| In the present study,12 feedstuffs commonly used in Chinese beef cattle feeding were selected to study the digestibilities, net utilization efficiency and degradation of crude proteins using in vivo digestion and metabolism test and in vitro tests including in situ, gas production and enzymatic method. Also the relationship between net protein utilization measured in vivo and protein degradation measured in vitro was established to provide a useful method for evaluation of protein values of beef cattle feedstuffs.Experiment 1. Using a digestion and metabolism trial, digestibilities and net protein utilization (NPU) of 12 feedstuffs from Exp.1 were determined. The result showed that, the highest protein digestibility (89.86%) and NPU (82.41%) were obtained in soybean meal, while the lowest CP digestibility(33.24%) and NPU (29.03%) were corn stalks, respectively. There was a high correlation between the NPU (Y) and the digestibility (X):Y= 0.9085X-0.0044 (R2= 0.9769, P<0.05), in which equation for energy feeds was Yenergy= 0.9502X-0.016 (R2= 0.9833, P<0.05), and that for roughage feed was Yroughage= 0.9174X-0.015 (R2= 0.9925, P<0.05), while the correlation for protein feeds was not significant (R2= 0.7017, P> 0.05). In summary, the digestion and metabolism test can accurately determine the digestibility and NPU of feed protein, and the digestibility and NPU showed a significant correlation.Experiment 2. An in situ technique was used for measuring ruminal protein degradability (ISPD) of 12 feedstuffs from Exp 1 and the correlation between ISPD and NPU. The results showed that there was a significant correlation between ISPD (X) and in vivo NPU (Y) with the equation of Y=1.6266X-54.54 (R2=0.7690, P<0.05). The correlation equations for energy feed, protein feed and roughage feed were Yenergy= 1.4179X-45.85 (R2=0.8331, P<0.05), Yprotein= 1.5617X-43.98 (R2=0.7232, P<0.05), Yroughage= 4.0166X-198.62 (R2=0.8955, P<0.05), respectively. In summary, the in situ method for ruminal degradation measurements had highly correlated values with in vivo NPU, suggesting the replacement for in vivio method for fast evaluation of the protein nutritional value of cattle feedstuffs.Experiment 3:A noval method, named as Bang-Bang method (BB) for determination of the in situ ruminal protein degradability of 12 feedstuffs from Exp.1 was developed to establish the correlation of ISPD between the traditional nylon bag method (NBM) and BB-method, and correlation between ISPD from BB-method and in vivo NPU. The results showed that:(1) the "a" value of the protein degradation parameters determined by the BB-method was higher than that of traditional NBM (P<0.05), and there was no significant difference in the "c" value of the feedstuffs (P>0.05). Moreover, the effective degradability (ED) measured by BB method (72.95%) was higher than of traditional NBM (70.59%); (2) there was a positive regression between BB method (X) and NBM (Y) with the equation of Y= 0.7801X+12.939, R2=0.8445. The BB method is a special designed apparatus for measuring the protein degradability and can be used instead of the traditional NBM. (3) The regression between ED (Xi) of energy feed, protein feed and roughage measured by BB method and the NPU were:Y=1.8312X1-83.85 (R2=0.9130, P<0.05), Y=0.9270X1+1.1745 (R2=0.9367, P<0.05), Y=1.4251X,-44.76 (R2=0.8564, P<0.05).Experiment 4. Based on the correlation between gas production and ammonia incooperated into rumen microbes, the ruminal protein degradability (IVPD) of 12 feedstuffs was determined by the gas production method in vitro, and the regression equations between IVPD, ISPD and NPU were established. There was a high correlation between in vitro gas production and NH3-N concentration and the highest correlation coefficient was observed at 6 h:Y= 0.1331X+0.133, R2= 0.9896 (P<0.05). There is a high correlation between feed protein degradation rate determined by gas production method (X) and in vivo NPU (Y) and the highest correlation coefficient were observed when incubated 6 h in vitro for three groups of feed (energy, protein and forage), and the correlation were Y= 1.6295X-17.70 (R2= 0.8909, P<0.05), Y= 0.8482X+34.81 (R2= 0.8650, P<0.05), Y= 0.5633X+33.20 (R2= 0.8517, P<0.05), respectively. There was a certain correlation between the results of protein degradation estimated from gas production method (X) and nylon bag method results (Z).The highest correlation coefficient was observed at 6 h for energy feed and the correlation was Z= 0.3042X+64.58, R2= 0.9781 (P< 0.05). For protein feed there were no significant differences of correlation coefficient of 6,12 and 24 h (P> 0.05). When forage feed incubated in vitro for 12 h, the highest correlation coefficient was observed and the regression was Z= 1.0571X+10.22 (R2= 0.8942, P<0.05). The in vitro gas production method could be used for rapid determination of protein value of different kinds of feed for there were significant correlation between gas production method and NPU, also could replace the nylon bag method for the measurement of protein degradation because of the correlation.Experiment 5. Using an enzymatic method including cellulase-pepsin, the protein digestibility at 6, 12 and 24 h of 12 feedstuffs from Exp.1 was determined and the correlation between protein degradation from the enzyme method, ISPD from in vitro and NPU from in vivo. The results showed that there were significant correlation for the three groups of feedstuff between enzymatic method (X) and NPU (Y) when incubated in vitro for 12 h and the correlation were Y= 0.5991X+10.89, R2= 0.8179 (P<0.05) (energy feed), Y= 0.6106X+28.27, R2= 0.7807 (P<0.05) (protein feed), Y= 1.2354X-30.27, R2= 0.8738 (P<0.05) (forage). There was a significant correlation between enzymatic method (X) and nylon bag method (Z) for energy feed when fermented for 6 h and the correlation was Z = 0.2797X+56.88 (R2= 8711, P<0.05). Significant regression were observed for protein feed and forage when incubated for 12 h and the correlation were Z= 0.2692X+57.07 (R2= 0.8354, P<0.05) and Z= 0.7625X+15.32 (R2= 0.7948, P<0.05). In conclusion, the protein degradation rate measured by cellulase-pepsin method showed significant correlation with nylon bag method and NPU, which could be used for rapid and accurate determination of the degradation rate of the feed protein. |
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