CARACTERÍSTICAS DEL LIDERAZGO TRANSFORMACIONAL

In the previous section it was concluded that crop rotations are an essential feature of carrot and onion production and as such the crop rotation feature has to be implemented in ECOSEDYN. When it comes to crop rotations there is no such thing as standard practice. The questionnaires and interviews with carrot growers and crop consultants (Carl Sharp and Tom Will) highlighted that a range of different crops are grown by the landowners on the land that is rented out but cereals are often grown in several years of the rotation. It was impossible to collect parameters for the entire range of crops grown in commercial fields, and for simplicity winter wheat was chosen as the single arable crop in rotation with carrot. More than 90% of wheat in the UK is now sown in autumn rather in spring (Robinson and Sutherland, 2002). Therefore, a simplified crop rotation is proposed for ECOSEDYN, consisting of one year of vegetable (carrot / onion) and five years of autumn sown wheat. On the one hand, simplifying the “system” results in a concession to the practical value of the model predictions. On the other hand, a simplified system is more easily comprehended and by understanding the basic principles, guidelines can still be given for more complex systems. The aim was to compare the weed seedbank after a total length of 24 years, i.e. 4 complete crop rotations, had passed.

The cultural control methods chosen as components for the weed management scenarios are:

• Crop sowing time

• Crop variety (maturity time)

Crop sowing time was chosen as the first cultural control component because the Defra project of which this Phd studentship was part, had as a primary aim to gain a better understanding of the importance of relative crop and weed emergence timing. Crop variety (maturity time) was chosen as the second cultural crop component because the few studies that explicitly included different times from sowing to harvesting show substantial effects on weed seed production.

If the availability and efficacy of weed management options for carrots is sufficient, then growers do not need to consider the weed management of the field in the years that the arable crop is grown. However, the diminishing range of post-emergence herbicides has contributed to an attitude where a more holistic or integrated view of weed management is being considered more positively (pers. communication Tom Will). Approximately 85% of the land that commercial vegetable growers use for

growing carrots is rented land (pers. communication Tom Will) and this shared ownership does not facilitate the integration of weed management. The selected components are therefore applied in both the vegetable as the arable phase of the crop rotation. The cultural control options are to be combined in a factorial to give a range of weed management scenarios assuming a ‘worst-case’ scenario for weed control (i.e. with herbicide efficacy values being 75% of normal efficacy values). Scenario components are referred to in this way: V_ST, V_MT, A_ST, A_MT where the prefix ‘A’ or ‘V’ indicates whether the vegetable or arable crop is concerned and the postfix ‘ST’ and ‘MT’ stands for sowing time and maturity time respectively. A particular weed management scenario might for example consist of V_ST=1, V_MT=3, A_ST=2, A_MT=3.

In comparing the weed management scenarios two main questions were posed:

1. Which cultural control practice, sowing time or crop variety, and applied in which crop, has most potential in alleviating long term weed seedbank levels? 2. Can cultural control practices applied in one crop (carrot or winter wheat)

maintain sufficient low weed seedbank levels or is the application in both crops required?

A more thorough explanation of the weed management scenarios and specific hypotheses are given in Chapter 5.

2.3.1

Implementation of crop sowing time

Five different sowing dates for carrot and onion and three for winter wheat were included in the weed management scenarios. Crop sowing and consequently seedbed preparation times vary markedly, both to provide carrots throughout the year and according to the type of market outlet (processing or fresh). Early carrots are sown from October to February and are grown under polythene. ‘Maincrop’ carrots are sown from April to mid June at densities of 600 to 800 thousand seeds / acre, approximately twice the density at which early carrots are sown (Elsoms, 2007c). In ECOSEDYN, early carrots (grown under polythene) will not be considered as no information is available about the effects of polythene on weed biology.

Sowing dates follow general practice in the UK and were obtained through the catalogues of breeding companies (Elsoms, 2007b (carrots); Elsoms, 2007a (onions))

from literature (Spink et al., 2000 (winter wheat)) and from trials from the Home- Grown Cereals Authority (HGCA, 2002 (winter wheat)), and are given in Table 2-1.

Table 2-1 Sowing dates and symbols used in ECOSEDYN to identify scenarios for the

different crops in the crop rotation

2.3.2

Implementation of crop variety (maturity time)

Three different crop varieties (maturity times) were selected. For carrot, the online product catalogue of Elsoms (2007b) was used to choose realistic values for the time from sowing to maturity: in the Nantes group, early (e.g. Norwich F1), intermediate (e.g. Nairobi F1) and late (e.g. Nerac F1) maturing varieties take between 98 and 130 days from sowing to maturity. Data on onion varieties is available from studies at Wageningen University (van den Broek, 2002). The variation in harvest time ranges between 115 and 129 days from sowing to 50% foliage senescence for an early and late maturing variety respectively. The time from sowing to harvest varied between winter wheat varieties by about 14 days (Gleadell, 2007; HGCA, 2007). Sowing and harvest dates varied depending on the location in the UK and resulted in a range of 301 to 349 days from sowing to harvest with an average of 320 days (HGCA, 2002). The maturity times are given in Table 2-2.

Table 2-2 Maturity times (days after sowing, DAS) and symbols used in ECOSEDYN to

identify scenarios for the different crops in the crop rotation

Symbol Carrot (DAS) Onion (DAS) Symbol Winter wheat (DAS)

V_MT = 1 98 115 A_MT = 1 313

V_MT = 2 112 122 A_MT = 2 320

V_MT = 3 130 129 A_MT = 3 327

Symbol Carrot / Onion Symbol Winter wheat

Date Day Date Day

V_ST = 1 1 March 152 A_ST = 1 1 October 1

V_ST = 2 15 March 166 A_ST = 2 19 October 19

V_ST = 3 29 March 180 A_ST = 3 8 November 37

V_ST = 4 12 April 194

It was assumed that the difference in time to maturity would be constant at each sowing time. It has to be emphasized that the values do not represent particular varieties since this assumption is unlikely to hold for specific crop varieties, i.e. variety A may mature 14 days earlier than variety B at ST = 1 but only 7 days earlier at ST = 3. The particular way in which the crop harvest date is determined in ECOSEDYN is explained in Section 4.7.1.2, after the novel modelling approach for ‘Biomass Increase’ has been explained.

In conclusion, given the various levels of sowing time and maturity time, a full factorial of combinations of cultural control could be created resulting in 135 weed management scenarios: V_ST (5) x V_MT (3) x A_ST (3) x A_MT (3).

2.3.3

Climate scenarios

As indicated in Section 2.1.2, plant populations are regulated by several factors that interact and changing climatic conditions are likely to interact with the effect of weed management scenarios. Weather projections for the UK estimate the annual temperature to rise between 2 ˚C and 3.5 ˚C by 2080, winters to become wetter and summers likely to become drier (Hulme et al., 2002).

Weather data collected at Warwick HRI were examined and 17 weather years (October-September) since 1989 selected (weather year 2001-2002 was omitted due to missing values for solar radiation). Over this interval no change in monthly or yearly rainfall could be detected although, contrary to the projections by Hulme et al.

(2002) there was a trend for the summer months May – August to be wetter (see Figure 2-7, left). There was a significant rise in temperature over the last 18 years

Figure 2-7 Cumulative rainfall (left) and cumulative day-degrees (right) measured at

(paired T-test, two-tailed: P=0.0051) with the largest differences for the months April- June and September-October (see Figure 2-7, right). The last six years belonged to the nine years with the highest monthly accumulated day-degrees. Two climate scenarios were therefore created based on the total accumulated day-degrees per year:

1. Scenario 1 - ‘No change’: Given that the total length of the simulation in

ECOSEDYN was 24 years and only 17 weather years were available, 7 weather years were randomly drawn from the 17 and added to the pool of 17 weather years. The 24 weather years were then permutated and this sequence was applied to each weed management scenario.

2. Scenario 2 - ‘Heating up’: The eight years with the highest accumulated day-

degrees above 0 ˚C from 1 April to the end of September were selected. The eight years selected were: 1994/95, 1996/97, 1998/99, 2002/03, 2003/04, 2004/05, 2005/06, 2006/07. Seven of the eight years would have been selected too if the cumulative day-degrees would have been based on the period from October to the end of September. Each of the eight years was then selected three times and a randomised sequence of 24 weather years was generated from this pool. The aim of including these two climate scenarios is to examine whether under the ‘Heating up’ scenario different combinations of cultural control should be applied compared to the default ‘No change’ scenario.

Since the weed management scenarios were run under both climate scenarios, the question was whether the questions as posed for the weed management scenarios (see pg. 29) would be any different under the two climate scenarios.