ABSTRACT

Canola (Brassica napus L) is very responsive to nitrogen, often requiring more than cereals in most situations. Nitrogen is crucial for determining the yield and oil quality of canola. Accurate recommendations of N are difficult to determine because of the influence of seasonal, soil and management factors.

Most N fertilizer for canola production is applied as two applications. An initial amount at seeding (10 - 15 kg/ha) with the remainder being top dressed at between 4-6 weeks after seeding. Usually between 30-40 kg/ha of N is applied all up in the low rainfall (< 325 mm) area of the central eastern district of Western Australia.

The seed yield, oil and protein content response of canola (cv Karoo) to the application of nitrogen, applied as urea (46%) was measured in a farm scale demonstration in the low rainfall central eastern district of Western Australia. This demonstration showed that by doubling the amount of N applied at top dressing to 46 kg/ha, increased yields by 22% from the standard paddock practice of 23 kg/ha. Regression analyses were conducted showing significant effects (p<0.05) of N rate on increasing yield and seed protein but lowering oil content. However, overall gross margins were still improved by $59/ha giving a 2.5 to 3 fold benefit to profit for every $1.00 invested on N fertilizer top dressed.

KEYWORD

Canola, nitrogen, top dress, urea, yield, quality

INTRODUCTION

Canola is a very new crop in the low rainfall central eastern district of Western Australia. Most growers would have only two or three years experience with its agronomy. As such there is much to learn about the response of canola to nitrogenous fertiliser applied in this environment. Holmes (1980) noted that the oilseed rape crop has a high requirement for nitrogen, needing considerably more than is provided by most soils and a generous use of nitrogenous fertiliser is necessary for optimum yield. Rates of nitrogen are not defined with any degree of certainty and there are also many other factors that come into play.

There have been attempts to define nitrogen requirements for canola using cross references to wheat responses (Mason, 1996) in an effort to fast track advice in this area. However, this is mainly confined to districts of greater rainfall. Early work in south eastern Australia has shown that applied nitrogen will result in higher yield and returns from canola (Ramsey and Hillman, 1993). Recent research by Brennan, et al (1997) has found that as the protein content of the canola seed increases, the oil content was found to decrease.

Research in medium and high rainfall areas of the state has shown that seed yield responds positively to increased nitrogen (Mason and Brennan, 1997). Only in 1994 has there been similar work in low rainfall (<350 mm) areas dedicated to investigating the effects of application time and rates of nitrogen with canola (Brennan, 1998).

Given that canola production is increasing in the district, growers are seeking greater information as to the nitrogen requirement of their crop. There is the need to balance the increase in yield with minimal oil depreciation to maximise profits. While seasonal, soil and management factors do influence crop responses, on farm demonstrations are an important way of being able to conduct some simple research that is relevant to the local growers particular circumstances. Farm based demonstrations are of great value because they are conducted by the farmers using their equipment. This way the results are seen as producing a realistic outcomes that are achievable by the growers themselves.

METHOD

A uniform site in a canola paddock was selected prior to seeding. Plots were marked out as 200 m long with 18 m centres. The site was seeded with the variety Karoo as part of the paddock program. All plots were given the same seed and basal fertiliser rates as seeding went across all plots. Pre and post herbicide and insecticide treatments were applied similarly. The site was measured for plant establishment using the metre stick method (van Rees and Ridge, 1994).

At seven weeks after seeding, top dress applications of urea (46% N) were applied to selected plots. Urea was top dressed down the plot at a 9 m spread, such that there were two runs of the spreader to cover the plot. Three treatments of N as urea were applied. These were a nil, 23 kg/ha and 46 kg/ha of N. The 23 kg/ha treatment was also the standard paddock practice employed by the farmer. Each treatment was replicated in two blocks. Each fertiliser treatment was adjoining a nil treatment. In November the plots were swathed using a 9 m Honey-BeeÒ front modified for swathing, that placed the windrows in the centre of each plot. The swathes were left to mature for about 16 days before harvesting using the same 9 m Honey-BeeÒ front.

Seed yield was recorded using a portable weigh trailer and seed samples collected from each plot for oil and protein analysis. Oil content was determined at the local CBH office at Merredin, while protein was determined from N content by Kjeldahl digestion.

Linear regression analysis Genstat (1995) was conducted on the response of seed yield, oil content and seed protein in relation to increasing N fertiliser rate.

RESULTS

Seasonal rainfall for the site was much greater than in an average year (Table 1). This was particularly for the months of June and July. The season began early with the opening rain falling on the 16^th of April and the crop sown by the 18^th April.

Table 1. Monthly growing season rainfall for the demonstration site and the long-term mean monthly rainfall figures of the Bureau of Meteorology site for Muntadgin within close proximity.

Month	April	May	June	July	August	September	October	Total
Site	35	21	88	83	31	33	16	307
Average	24	44	54	53	44	25	20	264

Plant density was determined at 61.6 plants/ m² for the site. The months of June and July were particularly wet and the site did experience some water – logging. Severe frosting was also experienced late in the season on September the 19^th and 23^rd giving visible effects at maturity.

Increasing the application of N fertiliser in the form of urea resulted in significant (p<0.05) increases in both seed yield and protein and a decrease in oil content of the canola. The initial addition of 23 kg/ha of N gave an increase of 83 kg/ha of seed (Figure 1). This equates to a 12% increase or 3.6 kg of seed for every 1 kg of N applied. Adding a further 23 kg/ha of N resulted in a further 167 kg/ha of seed or 22% yield increase. That equates to 7.3 kg of seed for 1 kg of N

supplied. Overall, on average there was a 38% yield increase, making a 5.4 kg/ha increase in canola seed yield for every 1 kg/ha of N supplied as the N rate increased to 46 kg/ha.

Figure 1. Canola seed yield response with N fertiliser rate. Regression analysis gives % variance accounted for = 78.6, N (p=0.012).

Oil content of the seed fell with the increasing N fertiliser rate (Figure 2), while seed protein content increased (Figure 3). There were good negative relationships between the protein content in the seed and that of the oil content.

Figure 2. Seed oil content of canola with increasing N fertiliser rate. Regression analysis gives % variance accounted for = 86.2 and N (p=0.005).

Figure 3. Seed protein content of canola with increasing N fertiliser rate. Regression analysis gives % variance accounted for = 85.2 and N (p=0.006).

DISCUSSION

The results of this demonstration confirms earlier assumptions, such as increasing the supply of nitrogen to the crop almost always reduces the oil content of the seed (Holmes, 1980). To the farmer the chief aim is to grow the most profitable crop and to arrive at the optimum seed yield and oil content that is possible. The value of the canola crop is shown in figure 4 using the current 1998/99 harvest prices with discount and bonuses for oil.

Figure 4. Value of the canola crop with increasing N fertiliser rate.

On the basis of nitrogen application being worth $0.80 /kg (Agriculture W.A., 1998) and canola is about $0.40 /kg, then the break even response is about 2 kg/ha of canola to 1 kg/ha of N applied. For the initial 23 kg of N, with every $0.80 invested, $1.47 was returned. This was further enhanced with the application of the further 23 kg/ha of N. Here for every $0.80 invested, $2.84 was returned. This is more than a threefold return for every $1.00 invested in supplying N to the crop.

This would indicate that in years of high potential, that is those having an early start and likely chance of greater than average growing season rainfall, investing in higher than normal rates of N are more likely to pay off. This is even more so, in the absence of environmental constraints such as frost which were experienced at the site. It is also possible that during the season there may have been leaching of N through the soil due to the June and July rain. This may have affected the performance of the crop at the nil and 23 kg/ha treatments to a larger degree.

CONCLUSION

Canola is a crop that can give a large yield response to applied nitrogenous fertiliser. Farm based demonstrations are a way of being able to verify previous research in a practical way and are useful for assisting in developing agronomic packages for local groups of growers operating in a similar climatic and environmental area.

ACKNOWLEDGEMENTS

Thanks is given to Mr G.P. Riethmuller for his assistance with the linear regression analysis of the results.

REFERENCES

Agriculture W.A. 1998. Farm Budget Guide 1998. Published by Farm Weekly.

Brennan, R.F 1998. Time of nitrogen fertiliser application effects on yield and oil of canola. In Highlights of oilseeds research and development in Western Australia, 1998. Ed P. Carmody. Perth, Western Australia, 18 – 20 February, 1998. p 17 and 18.

Brennan, R.F., Mason, M.G. and Walton, G.H. 1997. The effect of nitrogen fertiliser application on the seed yield of canola (Brassica napus) and the contents of oil and protein. In 11^th Australian Research Assembly on Brassicas. Ed G.H. Walton. Perth, Western Australia, 6 - 10 October, 1997. p103 –109.

Genstat 1995. Genstat 5 release 3.2. Lawes Agricultural Trust. Rothamsted Experimental Station.

Holmes, M.R.T. 1980. Nutrition of the oilseed rape crop. Applied Science Publishers Ltd, London.

Mason, M.G. 1996. Nitrogen fertiliser rates for canola. Agriculture Western Australia, Farmnote 14/96.

Mason, M.G. and Brennan, R.F. 1997. A comparison of the nitrogen uptake and yield of canola (Brassica napus) and wheat following application of nitrogen fertiliser. In 11^th Australian Research Assembly on Brassicas. Ed G.H. Walton. Perth, Western Australia, 6 - 10 October, 1997. p 110 – 117.

Ramsey, B.R. and Hillman, M.L. 1993. Response of canola to applied phosphorus and nitrogen. In 9^th Australian Research Assembly on Brassicas. Eds N. Wratten and R.J. Mailer. Wagga Wagga, New South Wales, 5 – 7 October, 1993. p 29 – 32.

Van Rees, H and Ridge, P. 1994. Mey-check. The crop monitoring manual. Department of Conservation and Natural Resources. Bendigo, Victoria.