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

Abstract

The global impact of an increased concentration of CO 2 in the atmosphere on plants has been studied extensively, but little information has been published on the effect of enrichment of atmospheric CO 2 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 CO 2 and low N supplementation and crops grown under ambient CO 2 with low levels of N supplementation. Increased N application increased sequestration of C (P<0.05) compared to straw from crops grown under ambient CO 2 concentration. Low levels of N application and elevated CO 2 led to straw containing similar concentrations of N to those grown under ambient CO 2 conditions. Increasing N application to crops grown under ambient concentrations of CO 2 elevated the concentration of N (P<0.01) whereas crops at elevated concentrations of CO 2 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 CO 2 on WSC, aNDF om, ADF om, hemicellulose, cellulose and lignin (sa) occurred. There was a small decline (−26 g/kg; P<0.05) in the concentration of aNDF om 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 CO 2 (33.5:1) compared with ambient CO 2 (24.6:1). No changes in the total C content occurred for grain samples in response to CO 2 concentration or supplemental N fertiliser. No interaction between supply of N and CO 2 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 CO 2 ( i.e., +8.6 g/kg; P<0.01) than for crops grown under ambient supply of CO 2 (+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 CO 2 levels. No effect of changing concentrations of CO 2 were observed for ADF om, lignin (sa), cellulose and neutral detergent cellulose digestibility but concentrations of aNDF om (P<0.05) and hemicellulose (P<0.05) were higher in grain grown under ambient concentrations of CO 2 irrespective of supply of N to the crop. Although effects of elevated concentrations of CO 2 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 CO 2 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 CO 2 in the atmosphere, and this may lead to a reduction in the total supply of crude protein in crops used by livestock.