1. Introduction
Crop rotation is among the most useful tools in order to control weeds [
1] and contribute to Integrated Pest Management (IPM). It allows varying the management of fields, creating an unstable and inhospitable environment that prevents weeds from proliferating [
1] as well as disrupting weed population dynamics [
2]. This is of special interest in organic farming, where no chemicals can be applied for their control. However, the range of crops used in rotation with cereals in Mediterranean semiarid dryland systems is scarce. Rotation with pea (
Pisum sativum L.) and sunflower (
Helianthus annuus L.), which intrinsically imply a sowing delay, have proven to be effective for weed control in these climatic conditions [
3,
4]. Nevertheless, the limited rainfall in semiarid drylands hampers the integration of some of these crops [
3].
In the driest part of the Ebro Basin, in northeastern Spain, annual rainfall is about 350 mm [
5] and usually presents an erratic distribution [
6], which highly affects cereal yields. The future projection of climate change is expected to increase evapotranspiration and decrease rainfall [
7]. For this reason, new drought-tolerant crops with the ability to optimize water efficiency and production are being promoted by the Spanish Government [
8].
Camelina is one such crop that may well fit with these requirements. Although it is an ancient crop [
9,
10], it was minimally produced for centuries until the last decade [
11]. Its unique oilseed composition has been widely highlighted [
12] and is useful for either industrial purposes, such as biodiesel [
13,
14], or for human consumption [
15,
16,
17]. It can be considered either as a winter [
18] or summer crop [
19,
20]. It has a short life cycle [
21], which varies from 58 to 134 days as a summer crop [
22,
23] and from 209 to 298 days as a winter crop [
15,
24]. Moreover, the hardiness of the crop [
24] makes it suitable for cropping in low input agricultural fields, such as organic systems. The capability of camelina to adapt to different environments [
11] makes it suitable for changing sowing dates for its production. In this respect, many studies have been performed in North America [
19,
25,
26,
27], but all these sowing dates (SD) are for spring-sown experiments done under temperate continental climates. By contrast, Berti et al. [
21] compared different autumn and winter SDs in humid Mediterranean climates and found that significant differences in yield and yield components were dependent on environment and latitude. Whether this also happens in a semiarid Mediterranean climate for autumn- and winter-sown camelina in Spain is unknown.
The objective of this work was to study the effect of different sowing dates of camelina on the control of weeds and its effect on camelina seed yield, so that the inclusion of this crop in rotation with organic winter cereals in the Mediterranean semiarid region of Spain could be evaluated.
4. Discussion
The hardiness of camelina to abiotic stresses, such as drought and low temperature, makes it a potential crop [
11,
12] for rotating with cereals, and thus, camelina may be highly beneficial for agricultural systems in semiarid climates, such as the Ebro Basin. Moreover, the high degree of weed suppression obtained in SD2 and SD3 with respect to SD1 in the present study indicates the potential of this crop to be used in organic farming.
In our work, weed coverage was effectively reduced with camelina SD, but no differences were observed with
R. In R1, WW coverage was reduced by up to 99.6% in 2015–2016 by delaying the SD from late October (SD1) to December (SD2), while the reduction was 98% in 2017–2018 and 66% in 2016–2017. The delay of SD to January (SD3) did not significantly improve weed coverage reduction, which was of 99.2%, 70.4%, and 99% respectively for 2015–2016, 2016–2017, and 2017–2018. Most WW emerge from October to February, depending on the species and the season; thus, the greatest amount of these weeds was killed with the soil tillage for late SDs. Although the WW species were not the most problematic in the area (
Avena sterilis and
Bromus diandrus were lacking, while
Lolium rigidum and
Papaver rhoeas were scarcely represented), the presence of mainly
D. erucoides, as well as
S. irio and
Hordeum murinum could be representative of a WW community, giving validity to the obtained results. Similarly, Garcia et al. [
30] also obtained 80–99% weed control with SD from October to December in
B. diandrus in the same area. In our conditions, these control levels would have been 0.0–79.4% in SD1, 9.1–37.7% in SD2, and 12.9–81.9% in SD3, depending on the season. Other emergence models already published for
A. sterilis [
31],
L. rigidum [
32], and
P. rhoeas [
33] indicate the usefulness of the SD in the control of the most important weeds in this area: In our climatic conditions, up to 83.9%, 46.9%, and 6.9% of
A. sterilis.,
L. rigidum., and
P. rhoeas emergences would have been killed by SD1, while these values would go up to 86.2%, 62.4%, and 17.8% respectively in SD2, and up to 91.9%, 77.7%, and 42.3% in SD3. Moreover, the competitive capacity of camelina, which was not studied in this work, is likely to have contributed to weed suppression. For this reason, camelina can be a good option to alternate with cereals, but this depends on the annual climatic conditions that highly affect the emergence of these weeds.
Not only did delaying SD contribute to the control of weeds, but it also delayed the harvest date. In our experiment, harvest was done between 11 May and 6 June for SD1 and between 25 May and 20 June for SD2 and SD3 (
Table 1). These time laps from sowing to harvest (120–220 days) are similar to those obtained by Berti et al. [
21] in Chile for autumn–winter sown camelina. Weeds are adapted to the cereal cycle, and most of them release seeds at harvest [
34]. This happens in the Ebro Basin from mid-June to July. Therefore, hastening the harvest of camelina by one month to mid-May would prevent weed seeds from maturing, thus avoiding the seed rain that contributes to the management of the seed bank. In this respect, camelina’s short life cycle coincides with that obtained by Gutiérrez and Albalat [
24] for SD1, while it was considerably shortened in SD2 and especially in SD3, in the present study.
Contrary to Berti et al. [
21], in the present study no significant yield loss was suffered by delaying the sowing date to SD2 or SD3 in any season. The differences with respect to Berti et al. [
21] could be partly explained by the climatic differences between the Chilean sites and Lleida Spain. Both winter (Dec–Feb/Jun–Aug) temperatures (6.7 °C < 7.3–8.8 °C) and rainfall (54 mm < 472–747 mm) were lower in Lleida, which could have slowed down the growth of SD1 and SD2, while in Chile the early SD could take advantage of the milder and wetter winter compared with the late SD. There was only some irregular yield in 2015–2016 between SDs. Rainfall in November 2015 was 71 mm, but 70 mm fell in only one day (with a lot of runoff), almost one month before SD2, followed by a December and January that were very dry. This could have contributed to the low yields obtained in SD2 that season. However, this reason does not fully explain yield results in SD2. The lowest yields in 2015–2016 were similar to those obtained by Gutiérrez and Albalat [
24] in the nearby county of Aragon, or those by Gesch & Archer [
18] and Obour et al. [
35] (0.5 Mg ha
−1), while others approximated more to French et al. [
36] (1Tn ha
−1), or even to McVay and Khan [
19] and Obour et al. [
35] (1.5 Mg ha
−1), as yields of 2016–2017. Meanwhile, yields from 2017–2018 were more like those from Gesch [
25] (2.3 Mg ha
−1) or Zubr [
37], who obtained yields of 2.6 to 3.3 Mg ha
−1 in Denmark. These differences in yield between seasons are the usual ones for a semiarid region, where climatic conditions can easily vary the productivity of the crop from one season to the next [
38], like in barley [
39].
In this study, seed oil content was not analyzed. Sowing date 1 plants matured before SD2 and SD3, which means that the climatic conditions for the seed maturation process were different between SDs. This could have affected oilseed quality. Gesch et al. [
40] observed higher seed mass, oil content, total carbon, and unsaturated fatty acid content for
Thlaspi arvense seeds produced in Spain than in Minnesota, and Pavlista et al. [
26] also found differences in oil content and quality in spring camelina, being the highest when sown in April. Whether this also happens in our conditions and how autumn–winter SD affects camelina seed oil content will require further study.
The lack of yield differences between SDs is a great advantage, because, depending on the climatic conditions of each season, mainly precipitation, weeds can significantly vary their emergence period. The flexibility of sowing of camelina without a yield penalty can be used as a strategy to contribute to better weed control. This sowing flexibility also allows for a wider window of opportunity to make decisions. Despite this, some productivity decreases have been reported [
35] in winter wheat the following season when it follows camelina in rotation. This aspect should also be studied to better understand this issue.