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The effects of elevated concentrations of atmospheric CO2 (e[CO2]) on the nutritive value of wheat vegetative matter and grain as a feedstock for ruminants were investigated in a study undertaken at the Australian grains free‐air CO2 enrichment (AGFACE) facility. The study included two commercial wheat cultivars (Janz and Yitpi) and two genetic selections from a Seri/Babex population (SB003 and SB062) which had previously been characterised for low and high water‐soluble carbohydrate accumulation efficiency. The trial was grown under ambient (~390 µmol/mol) and elevated (~550 µmol/mol) CO2 conditions, and plants harvested at tillering, anthesis and physiological maturity. Composition analyses to determine the nutritive value for ruminant feed were undertaken on stems, leaves and grain. Plant and grain nitrogen were reduced in the e[CO2] treatments, and as expected, the water‐soluble carbohydrates increased. All genotypes responded to e[CO2] with the effects of altered composition evident within 60 days of sowing. Determinants of ruminant feed quality such as neutral and acid detergent fibre and estimated in vitro metabolisable energy were not significantly affected. The reduced plant and grain N will impact on the nutritive value and supplementation may be required. The impact of e[CO2] on chemical composition of wheat plants may be greater if the predicted climate change is associated with concomitant abiotic stress such as high ambient temperature or low soil moisture. [ABSTRACT FROM AUTHOR]

Wheat production will be impacted by increasing concentration of atmospheric CO2 [ CO2], which is expected to rise from about 400 μmol mol−1 in 2015 to 550 μmol mol−1 by 2050. Changes to plant physiology and crop responses from elevated [ CO2] (e[ CO2]) are well documented for some environments, but field-level responses in dryland Mediterranean environments with terminal drought and heat waves are scarce. The Australian Grains Free Air CO2 Enrichment facility was established to compare wheat ( Triticum aestivum) growth and yield under ambient (~370 μmol−1 in 2007) and e[ CO2] (550 μmol−1) in semi-arid environments. Experiments were undertaken at two dryland sites (Horsham and Walpeup) across three years with two cultivars, two sowing times and two irrigation treatments. Mean yield stimulation due to e[ CO2] was 24% at Horsham and 53% at Walpeup, with some treatment responses greater than 70%, depending on environment. Under supplemental irrigation, e[ CO2] stimulated yields at Horsham by 37% compared to 13% under rainfed conditions, showing that water limited growth and yield response to e[ CO2]. Heat wave effects were ameliorated under e[ CO2] as shown by reductions of 31% and 54% in screenings and 10% and 12% larger kernels (Horsham and Walpeup). Greatest yield stimulations occurred in the e[ CO2] late sowing and heat stressed treatments, when supplied with more water. There were no clear differences in cultivar response due to e[ CO2]. Multiple regression showed that yield response to e[ CO2] depended on temperatures and water availability before and after anthesis. Thus, timing of temperature and water and the crop's ability to translocate carbohydrates to the grain postanthesis were all important in determining the e[ CO2] response. The large responses to e[ CO2] under dryland conditions have not been previously reported and underscore the need for field level research to provide mechanistic understanding for adapting crops to a changing climate. [ABSTRACT FROM AUTHOR]