| The initial nutritive value of cotton straw to sheep was evaluated by the methods of Nylon-bag technique, cumulative gas production technique and digestibility trail ,and was compared with that of the corn straw, the rice straw and the wheat straw．The nutritive value of cotton straw treated with chemical materials such as 1%AHP, 1%NaOH, 4%Urea , 4%Urea+4%Ca(OH)2 was also evaluated by the method of Nylon-bag technique. 1 The chemical compositions of cotton straw were determined in Experiment 1.The results showed that the cotton straw contained 9.6%(DM) of CP, 44.1% of Cellulose, 10.7% of Hemicellulose, 15.2% of ADL, 0.63% of Ca, 0.09% of P and 0.03% of free Gossypol．The cotton straw consisted of varying amounts of different parts with different nutritive contents. 2 The effective degradability and ME of the cotton straw were decided by in situ method in Trail 2．Provided that rumen outflow rate was 0.0275/h, the effective degradability(%) of the cotton straw ,corn straw, rice straw ,wheat straw were 33.28,61.47,49.15,37.71, respectively； The ME estimated by the DMD48 of the cotton straw,corn straw,rice straw ,wheat straw were 7.16,10.53,9.55,8.00 respectively．The ME in the cotton straw was low． 3 The cotton straws were treated by 1%AHP, 1%NaOH, 4%Urea and 4%Urea+ 4% Ca(OH)2 in laboratory in Trail 4. The variety of its structural carbohy drates and DMD48 were measured ,and the relationship between DMD48 and structural carbohydrates in straw was investigated．The result showed that the ADL in cotton straw decreased by 3.1(p<0.05) ,0.4 ,0.1 ,1.5(p<0.05) point respectively, and DMD48 increased by 14.9(p<0.05) ,7.2(p<0.05) ,3.4, 4.9 point respectively after chemical treatment．The multiple correlation between DMD48 and its structural carbohydrates was built by following regression equation:DMD48=-3.479×ADL+1.071×CEL+36.553 (R2=0.901, n=20) ．The ADL and CEL in cotton straw played a decisive role in preventing it from being digested in rumen． 4 In Trail 5 .The digestibility of nutrients in the cotton straw on practical condition was measured in vivo .The sheep were divided into three groups. Group 1 was fed with 76.3% of the cotton straw which were eaten ad libitum by Group 2, and the Group 3 were fed with corn silage freely, besides fed by the same amount of concentrate (350.8g). The results showed that the ADI (g) of DM ,OM, CP, Cellulose, HemiCellulose in the Group 2 were 1044±69.42, 968.29±63.68, 139.29±6.18, 286.85±31.39, 122.62±7.89 respectively ,and those in the Group3 were 1097.53±124.93,1003.31±111.41,137.83±12.95,278.11±39.38,222.85±29.07 respectively．Nothing but the ADI of the Hemicellulose of the Group 2 was significantly different from that of the Group3. The digestibility(%) of DM ,OM, CP, Cellulose, Hemicellulose in the Group 2 were 49.01±4.94, 54.46±4.06, 54.35±2.14, 22.78±6.43,49.33±6.09 respectively ,and those in the Group3 were 64.58±5.89, 69.79±4.97, 61.30±6.56, 63.59±6.85, 57.68±6.17 respectively．All was significantly different(p<0.05) except for the digestibility of CP in Group2 compared with Group3． The informations showed that sheep can eat 694g cotton straw with 350.8g of concentrate ,and the digestibility (%) of DM , OM, CP, Cellulose and Hemicellulose in the cotton straw decided by the indirect method though the Group1 and the Group 2 were 34.6, 39.7,43.0, 5.0, 65.8．The Cotton straw can be used for sheep as a roughage.
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