Abstract. Climate change projections for Australia predict increasing temperatures, changes to rainfall patterns, and elevated atmospheric carbon dioxide (CO 2 ) concentrations. The aims of this study were to predict plant production responses to elevated CO 2 concentrations using the SGS Pasture Model and DairyMod, and then to quantify the effects of climate change scenarios for 2030 and 2070 on predicted pasture growth, species composition, and soil moisture conditions of 5 existing pasture systems in climates ranging from cool temperate to subtropical, relative to a historical baseline. Three future climate scenarios were created for each site by adjusting historical climate data according to temperature and rainfall change projections for 2030, 2070 mid-and 2070 high-emission scenarios, using output from the CSIRO Mark 3 global climate model. In the absence of other climate changes, mean annual pasture production at an elevated CO 2 concentration of 550 ppm was predicted to be 24-29% higher than at 380 ppm CO 2 in temperate (C 3 ) species-dominant pastures in southern Australia, with lower mean responses in a mixed C 3 /C 4 pasture at Barraba in northern New South Wales (17%) and in a C 4 pasture at Mutdapilly in south-eastern Queensland (9%). In the future climate scenarios at the Barraba and Mutdapilly sites in subtropical and subhumid climates, respectively, where climate projections indicated warming of up to 4.48C, with little change in annual rainfall, modelling predicted increased pasture production and a shift towards C 4 species dominance. In Mediterranean, temperate, and cool temperate climates, climate change projections indicated warming of up to 3.38C, with annual rainfall reduced by up to 28%. Under future climate scenarios at Wagga Wagga, NSW, and Ellinbank, Victoria, our study predicted increased winter and early spring pasture growth rates, but this was counteracted by a predicted shorter spring growing season, with annual pasture production higher than the baseline under the 2030 climate scenario, but reduced by up to 19% under the 2070 high scenario. In a cool temperate environment at Elliott, Tasmania, annual production was higher than the baseline in all 3 future climate scenarios, but highest in the 2070 mid scenario. At the Wagga Wagga, Ellinbank, and Elliott sites the effect of rainfall declines on pasture production was moderated by a predicted reduction in drainage below the root zone and, at Ellinbank, the use of deeper rooted plant systems was shown to be an effective adaptation to mitigate some of the effect of lower rainfall.
Farmlets, each of 20 cows, were established to field test five milk production systems and provide a learning platform for farmers and researchers in a subtropical environment. The systems were developed through desktop modelling and industry consultation in response to the need for substantial increases in farm milk production following deregulation of the industry. Four of the systems were based on grazing and the continued use of existing farmland resource bases, whereas the fifth comprised a feedlot and associated forage base developed as a greenfield site. The field evaluation was conducted over 4 years under more adverse environmental conditions than anticipated with below average rainfall and restrictions on irrigation. For the grazed systems, mean annual milk yield per cow ranged from 6330 kg/year (1.9 cows/ha) for a herd based on rain-grown tropical pastures to 7617 kg/year (3.0 cows/ha) where animals were based on temperate and tropical irrigated forages. For the feedlot herd, production of 9460 kg/cow.year (4.3 cows/ha of forage base) was achieved. For all herds, the level of production achieved required annual inputs of concentrates of ~3 t DM/animal and purchased conserved fodder from 0.3 to 1.5 t DM/animal. This level of supplementary feeding made a major contribution to total farm nutrient inputs, contributing 50% or more of the nitrogen, phosphorus and potassium entering the farming system, and presents challenges to the management of manure and urine that results from the higher stocking rates enabled. Mean annual milk production for the five systems ranged from 88 to 105% of that predicted by the desktop modelling. This level of agreement for the grazed systems was achieved with minimal overall change in predicted feed inputs; however, the feedlot system required a substantial increase in inputs over those predicted. Reproductive performance for all systems was poorer than anticipated, particularly over the summer mating period. We conclude that the desktop model, developed as a rapid response to assist farmers modify their current farming systems, provided a reasonable prediction of inputs required and milk production. Further model development would need to consider more closely climate variability, the limitations summer temperatures place on reproductive success and the feed requirements of feedlot herds.
The phosphorus fertilizer requirements and long term productivity of nitrogen-fertilized Gatton panic (Panicum maximum cv. Gatton) pastures, grazed by lactating dairy cows, were evaluated over 7 years. Cows grazed at 2n6 cows\ha on pastures that received annually 100 or 300 kg N\ha at each of 0, 22n5 or 45 kg P\ha. Phosphorus treatments were applied as single superphosphate, balanced for calcium by applications of gypsum.The soil had an initial available soil phosphorus content of 40 mg\kg (bicarbonate extraction). At zero P fertilizer (0P), extractable soil P declined at the rate of 1n9 mg\kg each year ; at 22n5P it was maintained close to the original level while at 45P it increased at 6n6 mg\kg each year. Increased P fertilizer caused significant (P 0n01) increases in plant P concentration from year 2 onwards. In years 6 and 7 there was significantly less green pasture and leaf on offer in 300N pastures at 0P than with 22n5P and 45P. There was no influence of rate of P fertilizer at 100N on pasture quantity on offer in any year. There were clear trends at 100N of decreasing total pasture and green dry matter (DM) on offer over the 7 years, but not at 300N.Cows at 300N consumed more leaf in the diet in autumn and winter than at 100N. Leaf was 55-60 % of the diet in summer and autumn, but decreased to 21 % (100N) and 37 % (300N) in winter. Dead material in the diet was always higher at 100N. Pasture leaf percentage and leaf yield were the best individual predictors of leaf percentage in the diet. Diet P selected from pasture was reduced by the higher rate of N fertilizer in each season. Estimated P concentrations of the diet selected from pasture for summer, autumn and winter averaged 0n30, 0n38 and 0n28 % DM for 100N and 0n19, 0n24 and 0n18 % DM for 300N treatments, respectively.The response to P fertilizer was dependent on the rate of N fertilizer applied. The critical bicarbonate extractable soil P level for this soil type, below which pasture responses occurred, was 30 mg\kg at 300N. The critical level at 100N was not reached, but was 23 mg\kg P.
Dairy farms in subtropical Australia use irrigated, annually sown short-term ryegrass (Lolium multiflorum) or mixtures of short-term ryegrass and white (Trifolium repens) and Persian (shaftal) (T. resupinatum) clover during the winter–spring period in all-year-round milk production systems. A series of small plot cutting experiments was conducted in 3 dairying regions (tropical upland, north Queensland, and subtropical southeast Queensland and northern New South Wales) to determine the most effective rate and frequency of application of nitrogen (N) fertiliser. The experiments were not grazed, nor was harvested material returned to the plots, after sampling. Rates up to 100 kg N/ha.month (as urea or calcium ammonium nitrate) and up to 200 kg N/ha every 2 months (as urea) were applied to pure stands of ryegrass in 1991. In 1993 and 1994, urea, at rates up to 150 kg N/ha.month and to 200 kg N/ha every 2 months, was applied to pure stands of ryegrass; urea, at rates up to 50 kg N/ha.month, was also applied to ryegrass–clover mixtures. The results indicate that applications of 50–85 kg N/ha.month can be recommended for short-term ryegrass pastures throughout the subtropics and tropical uplands of eastern Australia, irrespective of soil type. At this rate, dry matter yields will reach about 90% of their potential, forage nitrogen concentration will be increased, there is minimal risk to stock from nitrate poisoning and there will be no substantial increase in soil N. The rate of N for ryegrass–clover pastures is slightly higher than for pure ryegrass but, at these rates, the clover component will be suppressed. However, increased ryegrass yields and higher forage nitrogen concentrations will compensate for the reduced clover component. At application rates up to 100 kg N/ha.month, build-up of NO3–-N and NH4+-N in soil was generally restricted to the surface layers (0–20 cm) of the soil, but there was a substantial increase throughout the soil profile at 150 kg N/ha.month. The build-up of NO3–-N and NH4+-N was greater and was found at lower rates on the lighter soil compared with heavy clays. Generally, most of the soil N was in the NO3–-N form and most was in the top 20 cm.
Dairy cows grazing a tropical grass pasture fertilized with 300 kg N\ha and with a 7-year history of phosphorus fertilizer at either 0 or 45 kg P\ha were given a P supplement in a 2i2 factorial experiment at Kairi Research Station, Queensland, Australia. Milk yield, fat-corrected milk yield, yields of milk fat, protein and lactose, and protein content of milk were increased (P 0n05) with P fertilizer. There was no response in milk yield or any component of milk to the provision of a P supplement. It is concluded that the milk response recorded in this experiment was due to P fertilizer leading to additional pasture on offer and increased pasture intake. The lack of response to additional P in the form of a supplement indicates that these pastures can supply adequate P for cows producing 20 kg\day even after 8 years without P fertilizer.
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