EFFECT OF BOUND CONDENSED TANNIN FROM COTTONSEED UPON IN-SITU PROTEINSOLUBILITY AND DRY-MATTER DIGESTION IN THE RUMEN

The effect of adding cottonseed hulls upon the solubility of protein i n unheated solvent extracted cottonseed kernels was studied using both in vitro incubation in mineral buffer and the in situ polyester bag t echnique. The latter technique was also used to study effects on rumen dry matter (DM) digestion. Effects attributable to condensed tannin ( CT) were assessed by making measurements in the presence and absence o f polyethlyene glycol (PEG; mol wt 3500), which binds and inactivates CT. Cottonseed hulls contained 51 g CT kg(-1) DM, with 56 and 20% of t he total CT being bound to protein and fibre, respectively; no CT was detected in kernel. Hulls and extracted kernel contained 33 and 509 g protein kg(-1) DM, and 887 and 289 g fibre kg(-1) DM. In the absence o f hulls, 42% of the total nitrogen (N) in cottonseed kernel was solubl e in mineral buffer in vitro, whilst potential in situ N solubility an d predicted rumen N solubility (corrected for rumen outflow rate) were 99 and 86%, respectively. Addition of hulls linearly reduced both in vitro N solubility and potential in situ N solubility, with 100% hulls addition reducing potential N solubility and predicted rumen N solubi lity to 94 and 79%, respectively. PEG addition had no effect upon the protein solubility of kernels, but increased N solubility in mixtures of hulls and kernels in vitro but not in situ. Two milligrams PEG mg(- 1) total CT was shown to reverse the effect of CT in reducing in vitro protein solubility. Potential in situ DM digestion and predicted rume n DM digestion (corrected for rumen outflow) were substantially lower for cottonseed hulls (41 and 33%) than for kernels (99 and 88%). Incre asing the addition of hulls to kernels lowered the rumen DM digestion of mixtures in a quadratic manner, with increasing rate of hulls causi ng progressively smaller depressions. Addition of PEG had no effect up on the digestion of kernel DM, but increased potential DM digestibilit y and predicted rumen DM digestion of hulls to 47 and 40%, respectivel y, and also produced an increase in mixtures of hulls and kernels. It was concluded that the high protein solubility of unheated solvent ext racted cottonseed kernels can be linearly reduced by the addition of c ottonseed hulls, with the magnitude of the reduction being small, and that the presence of bound CT in hulls substantially depressed fibre d igestion by rumen microorganisms. It is doubtful that CT plays a signi ficant role in the reduction of rumen protein solubility produced by c ottonseed hulls.