Weeds are one of the most significant threats to crop production worldwide. Crop losses in yield and quality due to weeds, as well as costs of control, have a significant economic impact on crop production. In Australia, Llewellyn et al. [1
] reported that weeds in their Mediterranean climate area cost Australian grain growers $
100/ha in expenditure and losses, with an average expenditure estimated at $
75/ha, including herbicide and non-herbicide practices. In Mediterranean rainfed areas of Spain, the reduction of grain crop yield due to competition by the major winter annual grass weeds such as Lolium rigidum
, Avena sterilis
, and Bromus diandrus
and broad-leaved weeds such as Papaver rhoeas
is the main concern of the farmers. We know of no data on economic impact of these weeds on the crops of this region, but yield losses in cereals have been quantified to be as large as 85% for severe infestations (1000 plants/m2
) of Lolium
], 50% for infestations of more than 300 panicles/m2
] and 71% for severe infestations (500 plants/m2
) of Bromus
has been reported to reduce crop yields between 6% and 70%, depending on crop density, degree of infestation and season [5
Farmers increasingly wish to control weeds by varying the application of herbicides to match the degree of infestation and positions of the weeds within individual fields [6
]. Such site-specific treatments depend on farmers’ knowing where (and when) those weeds are. This knowledge can be obtained in real time from sensors on tractors [7
], from weed maps created from aerial images taken by unmanned aerial vehicles [9
] or maps made by interpolation from weed counts in the field and subsequent geostatistical analysis [6
]. Mapping weed distributions from aerial imagery requires special software, such as that in geographic information systems (GIS) and the purchase of the images, which can be expensive. Mapping from weed surveys has the advantage of more accurate discrimination between weeds and crop plants, and it is better at detecting weeds early in the season when they are sparse. Although this technique seems to be time-consuming and laborious, the economic advantages of using these maps depends on the number of seasons in which a map can accurately depict the same weed patches, i.e., depends on weed patches’ being stable in location [12
]. Some species of weeds have been found to remain in place in diverse crops and under various forms of soil management from year to year. Examples include Abutilon theophrasti
], Solanum nigrum
and Chenopodium album
], Echinochloa crus-galli
] and Avena sterilis
]. Other weeds seem to be stable in some situations and not others. These include E. crus-galli
], Polygonum aviculare
and Papaver rhoeas
]. Detection of stability seems to depend on the density of the populations: the sparser are the weeds, the more difficult it is to detect them [14
]. Ecological factors such as wind dispersed seeds [14
] or post-harvest dispersal [18
] may also contribute to the lack of stability over time.
The studies cited above were made in fields where tillage has tended to homogenize the distribution of weed seeds throughout the field [19
]. Now, in rainfed Mediterranean agroecosystems, farmers are increasingly adopting conservation agriculture (CA) techniques based on the principles of minimal soil disturbance, permanent soil cover, and crop rotation. Minimal soil disturbance should lead to an increase or maintenance of the soil’s organic matter content and capacity to store water. This can be achieved without inversion of the soil, by harrowing or by direct drilling or no-tillage. Of the two, direct drilling is the most popular, especially where the previous crops have been harvested and the straw removed [20
]. But the CA systems, particularly those involving direct drilling, cause changes in the weed communities [21
] and a redistribution of the weed flora within the fields, because these practices disperse seeds less [19
]. As the seeds of the most important weeds in cereal crops are not airborne, we can expect these species to maintain their locations more in direct drilling systems than under tillage. For those species that remain in place, maps made in one year should be usable in subsequent years for targeted application of herbicide.
Although weed patchiness in CA systems have been studied [22
], we know of no data on weed patch stability under these systems. We therefore aimed to discover whether soil management (direct drilling versus harrow tillage) influences the spatial distribution of weeds and whether this spatial distribution remains stable over time. Our hypothesis is that direct drilling leads to stronger spatial structure (patchier distribution of weeds) than harrow tillage. To that end, we explored data on the spatial variation of weed cover in several fields. Our aim was to discover what spatial or temporal structures there were in the weed communities and how these might be affected by cultivation. We describe the data with statistical models and quantify both weed aggregation and spatial and temporal variation. If there is a strong spatial aggregation that persists from year to year then a farmer would be able to use maps of weed distribution made in one year for weed control in subsequent years.
Recall that our aims were to assess the feasibility of mapping weeds for site-specific spraying and to discover whether there were differences in the patch stability of weed associated with cultivating by direct drilling or harrow tillage. We can summarize our findings as follows.
Weed cover varied substantially across fields with greater variation generally in direct-drilled fields.
Aggregation was greater for Bromus than for Lolium, and both were more aggregated than Papaver, but the degree of aggregation differed from year to year and between fields.
Papaver was more aggregated in harrow-tilled fields than in direct-drilled ones.
Spatial correlation was stronger in the direction of traffic than the perpendicular direction.
In a few of the fields the patches of weeds were stable from year to year; most of these fields were harrow-tilled.
The spatial stability was more pronounced in the direction of field traffic than in the perpendicular direction for all three species.
Both weed cover and degree of aggregation varied substantially from one year to the next. Weed populations and their spatial distribution reflect cropping history and current management. The fields surveyed had been managed in the same ways, either direct-drilled or harrow-tilled, for between 4 and 14 years. According to Swanton et al. [32
], it may take 4–10 years for weed populations to reach equilibrium, and so, at least in part, the spatial distribution of the weeds is a consequence of soil management [33
for example, is symptomatic of direct drilling [35
], which creates favorable conditions for it [36
According to the results, weed cover and location seem to have been modulated by management factors such as herbicides and rotation. The herbicides applied differed between fields and changed between years for the same field, and that might explain changes in weed cover. For example, at Balaguer-T the increase in weed densities observed in the second year is likely to have arisen because the weeds were resistant to the herbicide (the farmer told us), and the reduction observed in the third year seems to have been caused by the much greater efficacy of the new herbicide. The increased population of weeds at Vilanova-D arose because no grass–weed herbicide was used. Another source of variation of weed cover in the fields was crop rotation. The field of Mas de Melons-D was left fallow in 2012 and weeds would be favored by the absence of crop and lack of herbicide. Sampling time may also have been important in fields such as Agramunt-D, Bellmunt-T, and Bellmunt-D where the sampling was done earlier than usual the second year. These factors might have contributed to the variation in the weed cover from year to year.
but particularly Papaver
were more aggregated (smaller k
parameter) in the harrow-tilled fields than in the direct-drilled ones. We did not expect this because the absence of soil disturbance in direct-drilled fields should allow seeds to remain closer to their mother plants, resulting in a more patchy distribution (quadrats with large proportions of both small and large counts). One possible explanation is that direct-drilled fields had some sites and seasons where no herbicide was applied or, if an herbicide was applied, it did not target the species analyzed (Table 1
). This allowed greater densities throughout the field that decreased the overall patchiness. The application or no application of herbicides may also explain the greater differences observed in the degree of weed aggregation between the direct-drilled fields (standard deviation of k
= 0.62) compared with the harrow-tilled ones (standard deviation of k
The spatial dependence was stronger in the direction of the field traffic generally than in the perpendicular direction. The timing of seed shedding in all species is generally between June and July [37
], coinciding with the harvest. Seeds of Papaver
that shed before harvest have a primary gravity-related dispersal in a limited space around the parent plants. Afterwards, they are dispersed in the direction of traffic by tillage, but the distance at which seeds move horizontally is limited to less than 2 m, and depends on the implement used [19
does not shed its seeds spontaneously before harvest, and even after harvest most seeds are dispersed as clustered spikelets or spike fragments [39
]. The seeds of Lolium
and the seeds of Papaver
that are still on the plant can be dispersed by combine harvesters. Combine harvesters have been reported to move seeds in the machine direction from their source up to 18 m for L. rigidum
] and up 30 m for Avena sterilis
and A. fatua
]. The smaller size and near-spherical shape of P. rhoeas
seeds might make them less likely to be dispersed than larger seeds such as those of Bromus
or seeds that remain attached to the spike at harvest time, such as those of Lolium
. These large seeds can remain in the interior of combine harvesters after entering the headers only to be displaced further away in the direction of traffic, making a significant contribution to seed dispersal. For such seeds, primary dispersal might be less relevant. The role of the agricultural machinery on the intensity and direction of weed dispersal has been widely reported [41
The shapes of weed patches can change (1), by expanding or shrinking radially as a result of population increase and dispersal (2), by intensifying or weakening as a result of an increase or decrease of the local population density without expanding or shrinking and (3), by shifting in space [14
]. The statistical techniques we used to analyze the spatio-temporal processes are based on the calculation of cross-correlations in two directions across space and time. Correlations across space characterize shifts and expansions or shrinkages by comparison of a given sample location with its neighbors in different years and characterize intensification by comparison of each sample location with itself in different years [27
]. All directions are accounted for, because all combinations of rows and columns are taken into account. The cross-correlation analyses did not detect temporal stability in most of the fields; only at Bellmunt-D with B. diandrus
and Bellmunt-T and Vilanova-T with L. rigidum
a significant stability was observed across years, being more perceptible in the direction of the field traffic and confirming the role of the traffic in modulating the spatial distribution of the weeds.
Some studies of spatial stability of weed patches in agricultural fields indicate that these remain remarkably stable over time [13
]. However, absence of spatial stability [14
] or stability over short times [11
] have also been reported. Heiting et al. [14
] attributed instability to both the dispersion mechanism of the species, being greater for weeds the seeds of which are dispersed by wind, and for species with sparser populations. The instability of patches in the fields we surveyed could be related to the concept of specialist and generalist plants. Species could be ranked along a specialist-to-generalist gradient based on their niche breadths [45
]. Specialist species tend to be aggregated, whereas generalists and species that have an intermediate degree of habitat specialization tend to be segregated [46
]. If we take into account the large distribution and abundance of B. diandrus
, L. rigidum
and P. rhoeas
in the Mediterranean cereal fields [47
], we can consider them to be generalists, and consequently, we can expect random aggregation patterns. Agronomic factors such as weed management rather than ecological-like niche requirements would mostly determine the location of these species. For example, in some of the fields that we studied (Agramunt-D, Bellmunt-D and Bellmunt-T) weed cover was greater towards the edges of the fields. Arable field edges have often been observed to support increased diversity and abundance of weeds compared with more central regions of the fields [48
]. This is assumed to be due to both a reduction in agricultural inputs towards the field edge or spatial mass effects associated with dispersal of weeds from the surrounding landscape or both [50
Aggregation was observed in most fields regardless of the soil management, but it did depend on both field and season. Herbicide applications, crop rotation, and traffic seem to affect weed populations strongly within fields, regardless of the soil management. The instability of the patches and the variation observed between years in the weed cover do not limit the application of site-specific weed control of these weeds in conservation tillage systems. The instability limits only the more-than-one-year-use of weed distribution maps based on discrete sampling. Other technologies such as real-time weed detection with optical sensors and on-board computer analyses can be more appropriate. We recommend that future research on site-specific weed management should be addressed to improve the detection of weeds in order to increase the efficacy of the control, particularly in CA systems with direct drill, with an enhanced reliance on herbicides. It is expected that this ground-based continuous sampling technology, neither labor-intensive nor time-consuming, can become more affordable in the future with the spread of its use.