ALCANCE: Trabajos de ACERCON en Perú INSTRUCTIVO / PROCEDIMIENTO
MATRIZ DE REQUISITOS PARA CALIFICAR COMO AUDITOR INTERNO
Several management factors are important in growing cover crops, including sowing rate, duration of growth, method of termination, residue handling and plant-back delay (i.e. the period between cover crop incorporation and planting of next crop). The effects of sowing rate and plant-back delay are of particular interest to growers aiming to maximise weed suppression during the cover crop phase, while also minimising any potential crop suppression in the subsequent cash crop phase.
The recently developed high-glucosinolate (GSL) brassica varieties have been anecdotally reported to suppress subsequent crop and weed growth in a range of farming systems in eastern Australia including wheat in Tamworth, New South Wales (J. Holland, pers. comm.) and sugar cane in northern Queensland (J. Kirkegaard, pers. comm.). However, these effects do not appear to have been tested nor the results published. While some recommendations exist on sowing rates for the newer cover crop varieties, these recommendations are based on results from trials on conventional farms where agronomic factors such as sowing rate, fertiliser input and weed control are likely to be managed differently compared with organic farms (Clark et al. 1998, Mäder et al. 2002) and where weed flora would be expected to be different to an organic farm (Bàrberi et al. 1998a, Hald 1999). The optimum timing of plant-back delay for the new brassica cover crop varieties is uncertain and is likely to depend on factors such as environmental conditions, farming system and plant species, i.e. cover crops, weeds and cash crops.
In general, a high density of cover crop plants with abundant biomass production is desirable for effective weed suppression (Nelson et al. 1991, Sustainable Agriculture Network 1998), although excessively high sowing rates may lead to spindly growth, greater disease incidence and/or lodging in some cover crop varieties (K. Light, pers. comm., Leach et al. 1999). The literature on brassica cover crops contains widely differing suggested sowing rates. Rates from about 6 kg/ha (Krishnan
et al. 1998) to 100 kg/ha (Gubbels and Kenaschuk 1989), and even 896 kg/ha (Vera et al. 1987) have been used in field trials for mustard and radish, depending on variety, although the most commonly recommended range is about 10 – 30 kg/ha (Gardner and Morgan 1993, Hafez et al.
1995, Stivers-Young 1998, Light 1999).
The lack of residual effects by the cover crops in the previous chapter was in contrast to some reports in the research literature of significant weed and/or crop suppression following a brassica cover crop (Boydston and Hang 1995, Al-Khatib et al. 1997, Krishnan et al. 1998). It is possible that the 4 week delay between cover crop incorporation and lettuce planting was too long for suppressive effects to be observed. Therefore, the plant-back delay treatments were included in the trials reported in this chapter to determine if the Indian mustard and fodder radish cover crops would suppress weeds or crops when a subsequent lettuce crop was planted immediately after incorporation of the cover crops. Various recommendations have been suggested for plant-back delay, ranging from 2 to 6 weeks (DPIWE 1996, Long 2000). Published field experiments using brassica cover crops have used plant-back delays from no delay, i.e. next crop planted within a day or two of incorporating a cover crop (Vera et al. 1987, Krishnan et al. 1998), up to about 6 weeks delay (Boydston and Hang 1995). In glasshouse trials where brassica residues have been incorporated into soils and a crop or weeds grown, there is usually no delay between incorporation and sowing or planting (Boydston and Hang 1995, Al-Khatib et al. 1997, Krishnan et al. 1998), although fresh residues are not always used (Mason-Sedun et al. 1986, Mason-Sedun and Jessop 1988).
Numerous researchers have highlighted the limitations of using bioassays in controlled environments to evaluate the allelopathic effects of crop residues, extracts or synthetic compounds, with considerable attention placed on separating allelopathy and resource competition (Harper 1977, Willis 1985, Williamson 1990, Inderjit et al. 2001). In order to address these issues, two
additional factors were used in the glasshouse experiment reported in this chapter. They were fertilisation level and cover crop residue removal.
The first group of experiments reported in this chapter was a series of field trials designed to assess the effect of (a) brassica cover crop sowing rate on weed suppression during the pre-crop phase and in a subsequent vegetable crop, and (b) the timing of plant-back of a vegetable crop after incorporating the cover crop. The second group of experiments was a series of glasshouse trials that tested cover crop sowing rate and timing of plant-back in a controlled environment. The glasshouse trials were designed to determine (a) the effect of sowing rate on cover crop biomass production and subsequent lettuce emergence and growth, (b) if fertiliser affected cover crop and subsequent lettuce growth, (c) if sowing lettuce immediately after cover crop incorporation was inhibitory to crop emergence and growth, and (d) if removing cover crop residue prior to sowing lettuce modifies the level of inhibition.
6.2 Methods
6.2.1 Experiment One: Cover crop sowing rate and plant-back delay in the field Field site descriptions
Details concerning the geographic location, recent land use history, soil types and climate during the research period are provided in the previous chapter. For each season, cumulative rainfall and degree day estimates were calculated using the methods outlined in Chapter 4. A summary of the cumulative rainfall and accumulated degree days during the pre-crop phase (i.e. fallows and cover crops) for experiment one is given in Figure 6.1. This figure also shows the cumulative rainfall plus 30 mm supplementary irrigation applied in 2001 to assist the germination of the cover crops. During the pre-crop phase, the cumulative rainfall was significantly greater in 2000 than the natural rainfall in 2001 (P = 0.006) and the natural rainfall plus irrigation (P = 0.019). There was no significant difference in accumulated degree days observed between the seasons (P = 0.997).
A B 0 50 100 150 200 250 0 2 4 6 8 10
W eeks after sowing cover crops
C u m u la ti ve r a in fa ll ( m m ) 20002001 2001 + irrigation 0 100 200 300 400 500 600 700 0 2 4 6 8 10
W eeks after sowing cover crops
D egr ee day s ( °Cd) 2000 2001
Figure 6.1 Cumulative rainfall (mm, graph A) and accumulated degree days (°Cd, graph B) during the pre- crop phase at Laureldale and Kirby in 1999 – 2000 (S) and at Kirby in 2000 – 2001 (). The dashed line in graph A shows the cumulative rainfall plus 30 mm supplementary irrigation in 2001.
Cumulative rainfall and accumulated degree days for the in-crop phase (i.e. lettuce) is presented in Figure 6.2. During this phase, the cumulative rainfall was similar up to about 3 WAP. In 2001, approximately 146 mm of rainfall was received in the 4th WAP of the delayed plant-back treatment, and the 8 WAP in the no delay plant-back treatment in 2001. The delayed treatment in 2001 was significantly higher than the other trials (P≤ 0.045), and the other two trials had similar
rainfall overall (P = 0.309). The differences in accumulated degree days were not significant (P≤ 0.926), although the period of the “no delay” treatment was slightly warmer in the period after lettuce harvest. A B 0 50 100 150 200 250 0 2 4 6 8 10
W eeks after planting lettuce
C u m u la ti ve r a in fa ll ( m m ) 2000 2001 2001 (nodelay) 0 200 400 600 800 1000 1200 0 2 4 6 8 10
W eeks after planting lettuce
D e gr ee day s ( °Cd) 2000 2001 2001 (no delay)
Figure 6.2 Cumulative rainfall (mm) and accumulated degree days (°Cd) during in-crop phase at Laureldale and Kirby in 1999 – 2000 (S), at Kirby in 2000 – 2001 () and at Kirby in 2000 – 2001 for the "no delay" treatment in experiment one (z). The closed symbols (S, , z) represent measurements taken up to harvest time, while the open symbols (U, , {) represent measurements after the lettuce was harvested, but while bolting was still being recorded for the remaining plants.
Preparation, maintenance and assessment of the field plots
Details about methods used in land preparation, plant species and materials used, sowing, management and incorporation of the cover crops, planting and management of the lettuces, and assessments used are given in the previous chapter.
A completely randomised design was used, with three replicates for each treatment randomly allocated to the field plots. Three replicates were used rather than four due to the larger number of sowing rate treatments used. In the first season (1999 – 2000), the pre-crop treatments were established on 28/9/99 and all plots were ploughed in on 8/12/99, and the lettuce seedlings were planted on 7/1/00 and harvested on 26/2/00. In the second season (2000 – 2001), the pre-crop treatments began on 19/9/00 and were ploughed in on 6/12/00, whilst the lettuce seedlings were either planted on 8/12/00 and harvested on 24/1/01 (i.e. no delay treatment), or planted on 8/1/01 and harvested on 28/2/01 (i.e. delayed treatment).
Treatments applied
The first treatment factor, cover crop type, consisted of the two brassica cover crops, Indian mustard (MU) and fodder radish (RA) as used in the previous chapter. The second treatment factor, sowing rate, consisted of sowing the cover crops at 0, 12.5, 25, 50, 100 or 200% of the recommended sowing rate at Laureldale and Kirby in 2000 and at 0, 50 or 100% rates at Kirby in 2001. The lower rates were used in order to evaluate the lower limits of possible sowing rates in regard to weed control. The sowing rates recommended by the seed suppliers were 10 kg/ha (~375 seeds/m2) for mustard (MU) and 20 kg/ha (~186 seeds/m2) for radish (RA). The actual rates used were 0, 1.25, 2.5, 5, 10 and 20 kg/ha for mustard (MU) and 0, 2.5, 5, 10, 20 and 40 kg/ha for radish (RA) in 2000, and 0, 5 and 10 kg/ha for MU and 0, 10 and 20 kg/ha for RA in 2001.
The third treatment factor, plant-back delay (Figure 6.3), was used at Kirby in 2001 and consisted of planting lettuces either (a) shortly after incorporating the cover crops and unweeded control ("no delay" treatment), or (b) 4 weeks after incorporation ("delayed" treatment). In the delayed treatment, the plots were not rotary hoed at 2 WAI as they had been in the previous year. Table 6.1 shows the treatment combinations used in each field trial.
Figure 6.3 Measuring weed density amongst lettuce seedlings planted 4 weeks after cover crop incorporation (“delayed” treatment) at Kirby in 2001. Lettuces in the plot in the foreground were planted 4 weeks earlier (“no delay” treatment).
Table 6.1 Cover crop sowing rates for mustard and radish, and plant-back delay (period between cover crop incorporation and planting following crop) treatments used in experiment one.
Cover crop type and sowing rate (% of recommended rate)
Mustard Radish Trial Plant-back delay (weeks) 0 12.5 25 50 100 200 0 12.5 25 50 100 200 Laureldale 2000 4 9 9 9 9 9 9 9 9 9 9 9 9 Kirby 2000 4 9 9 9 9 9 9 9 9 9 9 9 9 0 9 9 9 9 9 9 Kirby 2001 4 9 9 9 9 9 9
6.2.2 Experiment Two: Cover crops in the glasshouse
A pair of experiments was conducted under controlled environment conditions in 1999 – 2000 and 2000 – 2001 to assess the effect of (a) cover crop sowing rate and (b) plant-back delay. The glasshouse facility was located at the main campus of the UNE, Armidale. The glasshouse daily maximum temperature was maintained at 25 ± 3°C and the minimum temperature was maintained at 15 ± 3°C during the experiments. In both trials, a completely randomised design with 4 replicates was used. Randomisation of the pot layout was achieved using the method described in previous chapters. The first year’s trials will be referred to as 2000, and the second year’s trials as 2001, corresponding with the year in which the trials were completed.
Treatments
In 2000, two experimental factors were tested: sowing rate and plant-back delay. The cover crops were sown at the rates of 1, 2, 4, 8, 12 and 16 plants/pot, and a control treatment was also used (0 plants/pot). The plant-back delay treatments consisted of either sowing lettuce seeds on the same day that the cover crop phase was terminated ("no delay") or about 2 weeks after termination ("delayed").
In 2001, only one sowing rate was used (i.e. 4 plants/pot) for each cover crop, and a control treatment with 0 plants/pot was included. The delayed plant-back treatment was changed to 4 weeks for consistency with the delay period used in the field trials. Two additional factors were also used:
* a fertiliser treatment, consisting of fertilising (+FERT) or not fertilising (-FERT) the cover crop, and
* a residue treatment, consisting of incorporating the cover crop residues prior to sowing lettuce (+RES) or removing the residue and using the unamended potting soil from the cover crop phase (-RES).
Cover crop phase
The MU and RA cover crops used in the field trials above were used in the glasshouse experiments (Figure 6.4). In 2000, the cover crops were sown on 15/10/99 and incorporated on 9/12/99, while in 2001, the cover crops were sown on 17/10/00 and incorporated on 12/12/00. The cover crops were sown in black plastic pots (200 mm diameter × 200 mm high) at double the required sowing rate and thinned at 5 days after sowing. The potting medium was a sandy loam collected from the paddock at Kirby Research Station near where the field trials where conducted.
Figure 6.4 Layout of cover crop glasshouse trial, 12/11/99. Indian mustard and fodder radish were sown at 0, 1, 2, 4, 8, 12 and 16 plants/pot in 200 mm diameter pots.
Each pot in the +FERT treatment was fertilised with 0.45 g of gypsum (~0.06 g SO4) prior to sowing cover crops, and with 400 ml of a general liquid fertiliser (Maxicrop®, 4.6% nitrogen, 1.2% phosphorus, 3.1% potassium) diluted to 3 ml/L at 1, 2 and 3 WAS. Pots in the -FERT treatment received no fertiliser. Any emerged weeds were removed by hand at regular inspections every three days. All pots were placed on 240 mm diameter plastic saucers and irrigated by filling the saucers with tap water every 3 days, except when the saucer was already wet, e.g. in the control treatment. At 8 WAS, the cover crop phase was terminated. The plants and soil in each pot were removed, and the plants coarsely chopped and incorporated thoroughly and evenly in the soil. The amended soil was placed into two 140 mm diameter black plastic pots for use in the lettuce phase. Lettuce phase
The test plant used to evaluate the effects of the treatments in the cover crop phase was lettuce cv. Imperial Triumph, the same variety used in the field trials reported in this and the previous chapter.
Using the 140 mm diameter pots with soil (amended and not amended) from the cover crop phase, 20 lettuce seeds were sown at a depth of 5 mm per pot. Seeds were used as an alternative to transplants in order to test the effect of the treatments on lettuce emergence as well as growth. Each pot was fertilised with 400ml of Maxicrop®, diluted to 3 ml/L at 1 and 3 WAS, and the plants were irrigated every 3 days, as for the cover crops. The lettuces were grown for 5 weeks, and were weeded regularly during that time. In 2000, the lettuces were either sown on 9/12/99 and harvested on 13/1/00 ("no delay" treatment) or sown on 23/12/99 and harvested on 27/1/00 (2 week "delayed" treatment). In 2001, the lettuces were either sown on 12/12/00 and harvested on 16/1/01 ("no delay" treatment) or sown on 9/1/01 and harvested on 13/2/01 (4 week "delayed" treatment).
Assessment
The cover crops were assessed by measuring height (from soil surface to tip of tallest shoot) and fresh weight biomass of whole plants at 8 WAS the cover crops. The final biomass was determined by carefully removing excess soil from around the plants, washing the plants to remove all soil and residues, and weighing. Dry weights were not recorded as the cover crops were incorporated back into the soil for growing the lettuces. Lettuces were assessed at 5 WAS by counting the number of live plants and measuring height (from base of plant to tip of longest leaf) and dry weight biomass. The biomass was determined by carefully removing the plants, washing the plants, oven drying for 48 hours at 80°C and weighing.
6.2.3 Statistical analysis
Statistical analyses of the data were carried out using the procedures detailed in Chapter 4. In most cases, GLMs were used for variables measured at a single time point and GLMMs were used to test the effects of treatments on variables that were measured several times during the course of the trials. Contrast analysis was used to separate the means where significant effects were detected. The non-linear least squares regression function in S-Plus 2000® was used to fit data to several response curves (MathSoft 1999a). The suitability of the models was assessed by inspecting the plot of residuals versus fitted values. When the residual variance was not homogeneous, the generalized non-linear least squares regression function was used, with a power or exponential variance function being included in the model to account for the non-normal distribution of the data (MathSoft 1999c). The variability of the data was presented in graphs by using confidence limits with a probability level of 95%.
Cover crop and relative weed cover were modelled using the logistic function
)
exp(
1
m
WAS
a
RC
−
+
=
Equation 6.1where RC is the relative cover (%), WAS is the time variable, a is the asymptote of maximum relative cover, and m is the inflexion point of the curve (i.e. the time at which RC is 50% of the asymptote).
In experiment one, the effect of cover crop sowing rate on cover crop relative cover was modelled using non-linear least squares regression. In particular, the rectangular hyperbolic Michaelis- Menten equation (Thornley and Johnson 1990, MathSoft 1999a)
Rate
m
Rate
a
RC
+
=
*
Equation 6.2was used, where Rate is the sowing rate of the cover crop (% of recommended sowing rate), a is the upper asymptote and m is the Rate at which RC is 50% of the asymptote. An inverted form of Equation 6.2 was used to describe the effects of cover crop sowing rate on relative weed cover,
Rate
m
Rate
a
RC
+
−
−
=100
(100
)*
Equation 6.3where a becomes the lower asymptote.
Light transmittance in response to cover crop sowing rate for the six trial × cover crop type combinations (three trials with two cover crops each) was also modelled using the inverted rectangular hyperbolic function (Equation 6.3). Preliminary tests indicated that the alternative functions (exponential decay function [Equation 5.3] and sigmoidal function [Equation 5.4]) were less suitable compared with the hyperbolic function. The latter had significantly lower residuals sums of squares for all but one treatment. The anomalous response, MU at Kirby in 2000, fitted Equations 6.3 and 5.3 similarly well (P = 0.214), but Equation 5.4 provided a less adequate fit (P < 0.001).
6.3 Results
6.3.1 Experiment One: Cover crop sowing rate and plant-back delay in the field A visual evaluation of the weed and cover crop data suggested a possible spatial trend across the plots that was probably not related to the treatments. Therefore, GLMMs were used to analyse all data in this experiment, with the effects of the field plot columns and rows included as random terms in the analysis (Butler et al. 2000).
Cover crops
The density of the cover crops immediately before incorporation in each of the trials is presented in Figure 6.5. A test of the treatment effects using GLMMs indicated that density varied by trial, sowing rate and cover crop type, as well as all interaction terms (P≤ 0.003), except the rate × type interaction (P = 0.807). The correlation between cover crop sowing rate and final density was high for all trial × cover crop type combinations (r2≥ 0.81). The MU cover crop had higher densities than RA, and the slope coefficient was lowest for Laureldale and highest for Kirby in 2000 and 2001.
y = 6.6 + (0.41x) 0 50 100 150 200 250 Laureldale 2000Mustard y = -2.56 + (0.88x) Kirby 2000Mustard y = 1.64 + (0.71x) Kirby 2001Mustard y = -1.26 + (0.3x) 0 50 100 150 200 250 Laureldale 2000Radish 0 50 100 150 200 y = -0.57 + (0.48x) Kirby 2000Radish 0 50 100 150 200 y = -1.76 + (0.43x) Kirby 2001Radish 0 50 100 150 200
Cover crop sowing rate (% of recommended rate)
Cover crop density (plants/m²)
Figure 6.5 Cover crop density (plants/m2) in response to sowing rate (% of recommended rate) for mustard
and radish immediately prior to incorporation at Laureldale and Kirby in 2000 and Kirby in 2001. The circles show the data points, the solid lines are the linear regression curves, the dashed lines are the 95% confidence limits, and the equations describe the regression for each treatment combination.
The seedling survival of MU and RA from sowing to incorporation at 10 WAS was calculated from final cover crop density as a percentage of the initial sowing rate (Table 6.2). The survival of the