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1 E Irrigation 1 IRRIGATION PRACTICES IN THE QUEENSLAND SUGAR INDUSTRY K. C. Leverington Bureau of Sugar Experiment Stations Brisbane, Australia ABSTRACT The systems of irrigation used in the Queensland sugar industry are out- lined, and current research programmes are mentioned. It is pointed out that stability of production in areas of variable rainfall is a major factor con- tributing to the expansion of irrigation in Queensland. Water usage has fre- quently been wasteful in the past, but due to diminishing reserves of surface and underground water farmers are becoming more aware of the necessity for increasing irrigation efficiency. INTRODUCTION The production of sugar in Queensland is highly organized and coil- trolled, both at the farm and mill levels, by quotas which are renewed from time to time according to requirements for home consumption and export mar- kets. While there is no guarantee that cane grown in excess of an individual farm quota will be accepted for milling, frequently due to shortfalIs, over quota cane is milled.

2 The value of this cane is based on the price of sugar exported at world market prices and is much lower than for cane supplied within the quota. Consequently there is little incentive for farmers to produce large amounts ol cane in excess of their quotas. However, as the periodic review of farm quotas takes into account the farm's productivity, some cane growers are prepared to produce cane well' in excess of their quotas for comparatively little immediate return, but with the hope of an increased quota allocation in the future. On the other hand, where farms consistently fail to meet quota requirements, quotas may be reduced. Traditionally, the Queensland sugar industry has aimed to produce erect crops of cane. Although modern harvesting machines can handle recumbent crops efficiently, the harvesting of lodged cane inevitably results in tops, trash and other extraneous matter being supplied to the mill. Penalties for the inclu- sion of such materials have discouraged a philosophy of cane growing based on maximizing yields of canC to the detriment of milling operations.

3 The average farm size in the Queensland industry is about 90 acres, and on the majority of farms, with the exception of major operations such as plant- ing and harvesting, all of the management and labour requirements are carried out by the farmer himself. As there is no pool of labour which must be kept employed as in the case with large plantations, it is obvious that the average Queensland farmer will have a very different attitude towards labour costs from producers in most other sugar industries. These factors have an important bearing on the approach to cane growing 899 900 IRRIGATION in this country and the review of the irrigation practices detailed in this paper should be viewed against a background of these local influences. CLIMATE The industry, which extends over some 900 miles of the east coast of Queensland, was originally based on rain-grown crops; and, while the northern part of the cane-growing belt has a fairly reliable rainfall pattern, the Lower Burdekin, Mackay and Bundaberg districts, which grow more than half of Aus- tralia's sugarcane, have a variability of annual rainfall which is considered high by world standards.

4 The relative variability, , mean deviation percent of average rainfall, ranges from 25% at Bundaberg to 30% in the Lower Burdekin. Most of the irrigated cane in the State is grown in these 3 areas. While virtually all farms in the Lower Burdekin have been irrigated since before 1940, the areas under irrigation in Bundaberg and Mackay have been increasing rapidly in the past 15 years. The climatic data for these districts are shown in Table 1. Table 1. Climatic data for irrigated cane areas. Lower Burdekin Mackay Bundaberg Annual rainfall (in.) 39 69 41 Annual evaporation (in.) 82 78 65 Av max temp (OF) 84 8 1 80 Av min temp (OF) 64 6 1 60 Mean temp (OF) 74 7 1 70 I It should also be noted that in the Mackay and Burdekin districts some 80% of the annual rainfall is received during the wet season months of Dec-April. In Bundaberg the corresponding figure is 68%. As the wet season corresponds with the major growth period of the cane, failure of the wet season to produce reasonable falls of rain results in severe growth restrictions on non-irrigated farms.

5 DISTRICTS The Lower Burdekin District This is a flat area of alluvial soils of the levee bank and delta formation of the Burdekin River, a river with a large catchment area but which flows for only a few months of each year. Although the soils range, texturally, from sandy loams to clay loams, there is a preponderance of silty soils. All are very rich in available phosphorus and potassium, and nitrogen is the only nutrient required as fertilizer. The area is underlain by excellent aquifers carrying gen- erally fair to good quality water at shallow depths. The recovery of water from these aquifers is comparatively cheap and until recent drought years, when exces- sive demands on the water supply caused serious drops in the levels of the under- ground supplies, little thought was given to the efficient use of irrigation water in the area. Following the planting of cane in furrows, about 8 in. deep, during the I &. C. LEVERINGTON March-April period, it is the practice of many of the farmers to irrigate immedi- ately to assist in germination.

6 During the winter and early spring months up to 3 more waterings may be given by furrow irrigation down the row. The soils slump and compact with watering, and after each irrigation the soil in the inter- spaces and adjacent to the stools of cane is cultivated with discs and spring tyned implements to loosen the soil and control weed growth. When rapid growth of the crop commences about Oct, soil is thrown into the rows to form hills about 6-7 in. high leaving a flat area some 25 in. wide in the centre of the interspaces. ~11 later waterings are made down the interspace and, as the canopy is fairly well developed at this stage, it is not usual to cultivate the crop again. In ratooning the crops, the hills are cut down with a stubble shaver or rotary hoe and the field watered down the interspaces. Water frequently breaks through the remnants of the hills, and the field is virtually flooded. The soil of the interspaces is broken up to a depth of about 6 in. with tyned implements and on some farms, where water penetration is a problem, ripping to a depth of 18 in.

7 In the centre of the interspace is also carried out. As the ratoon cane grows, soil is again thrown into the rows to smother weeds as the interspaces are cultivated. By the time cane has grown sufficiently to prevent the access of equipment, about 3 irrigations will have been made and the hills reached their full height. On the majority of farms 10-12 irrigations are applied to each crop, both plant and ratoon, although according to soil type and the size of the crop the farmer is aiming to produce, 7-14 waterings may be given. Very few farmers in the area have any idea of the amount of water applied, and from the limited amount of reliable data available the rates appear to range from 4-10 , with mean and median values of about 7 in. The sur- face 3 ft of these soils hold a total of about in. of water in the available range, and it is obvious that irrigation efficiency is comparatively low. On more than half of the farms, row lengths are in excess of 1,000 feet; while shorter rows may bring about more efficient water application, the labour requirements would undoubtedly rise.

8 The falling of the levels of water in the aquifers in recent years has brought to farmers the realization that the water supply is not unlimited, and a number of techniques to improve water distribution are becoming widely used (Freshwater, 1970). Pipes of various materials, both underground and on the surface, have replaced earth ditches for the distribution of water around the farms, while gated pipes of aluminium, galvanized iron, rigid PVC, or flexible polythene are being used to control the flow of water into the rows and inter- spaces. In general, farmers in the area do not attempt to maximize production as they are conscious of the difficulties which may arise when very large crops are grown. Strong winds combined with wet soil conditions cause widespread lodg- ing, and this results in a lower sugar content of the cane as well as harvesting problems. Some farmers go as far as to allow the crops to reach fairly high levels of moisture stress in the early summer months in order to limit growth and reduce the likelihood or lodging during the wet season.

9 As the climatic condi- tions of the area favour high levels of sugar accumulation, drying off techniques are not necessary to achieve ripening. However, in the cool weather of winter and early spring just prior to harvest, watering is restricted so that the fields will 902 IRRIGATION be sufficiently dry to permit harvesting machinery to operate satisfactorily. Pro- longed withholding of water from mature cane at this stage must be avoided as excessive moisture stress causes deterioration of the crop to occur and a fall in the sugar content results. One feature of cane production in the Lower Burdekin was the preference, in the past, for growing only plant cane. In recent years, however, ratooning is being practised more widely, the average number of ratoon crops being This compares with an industry mean of ratoon crops. Ratoons in the ~urdekin produce less cane than the plant crops, probably due to soil compaction and water penetration problems. Irrigation research in the area has been limited to lysimeter studies of potential evapotranspiration.

10 The results, have shown that in a peroid 01 12 months it is possible to produce over 90 tons of cane and tons of recoverable sugarlacre. Between and tons of cane are obtained for each inch of evapo- transpiration. These yields are approximately double the present average com- mercial production for the area. The Mackay District A number of very dry years during the past decade has stimulated the use of irrigation to supplement rainfall. in the central district around Mackay; although the area under cane increased by only 50-60yo during that period, the area commanded by irrigation increased more than 12 times, from 2,000 acres in 1960 to over 25,000 acres in 1969. This area currently represents about llyo of the land under sugar in the Mackay district. A survey of irrigation practices in the area indicated that only 15y0 of farmers who are irrigating do so with the aim of maximizing production; the remainder are mainly interested in stabilizing production to prevent the wide fluctuations in yields which result from varying rainfall patterns, Irrigation is regarded by most growers as only supplementary.