Poor yields of sucrose from sugarcane are often associated with low sucrose:cane ratios, particularly when climatic conditions tend to favor stem elongation rather than sucrose accumulation. Plant growth regulators may be used to modify environmental effects on the growth of sugarcane and thus alter the balance between utilization and storage of sucrose. The objectives of this study were to investigate the influence of temperature, soil moisture, and the coadministration of plant growth retardants on the response of sugarcane to treatment with gibberellic acid. Single foliar applications of GA (gibberellic acid) commercial sugarcane varieties (interspecific hybrids of Saccharum) stimulated stem elongation for from 2 to 6 weeks, but this response was not reflected in higher cane yields unless the plants were harvested within 6 weeks of treatment. A second application of GA at 4 weeks prolonged the growth response. In the field low temperatures were associated with the more prolonged growth responses, which were of smaller amplitude. Moisture stress had no apparent effect on GA response. No correlations were evident between environmental conditions or application rates and the duration of growth responses in greenhouse trials, but the amplitude increased with application rate within the range of 25 to 200 ppm. Three to five immature or semimature internodes were affected. In field trials under warm conditions a period of growth retardation followed the initial GA‐induced growth stimulation. This did not occur in greenhouse trials. Stem‐sugar concentrations fell during the growthstimulation phase, but recovered as growth rates reduced. Attempts were made to accentuate this recovery, and so to increase the accumulation of sugars, by coadministration of growth retardants with GA. Dalapon had a delayed effect on cane growth, slightly curtailing the duration of the GA response, and suppressing growth thereafter. The phase of growth suppression was accompanied by increased stem sugar concentration in greenhouse‐, but not in field‐trials. Sodium silicate (at up to 200 ppm) had no significant effect on growth or sugar accumulation and did not modify the response to GA. Treatment with azauracil (at up to 400 ppm) strongly inhibited cane growth and increased stem‐sugar concentrations, but showed no significant interaction with GA.

A large series of trials involving phosphate fertilizer showed that yields of cane could be reduced by the application of superphosphate at rates of up to 2 cwt/acre in four distinct areas. In these areas, the yield response to phosphate was dependent on the rate of nitrogen fertilization; yield depression only occurred where the nitrogen supply was adequate; phosphate could increase yields at low nitrogen. In most cases, the yield depression was associated with a low calcium/ magnesium (Ca/Mg) ratio in the soil (a ratio of less than 3.0 in terms of milliequivalents). On soils with high Ca/Mg ratios, regressions of yield response on soil or leaf phosphate indicated critical levels similar to those accepted elsewhere. Significant regressions could not be obtained from soils with low Ca/Mg ratios. A few trials testing a phosphate x trace element interaction indicated that the phosphate yield depression in at least one area was due to an induced deficiency of some trace element. The trace element effect is independent of the Ca/Mg ratio effect.

Abstract. The literature indicates that cane yield‐tonnes cane per hectare (tc ha−1)‐is directly related to actual evapotranspiration (Et) and that a ratio 1 tc ha−1 cm−1 Et should be a reasonable objective; this is frequently not attained in commercial practice. Analysis of extensive data from non‐irrigated cane in upland Kenya demonstrated water use efficiencies of 0.5 to 0.6 tc ha−1 cm−1 Et in relationships which were remarkably precise; some of the reasons for the failure to achieve higher efficiencies at this location are discussed. The practice of irrigation is likely to introduce additional complications and so reduce water use efficiencies even further; thus responses to irrigation should be measured experimentally before capital expenditure is approved. The main additional complications are the interactions between irrigation and soil type on rooting and growth, in particular the influence of soil type on yield responses; and the fact that advective energy interferes with the convenient, direct relationships between radiation and evaporation and yield.

Random cane samples were collected from a number of commercial fields at harvest time. The canes were analysed individually to calculate the sample size required to give values within 5% of the mean at a 5% level of probability. The desired sample size to estimate quality (sugar content) varied from 12 to 618 canes; for percent fibre from 42 to 93 canes; and for average weight per cane from 204 to 397 canes. In the estimation of quality, the very large sample sizes were required only in old (19‐20‐month) crops; the sample size required was related to the degree of deterioration. In 12‐month crops, the sample size was related to the degree of lodging, being greatest (an 81‐cane sample) in a very heavily lodged crop. The high proportion of low quality, deteriorated, and dead cane in old, heavily lodged crops is discussed in relation to mechanical harvesting. Individual cane deterioration is attributed to the lodging of old, non‐vigorous canes.

Eleven replicated irrigation trials on sugar-cane, together with some preliminary observation tests, were conducted in the Bundaberg region of southern Queensland during four seasons, 1961–62 to 1964–65. Irrigation control was through actual measurements of potential evapotranspiration of cane (Etc) or grass (Etg) with lysimeters. In conjunction with the lysimeter work, pan evaporation (Ep), mean air temperature (Ta), and sufficient data to calculate Penman's potential evaporation (E0) were recorded. Etc consistently exceeded Ep, and Etg slightly exceeded Ep in summer. Et was fairly well correlated with Ep and Ta, but less well correlated with E0. The inclusion of Ta in regressions of Et on Ep or E0 improved these regressions; Ta presumably reflects the additional external source of energy which was causing the high Et values. Yield responses to irrigation were very different on two soil types: on a deep red volcanic loam they were 10 times as great as those on a heavy alluvial clay. The reasons for the poor response on the alluvial clay appear to be threefold: (i) a permanently moist (but saline) subsoil maintained some growth in dry weather; (ii) growth in soils kept moist by irrigation virtually stopped when heavy rain fell; (iii) heavy rain falling on previously dry soil resulted in an abnormal increase in growth rate, an effect which was associated with a dramatic increase in soil mineral nitrogen. The most satisfactory indicator of water use by cane was Etg; growth rate data indicated that 100% Etc is required, but yield data suggested that 66% Etc is adequate. The latter value approximates to 100% Etg or 100% Ep in the summer months. Growth, and therefore growth response to irrigation, is very severely curtailed when mean temperatures fall below 70°F; soil temperature appears to be the critical factor. Growth rate was too susceptible to factors such as temperature to be of any value as an indicator of the need for irrigation.