No abstract

Water stress at anthesis is the major cause of yield reduction or crop failure in grain sorghum [Sorghum bicolor (L.) Moench] in central Queensland. Rainfall is difficult to predict and it is impractical to substantially alter the timing and amount of water stored in the soil, so we focussed on whether crop ontogeny could be managed, ultimately giving farmers some capability to align anthesis with in-crop rain. It is widely considered that a signal, transported from the leaf to the shoot apical meristem, is integral to the onset of panicle initiation and reproductive development. We hypothesised that modifying the leaves may interrupt the signal and cause a delay in the onset of reproductive development. Delays in sorghum anthesis associated with leaf modification treatments applied before panicle initiation were found to be a consequence of delays in panicle initiation. The longest delays in panicle initiation were obtained by twice-weekly defoliation above the second ligule (15–45 days); delays were shorter when plants were defoliated above the third ligule (10–41 days) or when only the fully exposed leaves were removed (0–13 days), depending on genotype. Although panicle initiation was delayed, leaf initiation continued, so extra leaves were produced. Defoliation of fully irrigated plants, however, generally reduced green leaf area, plant dry weight at anthesis, and grain yield, all by 30–50%. The application of ethephon also delayed anthesis, and changed the pattern but not the area of leaf produced, and did not alter grain yield. In rain-fed agriculture, where grain yields are frequently <50% of irrigated controls, delaying panicle initiation by 2 weeks may provide a better rainfall environment during which anthesis and grain-filling will occur. Reductions in green leaf area, although reducing yield potential, may promote a more balanced use of water between vegetative and grain growth. There was sufficient evidence to indicate that defoliation before panicle initiation could provide simple post-sowing management to achieve this scenario.

Crop residues are an important source of nitrogen (N) for rice (Oryza sativa L.). The objective of this research was to determine how the supply of mineral N from different prior crops or fallow might affect the growth and yield of rice. The study also tested whether N use by rice might be improved by timing the application of inorganic fertiliser N to supplement the N mineralised after prior crops. Experiments consisted of fallow, or cereal or legume crops in the dry-season followed by wet-season rice; and fallow, or cereal or legume crops in the wet- season followed by dry-season rice. Urea at one-third of the rate required for optimum rice yield was applied at 3 times during the rice crop: sowing, permanent flood, and/or panicle initiation. The prior fallow and crop treatments significantly influenced the growth and yield of rice crops. After a fallow, the pattern of soil N mineralisation promoted vegetative growth but was limiting during grain-filling. In contrast, after a cereal crop, rice vegetative growth was limited but grain-filling was promoted. Legume prior crops promoted both vegetative and grain growth. The benefits derived from growing the cereal or legume crops before rice, in terms of replacing fertiliser N, were dependent on the time at which fertiliser N was applied to the rice crop. In particular, legume crops frequently nullified the rice growth responses to fertiliser N. The results demonstrated that fallow and prior crops can alter the amount and timing of mineral N supply to a rice crop. Farmers should consider including a legume crop in rotation with rice because legumes supply N, which increases rice yield and reduces the requirement for fertiliser N. Cereal crops also contribute N, although farmers who use a cereal rotation should monitor the soil and crop N status during early rice growth, and supply extra fertiliser N to alleviate N deficiency.

Four irrigation frequencies and six nitrogen (N) fertilizer rates (0-300 kg ha-1) were applied to cotton (Gossypium hirsutum L.) grown on three Vertisols in the Emerald Irrigation Area, central Queensland. The purpose was to describe lint production responses to the plant available water before irrigation and N fertilizer, in terms of the crop N content and the efficiency of crop N use for lint production. Lint yield was greatest when the plant available water before irrigation was 50-80010 of the plant available water capacity (PAWC) of each soil. The rate of N fertilizer for maximum yield varied with plant available water and soil type. Plant available water before irrigation >60% and <37% PAWC, and rain after irrigation reduced the crop N content at the time of maximum leaf area index. Relative yield generally responded to 130 kg crop N ha-', although the range from 101 to 141 kg crop N ha-1 reflected differences in the maximum yield of each treatment. If the crop N was <130 kg ha-1, yield was mostly determined by the crop N content, whereas if the crop N content was >130 kg ha-1, yield and the efficiency of crop N use for lint production was determined by the plant available water before irrigation and soil type. Nitrogen fertilizer strategies to achieve the maximum yield of cotton (var. Deltapine 61) should focus on obtaining 130 kg crop N ha-1. This crop N content produced maximum yields for a range of plant available water contents before irrigation, and for three soil types.

In earlier work, we found that the near complete defoliation of grain sorghum [Sorghum bicolor (L.) Moenchmp;rsqb; seedlings delayed panicle initiation and anthesis. Several aspects of the required defoliation remain unclear, however, including which parts of the seedling’s foliage need to be removed, the timing of defoliation, and what effects differing defoliation treatments have on the morphology of plants that re-form after defoliation is terminated. To answer these questions, sorghum plants (cv. Boomer) grown under natural (c. 11.5 h) or extended (14 h) photoperiods were defoliated during the vegetative development phase. Treatments removed the fully exposed leaf-blade and/or the partially exposed and still expanding leaves and were varied by commencing and ceasing defoliation at different times, by cutting the plants at different heights, and by leaving some green leaf area on the plant. All defoliation treatments, except the one in which only the fully exposed leaf-blade was removed, resulted in delays in panicle initiation and anthesis. Defoliation treatments terminating on the same date, yet commencing between the second and fifth leaf stages, the latter just prior to panicle initiation in control plants, gave the same delay to panicle initiation. Serial defoliation at 3–4-day intervals maintained the plants in a vegetative state. Subsequent plant development and growth were associated with the morphology of plants when defoliation was terminated, thus were influenced by the height at which defoliation was performed. Plants defoliated above the first ligule took longer to initiate reproductive development and re-formed bigger plants than did those defoliated above the second ligule. Defoliation did not always reduce the plant biomass at anthesis compared with that of control plants. We interpret these responses as evidence that the signal to initiate reproductive development in sorghum originates in the partially exposed expanding leaves and possibly the leaf primordia, and that removal of those leaves resets the plant’s developmental program to an earlier phase. For farmers of rain-fed crops this is an exciting result, since it now seems likely that post-sowing management, via defoliation, can be developed to control flowering time and adjust the yield potential of crops in line with the amount of in-crop rain.