EFFECT OF CONDENSED TANNIN IN COTTONSEED HULLS UPON THE IN-VITRO DEGRADATION OF COTTONSEED KERNEL PROTEINS BY RUMEN MICROORGANISMS

A 24 h in vitro rumen incubation procedure was developed to measure th e effect of adding cottonseed hulls (containing condensed tannin; CT) upon degradation of the 52 and 48 kDa major seed storage proteins pres ent in unheated solvent extracted cottonseed kernels (which does not c ontain CT). Proteins were fractionated using sodium dodecyl sulphate-p olyacrylamide gel electrophoresis (SDS-PAGE) and the protein bands qua ntified using imaging densitometry. Effects of CT were established by conducting incubations in the absence and presence of polyethylene gly col (PEG; molecular weight (MW) 3500), which binds and inactivates CT. A set of in vitro experiments was conducted, in which degradation rat e and potential degradability were measured. In the absence of hulls, potential degradability of both kernel proteins was very high (99%, SE 0.5), with approximately 97% (SE 0.4) of this taking place within 8 h , and the addition of PEG did not effect degradation. Increasing rates of hull addition reduced the potential degradability of both kernel p roteins in a linear manner, but did not affect degradation rate. Equal weights of hulls and kernel (ie 100% hulls) reduced potential degrada bility of the 52 and 48 kDa proteins by approximately 10% units. Addit ion of PEG increased degradation of both kernel proteins in incubation s involving mixtures of hulls and kernels, with 2 mg PEG mg(-1) CT bei ng required to maximise this effect. However, the increase obtained ac counted for only 50% of the depression in protein degradation caused b y the addition of hulls. In all experiments, the 52 and 48 kDa protein s were similarly affected by the treatments applied. It was concluded that in vitro rumen degradability of the 52 and 48 kDa storage protein s in unheated solvent extracted cottonseed kernel was very high and cl ose to 100%, and that this could be reduced by the addition of hulls i n a linear relationship, with approximately half of the depression in potential degradability caused by hulls being due to effects of CT. Ho wever, even in the presence of hulls, the degradation of the 52 and 48 kDa proteins was still very high.