Weed seed bank in agricultural science

A Comparative Study of Soil Weed Seed Bank Determination in Pothwar Region by using Different Methodologies

1 Department of Agronomy, Pir Mehr Ali Shah, Arid Agriculture University, Rawalpindi, Pakistan; 2 Ecotoxicology Research Institute, National Agricultural Research Centre Islamabad, Pakistan; 3 Crop Sciences Institute, National Agricultural Research Centre, Islamabad, Pakistan; 4 Research Farm Crops, Agriculture Department, AJ and K, Baldmas District Kotli, Pakistan.

Abstract | Soil weed seed bank is a natural source for weed infestation. Determination of soil weed seed bank has primary importance to get complete picture of weed seed reservoir in the soil profile. An experiment was conducted at University Research Farm, Chakwal Road, Rawalpindi to compare the techniques for determination of soil weed seed bank during winter 2012-2013 under rainfed conditions of Pothowar. Soil samples for the weed seed bank analysis were taken from the experimental field before wheat sowing from 0-10 cm, 11-20 cm and 21-30 cm soil depth. Two soil weed seed bank determination techniques were compared viz., sieving method and seedling emergence method. The data were collected on the seed density m -2 , seed frequency, diversity of weed species and relative importance value. The input and output data was also collected to find out the socioeconomic feasibility of the techniques. The comparative analysis of seed bank extraction methods revealed higher weed seeds density, weed frequency with more diversity of weed species under sieving method in comparison to seedling emergence method. Therefore, sieving method was considered superior over seedling emergence method. The feasibility analysis of seed bank extraction methods indicated that sieving was cost-effective, less time consuming, more user friendly with higher accuracy over seedling emergence method.

Received | September 29, 2017 ; Accepted | October 27, 2017 ; Published | November 21, 2017

*Correspondence | Ijaz Ahmad, Ecotoxicology Research Institute, National Agricultural Research Centre Islamabad, Pakistan; Email: [email protected]

Citation | Hussain, M., S. Ali, M.N. Tahir, G.A. Shah, I. Ahmad, M.A. Sarwar and S. Latif. 2017. A comparative study of soil weed seed bank determination in pothwar region by using different methodologies . Pakistan Journal of Agricultural Research , 30(4): 310-315.

Keywords | Comparative study, Weed seed bank, Sieving method, Seedling emergence method

S oil acts as storage house for different macro and microorganisms including insects, micro-organism, fungi, algae, spores, nematodes and seeds of different weeds. Weed plants after maturation shed their seeds and these weed seeds ultimately accumulated in the soil profile which form weed seed bank in the soil. Soil weed seed bank comprises of all viable, dormant and non-dormant seeds present in the soil profile ( Forcella et al., 2003 ). Knowledge of soil weed seed bank is important for population dynamics studied, establishment of appropriate weed management programs ( Ambrosio et al., 2004 ) and forecasting of weed infestations ( Ball and Miller, 1989 ; Creech et al., 2008 ).

Weed seeds may enter in the seed bank through many sources from plant seed production, together with primary and secondary dispersal such as farm equipment, contaminated crop seeds, animals, wind and manure ( Buhler et al., 1997 ). Among these sources, the largest source of weed seeds in the seed bank is plants producing seed within the field. Weed seeds also drive the spread of weed patches in fields, both for annual ( Steinmann and Klingebiel, 2004 ) and perennial weed species ( Blumenthal and Jordan, 2001 ), and are the only source of population increase for annual weed species. Decline in weed seed bank may occur by various factors such as germination, seed predation ( Van Mourik et al., 2005 ), seed decay and death ( Gallandt, 2006 ) and deep seed burial to layers from where emergence onto the soil surface is impossible ( Honda, 2008 ). These seed banks range from near 0 to as much as 1 million seeds m -2 ( Radosevich et al., 1997 ).

There are a number of methods that have been used to determine the density and composition of soil weed seed bank. These methods are categorized into two main techniques that are used to find out the number of seeds from the soil samples, i.e. (1) weed seed extraction method and (2) weed seedling emergence method. In direct seed extraction method, weed seeds are extracted by washing and floatation methods while in the second technique, weed seedling emergence, the soil sample is placed in the greenhouse or controlled environment, watered on regular basis in order to emerge the weed seedling and these emerged seedlings are then identified and counted ( Luschei, 2003 ).

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The objective of current study was the determination of soil weed seed bank of Pothowar region and to compare different methods to find out the most accurate, efficient, handy and economical technique for the determination of soil weed seed bank.

Material and Methods

A comparative study of soil weed seed bank determination methods was conducted during 2012-2013. Soil sampling was done from the different localities of the wheat field at University Research Farm, Chakwal Road Rawalpindi. Sampling of the soil was carried out before the sowing of wheat crop diagonally in four replications from three depths i.e. 0-10 cm, 11-20 cm and 21-30 cm. Soil samples were taken by using steel probe of 2.5 cm diameter for each method from each replication. The soil cores of same depth were bulked and mixed to make composite soil samples. These composite soil samples were divided into three working sub-samples having one hundred gram weight of each sample for soil weed seed bank analysis. The soil samples were then transported to laboratory and stored at room temperature until further processing. Two soil weed seed bank determination techniques i.e. seed extraction using sieving method and seedling emergence method were compared for extraction of weed seeds from soil. In sieving method, seeds were extracted from soil by sieving of soil sample through various sieves with different mesh sizes using method adopted by Konstantinović et al. (2011) . Each 100 gram soil sample was initially poured on sieve of 80 mesh size and placed in the water for softening the soil clods. This sample was then immersed in the sodium hexa-metaphosphate solution (40 g/L of water) in order to disintegrate the soil particles. The soil samples were shifted to the bucket having tap water and shook well so that almost all clay and silt particles were filtered out and removed from sample. The remaining material on the sieves was air dried and transferred on the filter paper so that samples become completely dried. These dried samples were then passed through a descending series of sieves i.e. mesh no. 10, 18, 30, 40, 50 and 80. Entire seeds remained on the sieves were collected for identification and further processing. Seeds of different weeds were collected from experimental area and its surrounding with objective of reference collection for the weed seeds identification. Seeds extracted from soil were compared with the collected seeds to identify seeds using high magnification lens (10X) and seeds of each species were counted. Viability of seeds was determined by using crushing method, i.e. gentle pressure was applied to the seeds with the help of forceps and seeds that resisted this pressure were considered as viable and counted. Seedling emergence trial was carried out in the growth chamber keeping the controlled environment for germination of different Rabi weed seeds. Soil core samples weighing 100 gram were spread onto petri dishes in separate sections according to their depth and replication. These petri dishes were placed in the germination chamber. The temperature in the germination chamber was maintained from 20 ˚C during day time (12 hours) to 8 ˚C during night time (12 hours). Petri dishes were watered on daily basis avoiding samples to dry. Emergence of weed seedlings was observed on weekly basis. Emerged weed seedlings were identified

Weed seed predation in agricultural fields

Weed communities in agronomic fields are dominated by annual species. Summer annuals initiate growth each spring from seeds found in the upper soil profile (Figure 1). In most fields, a small percentage of the emerging plants survive and contribute new seeds to the soil seedbank. Historically, most research of the annual weed life cycle has focused on seed dormancy and emergence (A), effect of control tactics on weed survival (B), and weed seed production (C). The fate of seeds between the time of maturation on the plant and entering the seedbank (D) has largely been ignored. However, current research at Iowa State University and other organizations has shown that significant seed losses routinely occur in agronomic fields, and these losses may influence the effectiveness of weed management programs. This article will provide a brief summary of some of the current research in this area and the potential importance of seed predation to weed management.

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Prairie deer mouse – a common seed predator.

Plant seeds are storage organs for high energy compounds that supply plant embryos the resources needed to germinate and develop into seedlings. These energy reserves are an excellent food source for a variety of animals that live in or near agricultural fields, including ground beetles (carabid beetles), crickets, mice and others. Estimates of cumulative seed losses due to seed predators have ranged from 20% for barnyardgrass and lambsquarter in a chisel plow system (Cromar et al. 1999) to 88% for giant ragweed in no-tillage (Harrison et al. 2003).

A common method of measuring seed predation involves lightly attaching seeds to sandpaper or a similar material and placing the seed cards in the field. After a few days the card is retrieved and the percentage of seeds removed is determined (Westerman et al. 2005). Averaged over 12 sampling periods from May through November, seed losses ranged from 7 to 22% per day depending on the crop present in the field in a study conducted near Boone, IA (Figure 2). The higher predation rates in small grain and alfalfa compared to corn and soybean may be due to differences in crop canopy development. The rate of seed predation typically increases as a crop canopy develops within a field. Corn and soybean canopies provide little protection for predators early in the growing season compared to small grain or alfalfa, and thus predators may seek other habitats when little canopy is present. Later in the season, predator activity is typically similar in corn and soybeans as in other field crops.

Insect predators (field crickets, ground beetles, etc.) are active during the growing season when temperatures are favorable for cold-blooded species, whereas field mice are active year round. Seed predators have a remarkable ability to locate seeds on the soil surface; however, once seeds move into the soil profile the threat of predation is greatly reduced. The highest rates of seed predation likely occur in late summer and early fall when weed seeds are shed from plants onto the soil surface. Tillage buries the majority of seeds at depths where predation is minimal. Avoiding or delaying fall tillage following harvest should increase seed losses due to predation. Seeds can also enter the profile due to the impact of rain droplets, by falling into cracks, or due to freezing/thawing cycles during the winter. Ongoing research at ISU is evaluating the fate of seeds on the soil surface and how long they remain available to predators.

Field crickets on seed card.

The preference of predators for different species of weed seeds in the field is poorly understood. When given a choice, seed predators often will feed preferentially on one species over another (van der Laat et al. 2006; Figure 3). A common question is whether seed predators pose a threat to crop seed. Seed size and depth of planting minimize risks of corn and soybean seed losses to predators. Small-seeded legumes and grasses are at greater risk for predation losses, but proper planting where the majority of seed are placed under the soil surface should minimize losses.

Significant numbers of weed seeds are consumed by predators in agronomic fields, but the full impact of seed predation on weed densities and weed management is poorly documented. Clearly, destruction of a significant percentage of the weed seeds produced in a field will impact the following year’s weed density. The impact of giant foxtail seed rain and seed predation on giant foxtail densities was evaluated near Boone, IA (Figure 4). Giant foxtail seed (750 / sq ft) were spread on the soil surface in standing corn in late September 2004. The field was planted to no-till soybean in 2005 and foxtail emergence monitored throughout the season. The experimental area had a history of good weed control, thus foxtail densities were very low (

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Modeling efforts at ISU have shown that seed predation can significantly affect long-term weed population dynamics within agricultural fields. For example, in a 4-year crop rotation (corn/soybean/small grain+alfalfa/alfalfa) the seed bank of giant foxtail rapidly increased from 2000 seed/m 2 to 4.3 million seed / sq m over an 18 year simulation period in the absence of predation (Figure 5). However, allowing for 25% seed predation resulted in a static seed bank, whereas any seed predation in access of 25% resulted in a decline in the seed bank density. The diverse rotation required 80% less herbicide than a conventionally managed corn-soybean rotation.

The value of intercepting weed seed before they enter the seed bank is somewhat of a forgotten control tactic. In the 1930’s and 40’s, combines were commonly equipped with a weed seed collector that separated and collected weed seed from chaff as the crop was harvested. When modern herbicides were introduced in the 1950’s, it was considered less expensive and more convenient to control weeds with chemicals, and these accessories quickly disappeared from combines. In Australia, seed collectors are again being used on combines due to widespread herbicide resistance and the loss of effective herbicides. Rigid ryegrass infestations have been reduced by as much as 70% through use of weed seed collectors during harvest (Gill, 1995). The effectiveness of weed seed collectors varies among weed species depending on timing of seed shed. Weed species that drop the majority of their seed prior to crop harvest would not be impacted significantly by use of weed seed collectors.

Weed seeds are an important food source for a variety of organisms that live within or adjacent to agricultural fields. It is clear that seed predation is an important form of biological control that influences weed communities within agricultural fields. Yet to be defined is how cropping systems can be manipulated to enhance the activity of seed predators and maximize their benefit, therefore allowing reductions in other more disruptive control tactics.

ISU research cited in this article was partially funded by:

The Leopold Center for Sustainable Agriculture

USDA National Research Initiative

References

Cromar, H.E, S.D. Murphyand C.J. Swanton. 1999. Influence of tillage and crop residue on postdispersal predation of weed seeds . Weed Sci. 47:184-194

Gill, G.S. 1005. Development of herbicide resistance in annual ryegrass in the cropping belt of Western Australia. Aust. J. Exp. Agric. 35:67-72.

Harrison , S.K., E.E. Regnier and J.T. Schmoll. 2003. Postdispersal predation of giant ragweed seed in no-tillage corn. Weed Sci. 51:955-964.

van der Laat, R., M. D.K. Owen and M. Liebman. 2006. Quantification of post-dispersal weed seed predation and invertebrate activity-density in three tillage regimes. J. Agric. Ecosys. Envir. Under review.

Westerman, P.R., M. Liebman, F.D. Menalled, A.H. Heggenstaller, R.G. Hartzler and P.M. Dixon. 2005. Are many little hammers effective? – Velvetleaf population dynamics in two- and four-year crop rotation systems. Weed Sci. 53:382-392.

Weed seed bank in agricultural science

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