Tannin is a key factor in the determination and prediction of energy content in sorghum grains fed to growing pigs

This study was conducted to determine and compare the DE and ME content as well as the apparent total tract digestibility (ATTD) of GE in 28 samples of sorghum grain fed to growing pigs and to generate prediction equations for DE, ME, and the ATTD of GE based on chemical analysis. The 28 samples included 8 low-tannin sorghum cultivars (LTS; tannin <= 0.16%), 10 medium-tannin sorghum cultivars (MTS: 0.67% <= tannin <= 0.98%), and 10 high-tannin sorghum cultivars (HTS; 1.11% <= tannin <= 1.51%) obtained from 9 provinces in China and 3 samples of LTS cultivars were obtained internationally. The 28 diets were formulated to contain 96.9% of 1 of the 28 sorghum grains and 3.1% of vitamins and minerals. Eighty-four barrows (36.5 +/- 3.4 kg initial BW) were used in a randomized complete block design conducted during 2 consecutive periods (n = 6 per treatment) with the pigs fed different diets during the 2 periods. For each period, pigs were placed in metabolic crates for a 5-d total collection of feces and urine following 7 d of adaptation to the diets. Among the 28 samples, GE, CP, and ADF content (DM basis) ranged from 4,363 to 4,496 kcal/kg, 8.26 to 13.41%, and 3.26 to 5.78%, respectively. The average tannin content was 0.08, 0.84, and 1.29% in the LTS, MTS, and HTS samples, respectively. The DE and ME content (DM basis) ranged from 3,861 to 3,989 and from 3,784 to 3,905 kcal/kg, respectively, in LTS; from 3,646 to 3,740 and from 3,588 to 3,669 kcal/kg, respectively, in MTS; and from 3,481 to 3,617 and from 3,418 to 3,538 kcal/kg, respectively, in HTS. The mean DE, ME, DE/GE, ME/GE, and ATTD of GE in MTS were less than those in LTS (P < 0.05) and greater than those in HTS (P < 0.05). The correlation coefficients between DE, ME, or the ATTD of GE and tannin content were all -0.96 (P < 0.01). The best-fit equations for DE and ME (kcal/kg DM) for the complete set of 28 sorghum grains were 6,974 - (236 x % tannin) - (43.27 x % ADF) + (24.96 x % CP) - (0.71 x % GE) for DE and 3,973 - (262 x % tannin) - (27.24 x % ADF) for ME. Our results strongly indicate that the DE and ME content of sorghum grains are related to their chemical composition, especially their tannin content. The DE and ME content could decrease by more than 200 kcal/kg when the tannin content is increased by 1% in the sorghum grain. Overall, tannin is a key predictor for the DE and ME content as well as the ATTD of GE in sorghum grains, and the potent antinutritive properties from tannin should not be ignored when sorghum is used as an energy source for pigs.