Hybrid forage sorghum is widely grown during the summer to supplement the feed from pasture in Northern New South Wales and Queensland, Australia. It can be managed as an uncut crop or cut several times as a multicut crop during a growing season. As forage sorghum is often grown in drier areas with low inputs, it can be subjected to water or nitrogen (N) stress at any time during crop growth. The thesis investigated environmental factors affecting the primary growth and regrowth of forage sorghum in the uncut and multicut conditions, respectively. The primary objective of this study was to examine whether the responses to the environmental stresses differ between uncut and multicut forage sorghum, and to elucidate reasons for such different responses. The secondary objective was to determine how forage quality of uncut and multicut forage sorghum was affected by different environmental conditions.
To achieve these objectives, six field experiments, one at Gatton and five at Redland Bay, in southeast Queensland were conducted in three seasons. In Experiments 1 and 2, grain sorghum, forage sorghum and maize were sown in October and December at Gatton and Redland Bay, respectively, with different N rates under well-watered conditions. In Experiment 3, there were four sowing dates, approximately 45 days apart, and three N levels. Experiments 4, 5 and 6 had two separate trials in each: a well-watered trial where water was supplied for the whole growing period, and a water stress trial where rain was excluded with the use of an automatic rainout shelter at different growth stages. In Experiment 4, water stress was imposed at 50 days after sowing (DAS) and there were four cutting time treatments. In Experiment 5, three sowing dates, 2 weeks apart, were employed to impose water stress at different stages of the primary grol1Jth before they were cut in the multicut. In Experiment 6, crops were grown with three N levels and water stress was imposed at 50 DAS.
Forage sorghum produced higher biomass in the uncut than in the multicut conditions irrespective of sowing date, N and water availability. Lower biomass in the multicut crop was related with a lag period of2-3 weeks after each cut in which time the canopy redeveloped, while the uncut crop continued to grow with its fully developed canopy. However, the reduction in biomass due to cutting was rather small under favourable conditions.
Under N limiting conditions, regrowth biomass production in the multicut crop was affected more than the biomass in the uncut crop. At the lowest N supply' level, cutting reduced the total cumulative biomass at the final harvest by 41-73% in different experiments. At the highest N level, the reduction was 12-49%.
Biomass response to N was higher in the "multicut than in the uncut crops under favourable conditions. The uncut biomass increase due to N application at the last harvest was 31, 30, 18 and 21%, in experiment 1, 2, 3 and 6, respectively. The corresponding values in the multicut crop were 80, 66, 117 and 37%. Due to higher leaf biomass in the multicut treatment, the demand for N was high and consequently responded more to the N application compared with the uncut treatment.
Similarly, reduction in biomass production due to water stress was always greater in the multicut than in the uncut crop for a given stress intensity. Greater reduction in the regrowth biomass was largely related to high sensitivity of leaf development under water deficit, as crop water status (leaf water potential) was more favourable in the multicut than in the uncut crop. In the uncut treatments, when stress was imposed at an earlier crop growth stage which tended to have a smaller canopy, reduction in biomass was greater compared with the crop that was exposed to water stress at a later growth stage with a fully developed canopy. Within the multicut treatments, regrowth was less affected when stress was imposed at the time of cutting compared to when the crop was already under water stress at the time of cutting. This was mainly because of high sensitivity of leaf area development to water deficit.
The time of sowing experiment has shown that regrowth was greatly affected towards the end of growing season when temperature and solar radiation became low. Thus, it can be generally concluded that multicut crops were more affected by adverse environmental conditions (water stress, low N and late season) than the uncut crop.
Although multicut crop produced lower cumulative biomass than the uncut crop, it had better forage quality in terms of tissue N concentration, leaf-stem ratio and in-vitro dry matter digestibility (IVDMD). The application of N increased forage quality in terms of protein content, but had small effect on IVDMD. However, water stress improved the forage quality by increasing the digestibility of tissue, particularly in the stem