Effect of Free Air Carbon dioxide Enrichment (FACE) on the chemical composition and nutritive value of wheat grain and straw

The global impact of an increased concentration of CO2 in the atmosphere on plants has been studied extensively, but little information has been published on the effect of enrichment of atmospheric CO2 on the nutritive value of grain and straw used as ruminant feeds. This paper reports the chemical composition and nutritive value of grain and straw harvested from the drought tolerant hard red spring wheat (Triticum aestivum L.) variety Yecora Rojo managed with two carbon dioxide regimes (ambient, 350 μl/l and elevated, 550 μl/l), two rates of nitrogen application (low N: 53 kg N/ha and high N: 393 kg N/ha) grown under a water-fed (i.e., no deficit) regime. Accumulation of carbon in straw did not differ among crops grown under elevated CO2 and low N supplementation and crops grown under ambient CO2 with low levels of N supplementation. Increased N application increased sequestration of C (P<0.05) compared to straw from crops grown under ambient CO2 concentration. Low levels of N application and elevated CO2 led to straw containing similar concentrations of N to those grown under ambient CO2 conditions. Increasing N application to crops grown under ambient concentrations of CO2 elevated the concentration of N (P<0.01) whereas crops at elevated concentrations of CO2 did not accumulate N to the same extent. Differences in the non-structural carbohydrate and cell wall content reflected the patterns for total C. No effect of increasing the concentration of CO2 on WSC, aNDFom, ADFom, hemicellulose, cellulose and lignin (sa) occurred. There was a small decline (−26 g/kg; P<0.05) in the concentration of aNDFom in straw from crops that had received high N input. The ratio of lignin to total N was higher in straw harvested from plots with elevated CO2 (33.5:1) compared with ambient CO2 (24.6:1). No changes in the total C content occurred for grain samples in response to CO2 concentration or supplemental N fertiliser. No interaction between supply of N and CO2 concentration occurred. Changes in the total N content of grain in response to treatments were similar to the changes observed in the straw fraction. The increases in concentration of N incorporated into grain were higher from crops grown under enriched concentrations of CO2 (i.e., +8.6 g/kg; P<0.01) than for crops grown under ambient supply of CO2 (+3.5 g/kg; P<0.05). Differences in concentration of starch in the grain with increasing supply of N from fertiliser occurred under FACE conditions (P<0.05), but not for grain harvested from those grown under ambient CO2 levels. No effect of changing concentrations of CO2 were observed for ADFom, lignin (sa), cellulose and neutral detergent cellulose digestibility but concentrations of aNDFom (P<0.05) and hemicellulose (P<0.05) were higher in grain grown under ambient concentrations of CO2 irrespective of supply of N to the crop. Although effects of elevated concentrations of CO2 on grain and straw quality were expected, this poses concerns for livestock production in systems that use lower levels of agronomic inputs. Elevated concentrations of CO2 in the ambient environment were beneficial for development of above ground biomass and grain yield as measured by thousand-grain weight. However, straw and grain quality, in terms of crude protein and the crude protein to energy ratio will be affected by increasing concentrations of CO2 in the atmosphere, and this may lead to a reduction in the total supply of crude protein in crops used by livestock.