Abstract: 【Objective】The study was conducted to investigate the effects of different dietary rapeseed levels on growth performance, rumen fermentation characteristics and blood biochemical indices in beef cattle to provide a basis for the rational use of rapeseed during beef cattle fattening. 【Method】 The trial has adopted one-factor completely random design. Forty healthy crossbred Simmental bulls at (447±35.67) kg initial live weight and approximately 18-month old were randomly allocated into 4 experimental diets containing varying proportions of rapeseeds in a completely randomized design. Four treatment diets with different inclusion levels of rapeseeds were: control diet with no rapeseed added, 8.7% rapeseed (low rapeseed, LR), 17.4% rapeseed (middle rapeseed, MR), and 26% rapeseed (high rapeseed, HR), respectively. The animals were allowed 10 days to adapt to the diets and experimental measurements were taken in 90 days. Cattle were weighed at the end of the experiment and feed cost was calculated individually. Blood and ruminal content samples were collected and volatile fatty acid (VFA), ammoniacal nitrogen (NH3-N), triiodothyronine (T3), thyroxine (T4) and thyrotrophin (TSH) were estimated in the four treatments using gas chromatography, colorimetric method and radioimmunoassay. Variance analysis was conducted using the general linear model of SAS 9.2. 【Result】The results showed that, no difference in liveweight before slaughtering among treatments (P＞0.05) was observed, the average daily gain (ADG) in MR group was the highest (1.33 kg•d-1), which was higher than that in HR group (P＜0.05), and 6.40% and 7.25% higher than that in control grounp and LR grounp (P＞0.05). Cattle in the HR group consumed the highest dry matter and LR group consumed the lowest, however there was no significant difference among four groups (P＞0.05). With the increase of rapeseed level, feed cost decreased (P＞0.05). Feed to gain ratio (F/G) was the lowest in MR group and the highest in HR group, however no significant difference among the four groups. Rumen pH, NH3-N and volatile fatty acid concentration did not get significant influence by the increased dietary rapeseed levels (P＞0.05), but NH3-N, butyric acid concentration and the ratio of acetic acid and propionic acid tended to decrease, propionic acid concentration tended to increase, and total VFA tended to increase first and then decrease. Serum TSH was significantly decreased when cattle received MR and HR diets, compared with control diet (P＜0.05). Serum AST was significantly increased when cattle received MR and HR diets, compared with control and LR diet (P＜0.05). UN was markedly decreased when cattle received MR diet, compared with control diet, whereas no differences with other groups. No significant differences were observed among all the groups in blood T3 and T4, ALT, creatinine and cholesterol (P＞0.05).【Conclusion】The data indicate that there is no significant effect on rumen fermentation when adding different diet rapeseed levels. Adding 17.4% rapeseed to beef cattle diets had no effect on ADG during fattening period. The metabolism of thyroid and liver could be aggravated when cattle received dietary with rapeseed.

Key words: rapeseed , beef cattle , growth performance , rumen fermentation , blood biochemical indices