Mountains can be considered unsuitable for ecological seed production of studied annual forage crops. Кey words: botanical structural elements; seed productivity; Ccreal and legume monocultures; drought–resistance; Central Balkan Mountains (Bulgaria).

THE YIELD STABILITY AND QUALITY OF LEGUMES DURING TWO CONSECUTIVE, EXTREMELY DRY YEARS

Abstract This study highlights the effect of drought and ambient temperature on performance and herbage quality of legume monocultures and grass-legume mixtures. In a field experiment, the total dry matter yield, seasonal pattern of dry matter yield distribution, content of crude protein and crude fibre of monocultures of red clover and alfalfa and grass-legume mixtures were investigated during two consecutive dry years (2011-2012). Alfalfa cultivars Kamila and Tereza grown as monocultures or as mixtures with Festulolium braunii (cultivar Achilles) outperformed the red clover cultivars Fresko and Veles and provided a well-balanced total and seasonal dry matter yield during both years. Across all experimental years, crude protein content was significantly higher at alfalfa monocultures and mixture when compared with clover monocultures (P &lt; 0.05). However, considerable lower content of crude fibre at clover monocultures in comparison with alfalfa ones was found. Responses of nutritive parameters of both legume species to weather variables were different. Crude protein content in red clover was independent of rainfall and temperature. In contrast, the crude fibre content correlated with temperature whereby the alfalfa monocultures showed stronger correlations (P &lt; 0.05) than red clover monocultures.

Winter cover crops influence Amaranthus palmeri establishment

Winter cover crops were evaluated for their effect on Amaranthus palmeri establishment and growth in cotton production. Cover crops examined included rye and four winter legumes: narrow-leaf lupine, crimson clover, Austrian winter pea, and cahaba vetch. Each legume was evaluated alone and in a mixture with rye. Cover crop biomass in monoculture was greatest for rye and lupine (>6750 kg ha−1), while clover, pea, and vetch were less and ranged from 2810 to 4610 kg ha−1. Cover crop biomass was more than doubled when rye was mixed with clover or vetch relative to the legume monoculture. In early-June, A. palmeri densities were 46 seedlings m−2 in the non-disturbed areas between cotton rows in the fallow, while populations were <4 seedlings m−2 with rolled vetch or pea and 18 and 29 seedlings m−2 in rolled clover and lupine. Rye and legume mixtures reduced A. palmeri densities to <3 seedlings m−2, while rye monocultures had 8 seedlings m−2. There were no differences in A. palmeri densities (≥144 plants m−2) in the cotton row among cover crop treatments. By late-June, rye and winter pea controlled A. palmeri in the row middle >80% relative to the non-cover crop fallow treatment, while control from clover, vetch and lupine ranged from 64 to 70%. The relationship between A. palmeri control in between cotton rows and cover crop biomass was described by a log-logistic regression model with 4530 kg ha−1 providing median weed control (Bio50); predicted A. palmeri control was 25, 50, and 75% from 2950, 4900, and 8600 kg ha−1 cover crop biomass, respectively. However, A. palmeri plants in the cotton rows prevented yield production in the absence of herbicides. Where A. palmeri was controlled with herbicides, the highest yields occurred following rye, with lower yields following lupin/rye mixture and treatments including pea. Management of herbicide resistant weed species requires diverse management tactics; this may include high-biomass cover crops to reduce weed establishment between crop rows. However, greater research effort is needed to devise weed management options for the crop row that do not rely exclusively on the diminishing array of herbicide tools.

Grazing Behavior of Steers on Different Annual Ryegrass and White Clover Forage Systems

ABSTRACTStudying grazing behavior of ruminants may help increase the understanding of the use of forage systems of varying forage types and composition. This study was conducted during 2008 at Raymond, MS, to evaluate grazing time by ruminants on various forage systems to examine any relationship with pasture composition and stocking rate (SR). Annual ryegrass (Lolium multiflorum Lam. ‘Marshall’) and white clover (Trifolium repens L. ‘Durana’) were used to create forage system treatments of either monoculture grass (MG) or monoculture legume (ML), a binary mixture of grass and legume (MIX), or a spatial system of adjacent grass and legume monocultures in a 50:50 land area ratio within the same paddock (SS). Two levels of SR (3 or 6 steers [Bos taurus] ha−1) were imposed on each of the four forage systems to give a 4 × 2 factorial arrangement of treatments with two replicate paddocks of each treatment combination in a completely randomized design experiment. Two Angus crossbred yearling beef steers (236 ± 24 kg initial body weight [BW]) were randomly assigned to each of the 16 paddocks. Time spent grazing per animal on continuously stocked pastures was recorded for 10 d selected randomly during the months of April and May, with each day of observation lasting for 14 h (0615 to 2015 h). Forage system had an effect (P &lt; 0.001) on time spent grazing daily. Animals grazing MG and MIX spent similar time grazing (mean = 442 min per animal), which was greater than daily grazing time on SS (380 min per animal). Grazing time was least on ML (341 min per animal). Also, SR had an effect (P &lt; 0.001) on daily grazing time. Animals on high SR (431 min per animal) spent greater time grazing than those on low SR (371 min per animal). There was no difference in time spent grazing on each component of the adjacent monocultures of grass and legume within the SS (P = 0.23; 197 [the monoculture legume component of a SS] vs. 182 min per animal [monoculture grass component of a SS]). Less grazing time by animal on SS than MG and MIX may have implications for performance of animals grazing such a system because of expected lesser energy expenditure for grazing.