Curve-linear relationship between altered carbohydrate traits with molecular structure and truly absorbed nutrient supply to dairy cattle in new hulless barley (Hordeum vulgare L.)

The objective of this study was to investigate the curve-linear relationship between altered carbohydrate (CHO) traits, molecular structures of protein and carbohydrates, and truly absorbed protein supplied to dairy cattle in newly developed hulless barley (Hordeum vulgare L.) varieties. Four hulless barley varieties (CDC Fibar, zero-amylose waxy; CDC Rattan, 5%-amylose waxy; CDC McGwire, normal amylose; HB08302, high-amylose) developed at the Crop Development Centre of the University of Saskatchewan, were used. The four cultivars varied in carbohydrates traits based on amylose (10-400 g/kg dry matter (DM)) and β-glucan (50-100 g/kg DM) contents. Samples (n = 3) of each variety, from three consecutive years were analysed. The results showed that: 1) intestinal (IDP) and total (TDP) digestible protein, intestinal digestible rumen undegraded starch (IDBST), total digestible starch (TDST) and total truly digestible carbohydrates (TDCHO) had a quadratic relationship with the altered amylose to amylopectin (A: AP) ratio and β-glucan contents. Notably, the contents of IDP and TDP decreased with an increases in A: AP ratio from 0.02 to 0.40, and then increased slightly with further increases in the ratio from 0.40 to 0.59. The TDST increased with increases in A: AP ratio from 0.02 to 0.40, and then decreased slightly with further increases in the ratio from 0.40 to 0.59. The content of TDP increased, while the content of TDST decreased with the increase in β-glucan contents from 42.8 to 100 g/kg DM. However, the rate of increase of TDP was slower, and the rate of decrease of TDST was much faster from 42.8 to 80 g/kg DM. 2) The DVE/OEB and NRC-2001 estimated fermentable organic matter (FOM), truly absorbed microbial protein (AMCP), total truly absorbed rumen undegraded protein (ARUP), total truly digested protein in the small intestine (DVE) and degraded protein balance (OEB) showed a quadratic relationship with the A: AP ratio and β-glucan content. The content of FOM and OEB increased, while the content of ARUP decreased with an increase in the A: AP ratio, with the highest and lowest values being observed at the A: AP ratio of 0.40. The FOM decreased with the increase in β-glucan content up to 80 g/kg DM, while ARUP and OEB consistently increased with an increase in the β-glucan content from 42.8 to 100 g/kg DM. 3) The advanced molecular spectroscopy (FT/IR) study revealed that protein structures, amide-I area and height, were linearly related to β-glucan content, while the amide-I area was linearly related to the A: AP ratio. The CHO peak-2 area and height showed a linear response to β-glucan content, and the CHO peak-2 area also had quadratic relationship with A: AP. 4) A synchrotron-based molecular spectroscopic study showed that the β-glucan peak area and height were cubically related to A: AP. The peak area of the cellulosic compounds presented a quadratic response to A: AP. In conclusion, the in situ and in vitro studies suggested that alteration of carbohydrate traits in the newly developed hulless barley significantly impacted the inherent molecular structures and showed significant curve-linear relationships with truly absorbed nutrient supply to dairy cows.