A. D. Swan

Changes in soil water content under annual- and perennial-based pasture systems in the wheatbelt of southern New South Wales

Nine pasture treatments differing in species composition were monitored for changes in soil water content at a depth of 0.10–1.70 m, at 2 sites (Kamarah and Junee), in the wheatbelt of eastern Australia. Treatments containing perennial species, viz. lucerne (Medicago sativa L.), phalaris (Phalaris aquatica L.), cocksfoot (Dactylis glomerata L.), mixture (lucerne + phalaris + cocksfoot), wallaby grass (Austrodanthonia richardsonii Cashmore.), and lovegrass (Eragrostis curvula (Schrader) Nees.), were sown with subterranean clover (Trifolium subterraneum L.). In addition, 3 treatments based solely on annual species were examined: subterranean clover (sown by itself and kept weed-free with herbicides), annual (sown to subterranean clover but weed invasion not controlled), and serradella (Ornithopus compressus L.). The experiment was conducted from 1994–97 at the Junee site (annual average rainfall 550 mm/year) and from 1995–97 at the Kamarah site (annual average rainfall 450 mm per year). At the higher rainfall site (Junee), there were few differences among pasture types in soil water content to 0.70 m. Below 0.70 m the soil profile was drier under all the perennial swards than under the annual pasture treatments by the end of the 4-year pasture phase. At the drier Kamarah site, where the pasture phase was shorter due to an initial sowing failure, all the perennials, except cocksfoot, dried the profile below 1.05 m. At both sites, lucerne dried the 1.05–1.70 m section of the soil profile more rapidly than the other perennials, which apparently took longer to reach this depth. At the Junee site, the soil water deficit in May (SWD(MAY), defined as field capacity (mm) – stored soil water (mm) at the beginning of May) was largest in the phalaris, mixture, lucerne, and cocksfoot treatments (155–162 mm), whereas as under a pasture of subterranean clover alone, SWD(MAY) was only 89 mm. At the drier Kamarah site, the largest SWD(MAY) was created by the lovegrass (114 mm) and lucerne (107 mm) treatments. The cocksfoot and subterranean clover treatments created the smallest SWD(MAY) at this site, at 79 and 72 mm, respectively. The study showed that currently available C3 and C4 perennial grasses can be as effective as lucerne in drying the soil profile to 1.70 m in the 450–600 mm rainfall areas of the southern NSW wheatbelt, creating a dry soil buffer to reduce the risk of deep drainage during subsequent cropping phases. As the rate at which grasses dried the profile was slower than lucerne, pastures based on perennial grasses may have to be retained longer to achieve the same level of dewatering.

Factors affecting the potential contributions of N2 fixation by legumes in Australian pasture systems

Abstract. The amounts of foliage nitrogen (N) fixed by various annual and perennial legumes growing in Australian pastures range from <10 to >250 kg N/ha.year. Differences in N2 fixation result from variations in the proportion of the legume-N derived from atmospheric N2 (%Ndfa) and/or the amount of legume-N accumulated during growth. On-farm surveys of %Ndfa achieved by legumes growing in farmers’ paddocks in Australia indicated that N2 fixation contributed >65% of the legume’s N requirements in three-quarters of the annual legumes examined, but this decreased to two-thirds of lucerne (Medicago sativa; also known as alfalfa), and half of white clover (Trifolium repens) samples. Factors such as low numbers or the poor effectiveness of rhizobial strains in the soil, water stress, high soil concentrations of N, and nutrient disorders contribute to poor nodulation and %Ndfa values <65%, but there is also evidence that the observed %Ndfa can be dependent on the legume species present, and whether the legume is grown in a pure stand or in a mixed sward. The accumulation of legume-N relates primarily to the legume content and net productivity of the pasture. For many legume species, ∼20 kg of shoot-N is fixed on average for every tonne of herbage dry matter produced. Legume productivity can be influenced by (i) sowing and establishment techniques and other strategies that enhance the legume content in pasture swards; (ii) the amelioration of soil constraints; (iii) the use of new legume species (and host–rhizobial strain combinations) that are more tolerant of hostile soil environments than subterranean clover (T. subterraneum) or annual medics (Medicago spp); and (iv) the inclusion of perennials such as lucerne to offset the year-to-year variability in productivity and N2 fixation that is a common occurrence with annual legumes.

Yield and grain protein of wheat following phased perennial grass, lucerne, and annual pastures

The effect of using 4 perennial grasses or lucerne (Medicago sativa L.) in the pasture phase on subsequent wheat grain yield, protein, and grain hardness was investigated at 2 sites (Kamarah and Junee) in the south-eastern Australian cereal belt. The 6 perennial treatments were 5 mixtures of subterranean clover (Trifolium subterraneum L.), with one of lucerne, phalaris (Phalaris aquatica L.), cocksfoot (Dactylis glomerata L), wallaby grass (Austrodanthonia richardsonii (Cashm.) H.P. Linder), or lovegrass (Eragrostis curvula (Schrader) Nees cv. Consol), or one mixture of cocksfoot, phalaris, and lucerne. The results were compared with wheat after one of 3 annual pastures consisting of either pure subterranean clover, subterranean clover with annual volunteer broadleaf and grass weeds, or yellow serradella (Ornithopus compressus L.). The duration of the pasture phase was 3 years at the drier Kamarah site (av. annual rainfall 430 mm) and 4 years at Junee (550 mm). The effect of time of removal of the pastures in the year prior to cropping (28 August–3 September or 6–7 November) and the effect of nitrogen (N) fertiliser application were also examined. In the absence of applied N, wheat grain yields at Kamarah were highest (4.7–4.9 t/ha) and grain protein lowest (10.3–11.1%) following phalaris, wallaby grass, and cocksfoot. Grain protein levels were highest (12.9–13.9%) in wheat following the 3 annual legume swards at both sites. Previous pasture type had no effect on wheat yields at the Junee site. Wheat grain protein and total N taken up by the crop were positively related to available soil N to 100 cm measured at sowing at both sites. Grain protein was inversely related to grain yield at both sites where additional N fertiliser was added, but not in the absence of fertiliser N. There was a positive response in grain protein to delayed time of pasture removal in second year wheat at Junee. The application of additional N fertiliser increased grain protein of wheat following all 9 pasture types at the drier Kamarah site, but at the Junee site there was only a positive grain protein response following phalaris, cocksfoot, and wallaby grass. Early removal of the pasture prior to cropping increased soil water (10–130 cm) at sowing by 18 mm, delayed wheat senescence, and increased crop yield by 11% (0.44 t/ha) at the drier Kamarah site. Early removal of the pasture at Junee increased soil water by 29 mm, crop yields by 2% (0.14 t/ha), and increased grain protein in wheat following cocksfoot, wallaby grass, and phalaris, but not following the 3 annual legume treatments. The study demonstrated that perennial grasses can be successfully incorporated into phased rotations with wheat without affecting grain yield, but protein levels may be lower and timing of pasture removal will be important to limit the effect of water deficits on grain yield.

Farmer experience with perennial pastures in the mixed farming areas of southern New South Wales: on-farm participatory research investigating pasture establishment with cover-cropping

Abstract. In 2009, 95 farmers in the mixed farming zone of southern New South Wales (NSW), average annual rainfall 450–700 mm, were surveyed about their use of perennial pasture species. Survey responses indicated that, on average, 52% of land was under crop, 29% contained perennial pasture and 19% annual pastures. The proportion of land sown to perennial pastures and the species used differed with rainfall. Farmers identified concerns about the cost of establishment and poor survival of perennial pasture species as constraints to wider adoption. The survey also revealed that cover-cropping (sowing pasture species under the final grain crop in a cropping phase) was the dominant method of pasture establishment. Large-scale, on-farm participatory experiments were sown with the farm machinery, three at Ariah Park and one at Brocklesby in southern NSW in 2009 (annual rainfall 100 mm less than long-term average), and a further two experiments (one at each location) commenced in 2010 (annual rainfall >200 mm above average). These experiments compared the effect of cereal cover-crop sowing rate (standard rates used by the collaborating farmer and half of the standard rate) on the establishment of the perennials lucerne (Medicago sativa), phalaris (Phalaris aquatica), cocksfoot (Dactylis glomerata), and chicory (Cichorium intybus) sown in different mixes and rates with various annual legume species. The persistence and productivity of individual species were monitored for 2 years after sowing. Results indicated little or no effect of the presence of a cover-crop on the initial establishment of any of the perennials, but pasture species survival were severely affected by cover-crop sowing rates as low as half of the farmer practice (10 kg barley or 12 kg wheat ha–1) in 2009. Despite higher than average annual rainfall in 2010 and 2011, the residual effect of establishing pastures under a cover-crop in 2009 was poorer persistence and lower productivity by lucerne at the standard cover-cropping rate, and by phalaris, cocksfoot and chicory at all cover-crop rates, and an increased incidence of weeds. Similar responses to cover-cropping occurred between 2010 and 2012, even with the wetter establishment conditions in 2010, for phalaris, chicory and weeds, despite demonstration at Ariah Park that higher populations of individual perennial species could be achieved by doubling the sowing rate of pasture seed in 2010. Lucerne compensated for lower plant numbers by increasing herbage growth in response to rainfall, but phalaris could not and total pasture productivity over the first 2 years after establishment was greatly reduced by the use of cover-crops in both 2009 and 2010. Cover-cropping also reduced annual legume seedset, which could have implications for future pasture performance. Lucerne was the most consistently productive perennial pasture species evaluated regardless of establishment technique or climatic conditions.

Effect of gypsum on establishment, persistence and productivity of lucerne and annual pasture legumes on two grey Vertosols in southern New South Wales

Changes in pasture yield and botanical composition due to gypsum application were examined on Vertosols at two locations of differing soil sodicity, Grogan and Morangarell, in southern New South Wales. Two pasture treatments were examined. One was an annual pasture comprised of 3 annual legumes (2 subterranean clover Trifolium subterraneum L. cultivars, Clare and Riverina, and balansa clover T. michelianum Savi cv. Paradana), while the second treatment consisted of lucerne (Medicago sativa L.) cv. Aquarius sown in a mixture with the same annual legumes. Gypsum had no effect on the establishment or persistence of lucerne at either site. Gypsum increased the number of subterranean clover seedlings present in autumn in annual swards at the more sodic Grogan site in each of the 4 years, but provided no difference when the clover was in a mixture with lucerne. Annual legume seed yields in annual-only swards increased with gypsum by up to 58% at Grogan and 38% at Morangarell. Seed yields of both cultivars of subterranean clover declined as a proportion of the total annual legume seed bank when lucerne was included in the mixture, in contrast to balansa clover (at Grogan) and the naturalised annual legumes, burr medic (M. polymorpha L.) and woolly clover (T. tomentosum L.), which all increased in relative seed yield in the presence of lucerne. Total pasture production at the Grogan site increased with gypsum by up to 15% per annum in annual swards and 36% in lucerne swards depending on the season. Yield responses to gypsum by the lucerne component were observed in 10 of the 13 seasonal yield measurements taken at Grogan. However, total pasture yield and seasonal yields were unaffected by both gypsum and pasture type at the less sodic Morangarell site. It was concluded that sowing a diverse mixture of annual legumes or polycultures was conducive to maintaining productive pastures on these spatially variable soils. Lucerne dried the soil profile (0.15–1.15 m) more than annual pastures at both sites. The combination of gypsum and lucerne enhanced water extraction at depth (0.6–1.15 m) at the Grogan site increasing the size of the dry soil buffer whereas gypsum increased soil water at depth (>0.6 m) under annual swards.

Soil mineral nitrogen benefits derived from legumes and comparisons of the apparent recovery of legume or fertiliser nitrogen by wheat

Nitrogen (N) contributed by legumes is an important component of N supply to subsequent cereal crops, yet few Australian grain-growers routinely monitor soil mineral N before applying N fertiliser. Soil and crop N data from 16 dryland experiments conducted in eastern Australia from 1989–2016 were examined to explore the possibility of developing simple predictive relationships to assist farmer decision-making. In each experiment, legume crops were harvested for grain or brown-manured (BM, terminated before maturity with herbicide), and wheat, barley or canola were grown. Soil mineral N measured immediately before sowing wheat in the following year was significantly higher (P < 0.05) after 31 of the 33 legume pre-cropping treatments than adjacent non-legume controls. The average improvements in soil mineral N were greater for legume BM (60 ± 16 kg N/ha; n = 5) than grain crops (35 ± 20 kg N/ha; n = 26), but soil N benefits were similar when expressed on the basis of summer fallow rainfall (0.15 ± 0.09 kg N/ha per mm), residual legume shoot dry matter (9 ± 5 kg N/ha per t/ha), or total legume residue N (28 ± 11%). Legume grain crops increased soil mineral N by 18 ± 9 kg N/ha per t/ha grain harvested. Apparent recovery of legume residue N by wheat averaged 30 ± 10% for 20 legume treatments in a subset of eight experiments. Apparent recovery of fertiliser N in the absence of legumes in two of these experiments was 64 ± 16% of the 51–75 kg fertiliser-N/ha supplied. The 25 year dataset provided new insights into the expected availability of soil mineral N after legumes and the relative value of legume N to a following wheat crop, which can guide farmer decisions regarding N fertiliser use.

Effectiveness of grazing and herbicide treatments for lucerne removal before cropping in southern New South Wales

The difficulty of reliably removing an established lucerne pasture before cropping has been identified as a major problem with phase-farming systems on mixed farms. A series of experiments were undertaken on established lucerne stands at the Ginninderra Experimental Station in the Australian Capital Territory (ACT) and at the Temora Research Station in southern New South Wales (NSW) to compare the ability of grazing, either alone or in combination with herbicides, to remove a lucerne pasture. A pilot study at the Ginninderra Experimental Station in 1998–99 utilised a high stocking rate (30 dry sheep equivalents [dse]/ha), while the main study at the Ginninderra Experimental Station and the Temora Research Station in 1999–2000 used a lower stocking rate (10–12 dse/ha) considered to be closer to farmer practice in the region. Continuous grazing at the high stocking rate removed 73% of the lucerne stand over 3 months. In contrast, the stocking rates applied in the main study proved too low to substantially impact on lucerne survival at both the ACT and NSW sites (13–23% removal) under the good seasonal conditions experienced in 1999–2000 even though the pastures were continuously grazed for 8–9 months. The use of herbicides both alone or in conjunction with grazing greatly improved lucerne removal in both studies. However, herbicide efficacy was variable (53–100% removal), and seemed to be related to the time of year it was applied, the period of lucerne regrowth or the amount of rainfall before herbicide application. An additional on-farm study was undertaken near Junee Reefs in southern NSW between 2001 and 2003 that compared the survival of a range of lucerne cultivars under simulated hay-cutting (mown) and commercial grazing regimes. Dry conditions during 2002–03 resulted in a decline in lucerne frequency at about 50% in mown control plots. Grazing increased the stand decline, particularly for many winter-active cultivars where lucerne frequency was reduced by up to 70–93%. It was concluded that: (i) continuous grazing for prolonged periods can be effective at removing lucerne, but the rate of lucerne loss will be influenced by both stocking rate and rainfall; (ii) the application of herbicides can improve lucerne removal either in association with, or in the absence of grazing; however, herbicide efficacy appeared to be dependent upon the physiological status of the lucerne plants and/or the environmental conditions before application; and (iii) cultivar responses indicated that lucerne types could potentially be developed for phase-farming systems with increased susceptibility to grazing mismanagement.

Prospects to utilise intercrops and crop variety mixtures in mechanised, rain-fed, temperate cropping systems

Abstract. Despite the potential productivity benefits, intercrops are not widely used in modern, mechanised grain cropping systems such as those practised in Australia, due to the additional labour required and the added complexity of management (e.g. harvesting and handling of mixed grain). In this review we investigate this dilemma using a two-dimensional matrix to categorise and evaluate intercropping systems. The first dimension describes the acquisition and use of resources in complementary or facilitative interactions that can improve resource use efficiency. The outcome of this resource use is often quantified using the land equivalent ratio (LER). This is a measure of the relative land area required as monocultures to produce the same yields as achieved by an intercrop. Thus, an LER greater than 1 indicates a benefit of the intercrop mixture. The second dimension describes the benefits to a farming system arising not only from the productivity benefits relating to increased LER, but from other often unaccounted benefits related to improved product quality, rotational benefits within the cropping system, or to reduced business risks. We contend that a successful intercrop must have elements in both dimensions. To date most intercropping research has considered only one of these two possible dimensions. Intercrops in large, mechanised, rain-fed farming systems can comprise those of annual legumes with non-legume crops to improve N nutrition, or other species combinations that improve water use through hydraulic redistribution (the process whereby a deep-rooted plant extracts water from deep in the soil profile and releases a small proportion of this into the upper layers of the soil at night), or alter disease, pest or weed interactions. Combinations of varieties within cereal varieties were also considered. For our focus region in the southern Australian wheatbelt, we found few investigations that adequately dealt with the systems implications of intercrops on weeds, diseases and risk mitigation. The three main intercrop groups to date were (1) ‘peaola’ (canola-field pea intercrops) where 70% of intercrops (n = 34) had a 50% productivity increase over the monocultures, (2) cereal-grain legume intercrops (n = 22) where 64% showed increases in crop productivity compared with monocultures and (3) mixtures of cereal varieties (n = 113) where there was no evidence of a productivity increase compared with the single varieties. Our review suggests that intercropping may have a role in large rain-fed grain cropping systems, based on the biophysical benefits revealed in the studies to date. However, future research to develop viable intercrop options should identify and quantify the genotypic differences within crop species for adaptation to intercropping, the long-term rotational benefits associated with intercrops, and the yield variability and complexity-productivity trade-offs in order to provide more confidence for grower adoption. Farming systems models will be central to many of these investigations but are likely to require significant improvement to capture important processes in intercrops (e.g. competition for water, nutrients and light).

Factors influencing herbicide efficacy when removing lucerne prior to cropping

Farmers often experience inconsistent responses when using herbicides to terminate an established lucerne pasture prior to cropping. In an attempt to redress this problem, a series of field experiments were conducted between 1999 and 2002 at various locations in southern and northern New South Wales, the Australian Capital Territory, and south Western Australia that aimed to identify management guidelines that improved the efficacy of herbicide mixtures commonly used to remove lucerne. Collectively, these studies indicated that herbicides were generally less effective when applied either early (less than 2 weeks) or late (6 weeks or more) in the regrowth cycle of lucerne after defoliation. Herbicide efficacy tended to be greatest if applied to regrowth 3–5 weeks after defoliation, which corresponds to a time when the lucerne crown and root reserves are likely to be in the process of being replenished by photoassimilates transported from the shoot. The impact of timing of herbicide application in relation to season was compared at a number of locations. Across all the sites and years, spring herbicide applications were generally the most effective, removing on average 87% of the lucerne (range 53–100%) compared with 72% in summer (24–100%) and 60% in autumn (7–92%). Spring applications were also more consistent in their effect, removing >80% of the lucerne plants in 9 out of 12 experiments, whereas similar rates of removal occurred on 4 occasions in 9 summer applications and only twice in 8 autumn applications. Some of the seasonal variation could be explained by differences in the amount of rainfall prior to herbicide applications. It was assumed that the relationship between rainfall and herbicide efficacy reflected the stimulation of lucerne shoot and root growth by the additional soil moisture before herbicide treatment. Herbicide mixtures that contained ingredients such as picloram that retain residual activity in the soil tended to be more effective and were less influenced by lucerne growth and season than those herbicides with little or no residual activity. However, such chemicals could potentially restrict which crops can subsequently be grown after a lucerne pasture has been removed. It was concluded that >80% of lucerne plants were likely to be removed using herbicides provided that the herbicide treatment was applied to actively growing lucerne 3–5 weeks after defoliation, and when greater than 70–95 mm rain had fallen in the 6–8 weeks prior to application.