Annual Crop Fields Research Articles

Temporal habitat variability and the maintenance of sex in host populations of the pea aphid

The evolutionary maintenance of sex, despite competition from asexual reproduction, has long intrigued the evolutionary biologists owing to its numerous apparent short-term costs. In aphids, winter climate is expected to determine the maintenance of sexual lineages in the high latitude zones owing to their exclusive ability to produce frost-resistant eggs. However, diverse reproductive modes may coexist at a local scale where climatic influence is counteracted by microgeographical factors. In this study, we tested the influence of local habitat characteristics on regional coexistence of reproductive modes in the pea aphid, Acyrthosiphon pisum. In the laboratory, the induction of sexual morph production of many pea aphid genotypes from the local fields of annual (pea and faba bean) and perennial (alfalfa and red clover) crops in Western France indicated that A. pisum lineages from annual crops had a significantly higher investment in sexual reproduction than A. pisum lineages from the perennial hosts. We propose that temporal habitat variability exerts a selective pressure to maintain the sexual reproduction in A. pisum. The ecological and evolutionary consequences of the association between the mode of reproduction and the host population on gene flow restriction and on ecological specialization are discussed.

Effects of landscape complexity on farmland birds in the Baltic States

Data on birds occurring in farmland in the Baltic States of Estonia, Latvia and Lithuania were related to the spatial organisation of farmed habitats in three different agricultural landscape types. Species richness, abundance, and diversity of farmland bird communities, as well as abundance of the most frequently observed species were positively related to the number of residual non-cropped elements within farmland, the local mixture of annual crop and grass fields, and the variety of field types. The positive association of the species richness and abundance of the farmland bird community with richness in residual habitats and crops was most prominent in open landscapes. The results suggest that, by simplifying farmland structure and making it more homogenous, EU agricultural policies will have a detrimental effect on farmland bird populations in Eastern Europe. Ways of better targeting of the agri-environment schemes are suggested.

Manipulating plant resources to enhance beneficial arthropods in agricultural landscapes

Annual crop fields typically are simple habitats dominated by a few plant species where pesticides play a major role in managing weed and insect infestations. Recently, there has been significant interest in the potential to reduce reliance on pesticides by manipulating plant species and communities to benefit natural enemies of insects and weeds. Such efforts aim to enhance natural enemy impact by providing appropriate food, shelter, and hosts, and efforts typically are accomplished by manipulation of plant species, populations, or communities. Habitat management is generally viewed as an important factor in maintaining stable insect and natural enemy populations in agricultural systems and may have a similar function in increasing weed seed predation. Crop and noncrop habitats provide resources to natural enemies either directly through floral nectar and pollen, indirectly by increased host or prey availability, or through emergent properties of the habitat such as by moderating the microclimate. These critical resources for natural enemies can be provided in agricultural ecosystems at several scales: within fields, at field margins, or as a component of the larger landscape. Because individual natural enemy species may require quite specific resources at different times and spatial scales, not all attempts to manipulate habitat diversity are equally effective. We review the role of plant resources, including weeds, in supporting natural enemy communities and provide case studies of how varying plant diversity at different spatial scales can influence the effectiveness of biological control in agricultural landscapes.

Rhizome dynamics in Calystegia japonica Choisy and Calystegia hederacea Wall. in relation to overwintering

Calystegia japonica Choisy and Calystegia hederacea Wall., common perennial weeds throughout cropped and uncropped land in Japan, are known to regenerate mostly by rhizomes. The rhizome dynamics of the two species were investigated, focusing on their overwintering characteristics. The dry weight of rhizomes continued to increase for 6–7 weeks after the decrease in weight of the aerial parts began, then it decreased in winter because of the partial death and consequent fragmentation of the rhizomes. Overwintering as naturally fragmented rhizomes is a unique characteristic of the two Calystegia spp. as it is not observed in other rhizomatous weeds that maintain systematic connections at least during the following regeneration in spring. In C. japonica, rhizome degradation was more pronounced and more systematic than in C. hederacea, where thicker fragments that included apices remained. Qualitative changes in C. japonica rhizomes, such as increases in dry matter accumulation and starch, and a temporary decrease in sprouting ability, seem to relate to this specialized survival mechanism. Calystegia hederacea did not show either specialization or preparation for overwintering in its rhizome. The adaptability of Calystegia weeds to annual crop fields can be attributed to their capacity to regenerate from naturally fragmented rhizomes.

Modelling catchment‐scale erosion patterns in the East African Highlands

AbstractPrompt location of areas exposed to high erosion is of the utmost importance for soil and water conservation planning. Erosion models can be useful tools to locate sources of sediment and areas of deposition within a catchment, but the reliability of model predictions of spatial patterns of erosion at catchment scale has seldom been validated against observations. This study aimed to evaluate the performance of a simple empirical model (Morgan, Morgan and Finney model, MMF) in predicting spatial patterns of erosion at two small catchments in the East African Highlands: Kwalei (Tanzania) and Gikuuri (Kenya). Erosion maps predicted by the MMF model were compared with erosion maps obtained by direct survey. In Kwalei, erosion features were especially frequent in fields of annual crops. In Gikuuri, slope was the critical erosion factor, with estimated erosion rates >10 kg m−2 a−1 on slopes >18 per cent. Predicted erosion rates were mainly transport‐limited and ranged from <0·01 to 13·50 kg m−2 a−1 in Kwalei and 9·29 kg m−2 a−1 in Gikuuri. The performance of the MMF model in predicting the spatial patterns of erosion was acceptable in Kwalei, but poor in Gikuuri. However, by excluding the elements at the valley bottoms in Gikuuri Catchment, the performance of the model improved dramatically. The spatial pattern of erosion predicted by the MMF model was driven by the accumulation of surface runoff, which did not consider the possibility of re‐infiltration along the slope. As a result, the MMF erosion patterns predicted by the model increased invariably from the ridges to the valley bottoms, hampering the model suitability for locating areas subjected to high and very high erosion. It is concluded that the model predictions could be substantially improved by introducing a more realistic hydrological component for the prediction of surface runoff along the hill‐slope. Copyright © 2005 John Wiley & Sons, Ltd.

Dynamics of banana-based farming systems in Bukoba district, Tanzania: changes in land use, cropping and cattle keeping

Abstract The spatial and temporal changes of land use, cropping patterns and cattle keeping were assessed for the period 1961–1999 in Kyamtwara division, Bukoba district, Tanzania. The assessment was based on interpreting aerial photographs, surveys and a review of historical statistical data. The area of grassland declined by 40% with a concomitant increase in annual crop fields and forest of 225 and 36%, respectively. The cropping pattern changed from a predominance of banana/coffee/beans to a complex mixed cropping of banana/coffee/beans/maize and root crops in the homegarden, and increased cultivation of maize and root crops in pure stands. Farmers stopped cultivating sorghum and finger millet. The population of indigenous cattle decreased by 50% and an equal percentage of dairy cattle was introduced, but cattle-owning households decreased by 85%. Nutrient balances of homegardens ranged between −27 and 17 kg N ha −1 yr −1 , −1 and 7 kg P ha −1 yr −1 and −5 and 12 kg K ha −1 yr −1 , with the positive balances achieved by resource-rich households. Nutrient balances of crops in pure stand ranged between −15 and −2 kg N ha −1 yr −1 , −2 and −1 kg P ha −1 yr −1 and −14 and −1 kg K ha −1 yr −1 , with more negative balances observed with maize, implying that soil nutrient stocks are decreasing. Increasing population density, coupled with an unequal distribution of resources among households, land tenure, economic policies and poor crop markets were identified as major causal factors of the above changes. Reversing soil fertility decline requires external inputs of nutrients. Within the current poor economic situation of the farming community, different potential soil fertility improvement strategies, including exploitation of N 2 -fixing legumes are discussed.

Are landscape complexity and farm specialisation related to land-use intensity of annual crop fields?

Abstract Little is known about the predictive value of landscape complexity and farm specialisation for land-use intensity, although this is critical for regional agri–environmental schemes and conservation of biodiversity. Here, we analysed land-use intensity of annual crop fields of 30 farms in northern Germany that were located in 15 landscapes differing in structural complexity ranging from 65% non-crop habitats. The proportion of arable land per landscape was used as simple predictor of landscape complexity due to its close correlation with habitat-type diversity, and the proportion of arable land per farm acted as an indicator for farm specialisation due to its negative correlation with stock farming. Land-use intensity was quantified using questionnaires. Landscape complexity and farm specialisation were related to several but not all indicators of land-use intensity. Structurally simple landscapes were related to more nitrogen input and higher crop yields, and farms specialised on annual crops had reduced crop-species diversity, larger fields, higher crop yields and more pathogen species. In contrast to general expectations, pesticide use in annual crop fields was exceptionally high and not a function of landscape complexity or farm specialisation. Our results show that generalisations such as “farms specialised on annual crops and structurally simple landscapes show increased land-use intensity” may be misleading.

Soil Nutrient Relationships with Topography as Influenced by Crop

Variable-rate fertilizer application is often based on grid soil sample data from a single year of data in an annual crop rotation. The objectives of this study were to determine if crop history influences spatial dependence (the degree of spatial variability) of nutrients in a rotation including both annual crops and alfalfa, and to compare grid-based and topography-based sampling strategies for representing within-field nutrient levels. A site in the Red River Valley of North Dakota was observed over three years from 1994–1996. The site was divided into one field of continuous alfalfa (Medica sativa L.) and an adjacent field seeded to spring wheat (Triticum aestivum L.) in 1994, barley (Hordeum vulgare L.) in 1995 and alfalfa in 1996. Samples were taken from a 16.2-ha site each fall in a 33-m grid and analyzed for NO3-N, P, SO4-S and Cl. Topography was determined by measuring elevation in a 33-m grid with a laser-surveying device. Spatial dependence was determined by calculating the semivariogram and using regression analysis to assess the relationship between the semivariogram and the semivariogram model. Spatial dependence of NO3-N and P was strongest following spring wheat and barley, while spatial dependence for SO4-S and C1 was strongest for vigorous stands of alfalfa. When the continuous alfalfa stand declined following winter kill, NO3-N and P spatial dependence intensified. Topography based sampling was correlated with the 33-m grid by giving each 33-m sampling location its value as directed by a topography sampling, then correlating that topography based value with the original 33-m sampling value. Topography-based sampling was correlated with the 33-m sampling grid for all nutrients following spring wheat and barley, but not in continuous alfalfa until the stand began to decline in vigor. Following alfalfa seeding in the annual crop field, topography relationships with NO3-N and P decreased, while topography relationships with SO4-S and Cl increased. Topography samplings of sulfate-S and chloride were most highly correlated to 33-m grid values in vigorous alfalfa. Lack of NO3-N spatial dependence in the vigorous alfalfa stands suggests that a composite or field average soil test might be sufficient to provide soil NO3-N information under similar conditions.

Agroforestry opportunities for the United States of America

Agriculture in the United States makes intensive use of large portions, of the nation's arable landscape. This landscape is dominated by large fields of annual crops with few perennial buffering communities within them. Agroforestry systems such as riparian buffers, alleycropping, windbreaks, tree/pasture systems, and forest farming provide buffering opportunities within these landscapes. Riparian buffers and alleycropping systems provide two unique opportunities toward sustainable production by reducing nonpoint source pollution while increasing ecological diversity. The major impediment to agroforestry in the United States is a lack of identity. Agroforestry as a practice is not officially recognized by federal and most state agencies and thus does not qualify for cost-share support or funding for research and establishment of demonstrations. A recent white paper, prepared by representatives from government agencies, academic institutions, and nongovernment organizations, identified eight major actions that could provide the support, for making agroforestry an acceptable alternative to nonsustainable agriculture. Ames, Iowa. Project No. 3209.

Seed coats of someBrassica andSinapis weedy and cultivated in Canada

A number of species of Brassica and Sinapis are important cultivated plants or weeds in Canada. Turnip Rapeseed, B. campestris L., and Swede Rapeseed, B. napus L., were grown on 3,200 acres in Canada during 1943 1944, but by 1969 1970 the acreage had increased to 2,012,000 (Downey et al. 1970). In 1969 1970, 37,020,800 bushels of rapeseed were produced on the Canadian prairies. In 1969 1970, 200,000 acres of White Mustard, S. alba L. and Indian Mustard, B. juncea (L.) Czern. were also grown in the three prairie provinces. Other species of Brassica commonly cultivated in Canada are: Turnip (B. campestris), Black Mustard (B. nigra (L.) Koch), and Kale, Cabbage, Kohlrabi, Brussels Sprouts, Cauliflower and Broccoli (all varieties of B. oleracea L.). In addition, Wild Mustard, S. arvensis L., and Bird Rape, B. campestris, are serious weeds in Canada and Indian Mustard, B. juncea, Black Mustard, B. nigra, and Swede Rape, B. napus, are occasionally weedy (Frankton & Mulligan 1970; Mulligan & Bailey 1975). Wild Mustard and Bird Rape are frequently weeds in cultivated fields of Brassica, Sinapis and other annual crops. In addition, cultivated Brassica and Sinapis volunteer as weeds in other annual crops, including fields sown to other cultivated Brassica and Sinapis. Since seed of these annual crops may contain seed of one or more weedy Brassica or Sinapis, it is important to be able to identify seed of the different Brassica and Sinapis. Although morphological differences are outlined in Musil (1948), McGugan (1948)