Recent years have seen a substantial increase in durum wheat production in Australia. Durum wheat has excellent cooking qualities, which make it suited to the production of pasta. As a result, it attracts price premium over bread wheat. The development of new varieties with crown rot resistance has enabled durum wheat to be considered for production in western Victoria.
Durum requires a high protein content (13%) to ensure it delivers appropriate cooking qualities, so that it is important that crops be supplied with adequate nitrogen (N). For bread wheat, the later N is applied the more it is likely to raise grain protein. This being the case, practices that allow wheat crops to take up N late in the season are advantageous.
Fertiliser inputs represent a significant proportion of total variable costs in grain production, and when rain forecasts fail to eventuate after top-dressing, yield responses can be negligible. There are at least two possible reasons for this. Either the N applied is lost through volatilisation so that less N is available to the crop, or the N applied is not taken up because of the dry soil.
One of the most common questions asked by producers during the cropping season is: “How much N is lost when fertiliser, particularly urea, is applied and follow-up rain does not occur immediately after application?” Proponents of fluid N products, such as Urea Ammonium Nitrate (UAN), have suggested that liquid applications can reduce the potential for volatilisation. The perception is that a large portion of the N, when applied as UAN, is taken up by the crop leaves and that there is a reduced need for follow up rain.
Urea is prone to volatilisation because it is highly reactive with water. Before N from urea can become available for plant use, it must pass through a two stage reaction. The first stage of the process is the conversion of urea [(NH2)¬2CO], with the addition of water, to ammonium (NH4+) as ammonium carbonate. This is called urea hydrolysis. The second stage occurs when ammonium is in its plant available form. Depending on conditions, this ammonium can be converted to ammonia (NH3) which is a heavier than air gas, or to plant available nitrate (NO3-).
The rate of the first stage of the reaction depends on the presence of the soil enzyme urease and is usually quite rapid. The outcome from the second reaction depends on a number of factors. The production of ammonia, rather than ammonium or nitrate, is favoured by warmer temperatures, high soil pH (especially where carbonate is present), low moisture, low clay content and the presence of organic materials.
If the soil is dry, the second stage of the reaction will not progress, leaving the N in a gas form (ammonia) that is susceptible to volatilisation into the atmosphere.
The perception that UAN liquid fertiliser – a solution of urea (35%) and ammonium nitrate [NH4NO3] (45%) – can reduce volatilisation because some of the urea is quite rapidly taken into the plant, although the nitrate fraction of the UAN is not likely to be absorbed into the leaves, and the ammonium fraction is only slowly taken up.
When in the root-zone, both ammonium and nitrate are immediately available to the plant, but when applied to leaves, the urea is taken up 10 times more rapidly than the ionic N forms. Even so, only a relatively small amount can be taken up without causing leaf damage due to urea toxicity and salt damage. In addition, in some circumstances, not all of the liquid fertiliser will make contact with the crop leaves; some may pass through to the soil thus reducing the actual percentage of N immediately available to the crop to less than 50%. The density of the crop canopy also has a significant impact on the amount of N immediately available to a crop. In a dense canopy, some ammonia that is produced through the above soil reactions can be absorbed directly by the leaves.
TAKE HOME MESSAGES
- In this trial, contrary to expectation, the durum wheat achieved an average yield only 0.1t/ha lower than bread wheat (Scout).
- During 2014 there was a price differential of between -$83 and $277 for durum (DR1) over bread wheat (H1)
- The later N is applied in the season, the more it contributes to grain protein.
AIM
To assess liquid and granular N products and alternative application methods on crop N uptake in bread and durum wheat.
TRIAL DETAILS
Location: Horsham
Soil type: Clay no sub-soil constraints
Annual rainfall: 253mm
GSR (Apr-Oct): 210mm
Crop types: Scout (bread wheat) and UAD0951096 (durum)
Sowing date: 14 May
Seeding equipment: Knife points, press wheels, 30cm row spacing
Target plant density: Scout – 150 plants/m²; UAD0951096 –220 plants/m2
Harvest date: 29 November
Trial average yield: Scout – 2.5 t/ha; UAD0951096 – 2.4t/ha
TRIAL INPUTS
Fertiliser:
At sowing Granulock Supreme Z @ 55 kg/ha
20 June (GS15) Urea @ 90 kg/ha
19 September (GS45) See Table 1
Herbicide:
Pre-sowing Glyphosate @ 1.2 L/ha + Treflan @ 1.5 L/ha + Avadex @ 1.5 L/ha
In-crop Ammonium Sulfate @ 0.8 kg/100L + Precept 300 @ 1.5 L/ha + Lontrel Advance @ 70 ml/ha + Hasten 1% v/v
METHOD
A replicated field trial was established at BCG’s Horsham research site. The trial compared the impact of liquid and granular N products, and alternative application methods, on crop N uptake in bread and durum wheat. Scout bread wheat and durum (UAD0951096) was sown in two blocks on 14 May and treatments applied in a complete randomised block design. Soil samples to 1.3m were conducted pre-sowing (26 March) to measure soil water and available N. Trial assessments included a measure of greenness (NDVI, refer to ‘Appendix 1: acronyms and abbreviations’ pp 215) prior to, and after, treatment applications and yield and quality parameters. All plots received consistent agronomic management with full weed and insect control. The only differences were in the N treatments applied (Table 1).
Granular fertiliser treatments were applied using garden hand spreaders and liquid fertilisers using a hand-held 1.5m boom with either a streaming (TeeJet SJ-04-VP) or a standard nozzle (TeeJet AIXR110025). A modified hand-held boom with modified nozzles (single stream) and set to the crop inter-row was used for the inter-row treatments.
RESULTS AND INTERPRETATION
initial CONDITIONS AND CROP AVAILABLE n
Soil test results from samples taken in March showed only 6mm of plant available water (PAW) and 37kg N/ha, which is very low for both. Prior to the treatment applications, 5kg N/ha was applied at sowing, with a further 41kg N/ha top-dressed during tillering. Excluding mineralisation, the total available crop N prior to treatment applications was 84kg/ha. Assuming that a wheat crop requires 40kg N/ha to produce one tonne of grain, this was sufficient N to produce slightly over 2t/ha.
seasonal CONDITIONS
Rainfall at the site from 26 March (soil sampling date) until late July was tracking close to Decile 5 (Figure 1). However, after 1 August very little rainfall (38.8mm) was received, and at the end of the growing season the total recorded was at Decile 1 (Figure 1). The spring rainfall at the site was the third lowest on record (114 years). The 2014 season produced the worst possible outcome for managing seasonal risk. Conditions were favourable early, followed by one of the worst finishes on record. This lack of rainfall in spring was also unsui ted to late applications of N.
CROP BIOMASS
There was no difference between treatments in the level of green cover (NDVI, a substitute for crop biomass) conducted prior to the treatment applications (19 September). From this it can be concluded that all treatments prior to the first treatment had the same level of biomass and that any differences observed from this point forward were the result of treatment effects. A repeat NDVI conducted on 21 October, after treatments had been applied, again revealed no treatment differences. This is probably due to the dry conditions reducing the greenness of the crop for all treatments, irrespective of the inputs applied.
TREATMENT APPLICATION AND SUBSEQUENT CONDITIONS
The treatment applications were conducted on 19 September when the crop was in mid to late booting. The total rainfall and average maximum temperatures for the 10 days following application were 4.4mm and 22.1°C, respectively. With such a small amount of rain and the relatively high maximum daily temperatures over the 10 days, the potential for loss through volatilisation was considerable. In similar work, conducted in a laboratory, using incubations with urea in a Kalkee clay loam, urea was completely hydrolysed by Day 7 at 25°C, by Day 10 at 15°C, and by Day 15 at 5°C. (Suter et. al., 2011).
YIELD, QUALITY AND GRAIN N RESULTS
Neither yield nor quality effects were realised in response to product type or application method in either of the durum or bread wheat trials, with the exception of protein and test weight in the durum trial (Tables 2 and 3). In the durum trial, the test weights of some treatments were higher than others, but all treatments achieved values well in excess of the threshold of 76kg/hl.
In the durum trial, the protein content of grain was higher in treatments in which Urea Solution (US) and UAN were applied with a standard nozzle and were the same as the control. At first glance, these results would suggest an effect of leaf uptake through the applications through standard nozzles. However, the control, which had no additional N, yielded the same increase in protein. Given that these differences did not carry through to the total grain N, it is likely that there was a dilution effect occurring in which higher protein was correlated with a slightly lower yield.
Total average grain N was calculated by multiplying yield by the protein percentage and applying a multiplier ([protein %]/5.7) (Table 2 and 3). With no effect on total grain N in either trial, it is apparent that none of the treatment applications generated any increase in N accumulation in the grain. This is not unsurprising given the dry finish. However, it also suggests that even if leaf uptake from foliar applications had occurred, it did not lead to increases in grain N.
While the design of this trial does not enable direct comparison of the bread and durum wheat, it is interesting to note that the average yield and quality results are not vastly different (Table 4). Previous experience suggests that durum wheats are lower yielding than bread wheats, but in this trial the yields were similar.
COMMERCIAL PRACTICE
In this trial the durum and bread wheats, which received identical management, produced similar yields. There was no indication from the results that the durum or bread wheats had different responses to N application methods. This may be a consequence of the very dry finish to the season.
Some of the key things growers should consider in the production of durum wheat include
- agronomic management for durums. In 2014, the GRDC produced a factsheet (Durum quality and agronomy), which outlined tips for achieving higher durum quality and yields.
- the availability of seed. Seed can be purchased from a number of seed suppliers.
- marketing. There are limited receival sites that accept durum. Growers should ensure they have a market for their durum prior to production and that any extra delivery cost is taken into consideration.
In Australia, durum wheat production has traditionally been dominated by New South Wales and South Australia. The South Australian Durum Growers Association is an organisation with significant experience in durum production and is a reliable resource for those seeking more information.
ON-FARM PROFITABILITY
The motivation for growers to grow durum wheat lies in the premium that it attracts. For much of 2014, durum (DR1) received a premium over bread wheat (H1) of up to $280/t (Figure 2). Had the grain been sold at harvest in 2014 growers, would have realised a significant advantage (~$200) over bread wheat. This is an exceptional premium and could not be expected every year. That said, it does highlight that growers in Western Victoria should consider growing durum, with the added attraction of diversification benefits.
ACKNOWLEDGEMENTS
This trial was funded by BCG members through their membership, in conjunction with Agritech Rural, Horsham.
