This overview paper presents a description of the National Windbreaks Program (NWP) — its objectives, the main methods used to achieve these objectives and a summary of the key results. It draws these from the individual papers appearing in this special issue, which provide detailed descriptions and discussion about the specific research sites and research methods used, in addition to interpreting and discussing the results. The key findings were the following: (i) Two broad areas of crop and pasture response can be identified downwind of a porous windbreak: a zone of reduced yield associated with competition with the windbreak trees that extended from 1 H to 3 H, where H is the windbreak height, and a zone of unchanged or slightly increased yield stretching downwind to 10 H or 20 H. (ii) Averaged over the paddock, yield gains due to the effect of shelter on microclimate were smaller than expected — especially for cereals. Yield simulations conducted using the APSIM model and 20 years of historical climate data confirmed this result for longer periods and for other crop growing regions in Australia. Larger yield gains were simulated at locations where the latter part of the growing season was characterised by high atmospheric demand and a depleted soil water store. (iii) Economic analyses that account for the costs of establishing windbreaks, losses due to competition and yield gains as a result of shelter found that windbreaks will either lead to a small financial gain or be cost neutral. (iv) Part of the reason for the relatively small changes in yield measured at the field sites was the variable wind climate which meant that the crop was only sheltered for a small proportion of the growing season. In much of southern Australia, where the day-to-day and seasonal variability in wind direction is large, additional windbreaks planted around the paddock perimeter or as closely-spaced rows within the paddock will be needed to provide more consistent levels of shelter. (v) Protection from infrequent, high magnitude wind events that cause plant damage and soil erosion was observed to lead to the largest yield gains. The main forms of direct damage were sandblasting, which either buries or removes seedlings from the soil or damages the leaves and stems, and direct leaf tearing and stripping. (vi) A corollary to these findings is the differing effect that porous windbreaks have on the air temperature and humidity compared to wind. While winds are reduced in strength in a zone that extends from 5 H upwind to at least 25 H downwind of the windbreak, the effects of shelter on temperature and humidity are smaller and restricted mainly to the quiet zone. This means that fewer windbreaks are required to achieve reductions in wind damage than for altering the microclimate. (vii) The wind tunnel experiments illustrate the important aspects of windbreak structure that determine the airflow downwind, and subsequent microclimate changes, in winds oriented both perpendicular and obliquely to porous windbreaks. These results enable a series of guidelines to be forwarded for designing windbreaks for Australian agricultural systems.

There is great interest in quantifying and understanding how shelter modifies crop growth and development under Australian conditions. Small constructed enclosures (shelters) can consistently reduce wind speed, allowing experiments to be run with replicated sheltered and unsheltered treatments in close proximity. The aim of this study was to quantify the effect on microclimate of consistently reducing wind speed by 70% and explain the consequences for dryland wheat (Triticum aestivum), lupin (Lupinus angustifolius) and mungbean (Vigna radiata) growth and development, at sites in Queensland, Victoria and Western Australia. Crops were grown inside and outside of artificial shelters, 10 by 10 m and extending 1 m above the crop canopy throughout the growing season. Mean daily air and soil temperatures and atmospheric vapour pressure inside the shelters were largely similar to unsheltered conditions. However, clear diurnal trends were evident; daily maximum temperature and vapour pressure deficit (VPD) were increased in shelter when crops were establishing or senescing. When leaf area index (LAI) was reduced in the shelters, soil temperature was greater than in the open, however when LAI was increased in the shelters, soil temperature was less than in the open. Grain yield in shelters ranged between 78 and 120% of unsheltered yield, depending on seasonal conditions and crop species; the mean yield for all sites, crops and years was 99% of unsheltered yield. In the absence of waterlogging, sheltered crops tended to develop more leaf area than unsheltered crops, with an increase in the ratio of leaf area to above-ground biomass. This greater leaf area did not increase soil water use. While LAI was increased by shelter, only 2 of the 6 sheltered crops that were not waterlogged yielded significantly more grain than the unsheltered crops. This may be because the sheltered crops experienced greater maximum temperatures and VPD during anthesis and grain filling than unsheltered crops. Also, net photosynthesis may not have increased in the shelters after canopy closure (LAI>3–4). Lupins, which developed more leaf area inside shelters, may have experienced strong competition for assimilates between developing branches, flowers and fruit. When rainfall was above average and the soil became waterlogged for part of the growing season, grain yield was reduced inside the shelters. Reduced evaporation inside the shelters may have extended the duration and severity of waterlogging and increased stresses on sheltered plants when potential yield was being set. The reductions in wind speed achieved inside the artificial shelters were greater than those likely in conventional tree windbreak systems. Analysis of crop growth illustrated that microclimate modification at this high level of shelter can be both beneficial and harmful, depending on the crop species and climatic conditions during the growing season.

A grazing experiment was conducted on an area with high groundwater recharge potential in northeastern Victoria from 1988 to 1992. Merino wether weaners were grazed on either lucerne (Medicago sativa) or a pasture consisting of annual species (Trifolium spp., Lolium rigidum, Vulpia bromoides, Hordeum leporinum) at 5.0, 8.75, or 12.5 wethers/ha. Lucerne pastures were rotationally grazed and annual pastures were set-stocked. Measurements included herbage mass, lucerne plant density, sheep liveweight, wool production and fibre diameter, and soil moisture potential. Herbage mass in both pasture treatments decreased with increasing stocking rate, but at 12.5 wethers/ha, lucerne herbage mass was significantly (P<0.001) greater than annual pasture herbage mass. Sheep liveweight showed similar trends. On annual pastures, wool production per sheep decreased with increasing stocking rate, whereas on lucerne, it was maintained as stocking rate increased from 8.75 to 12.5 wethers/ha. Mean wool production per ha on lucerne pasture was 14.5, 23.5, and 32.1 kg for 5.0, 8.75, and 12.5 wethers/ha, respectively, and on annual pasture it was 13.8, 21.7, and 24.8 kg. Supplementary feeding was needed on annual pastures every year at the high stocking rate. On lucerne pastures, it was unnecessary except for 3 weeks in 1991. Soil moisture under lucerne was less than under annual pasture, indicating that lucerne used more water than the annual pasture and created a larger soil water deficit, thus leading to potentially less groundwater recharge. In this environment, lucerne could be productively stocked at 12.5 wethers/ha, whereas annual pastures were less productive and could only be stocked at 8.75 wethers/ha. Furthermore, lucerne was more effective at using soil water and reducing the potential for groundwater recharge.

A field experiment located in southern New South Wales compared the component yields of cereal–lucerne companion crops (cereals sown into established lucerne) with the yields of cereal and lucerne monocultures. In-crop lucerne herbicide suppression, cereal crop types (wheat and barley), and top-dressed nitrogen (N) were evaluated for the potential to improve cereal production in the presence of lucerne. Plant populations and biomass, cereal grain yields, and grain quality (protein, screenings, and contamination) were measured. Over the 3-year study, cereals sown into established lucerne (4 years of age at the commencement of the experiment) yielded 17% less (P < 0.05) grain than the cereal monoculture. Companion cropping also resulted in a 71% reduction (P < 0.05) in lucerne biomass over the growing season compared with the lucerne monoculture, but a 3-fold (P < 0.05) increase in total (cereal and lucerne) biomass production. There were no differences between wheat and barley crops in the presence of lucerne, although extensive lodging in the 2003-barley monoculture did result in a significant main treatment (+/0 lucerne and +/0 in-crop lucerne suppression) × crop type (wheat and barley) interaction in grain yield, but not cereal biomass. N top-dressed after tillering onto cereal–lucerne companion crops did not increase grain yield, although it did increase cereal biomass in 2003. Whilst in-crop lucerne suppression did not increase cereal grain yields, it did increase (P < 0.05) cereal biomass and reduced lucerne biomass at cereal maturity and contamination (lucerne pods and flowers) of the cereal grain. However, this practice reduced (P < 0.05) lucerne populations, and therefore potentially threatens the longer term viability of lucerne stands so more research is recommended to develop less detrimental strategies for achieving effective in-crop lucerne suppression. This study combined with results from others, suggests that rainfall was a major factor determining cereal responses in the presence of lucerne, and although there were responses in cereal biomass to additional N and herbicide suppression, these strategies appear to only have potential under favourable growing-season conditions.

No abstract