Precipitation of proteins in the rumen fluid using tungstic and trichloracetic acid after feeding sheep thermally treated or untreated soybean meal

The aim of the study was to quantify the protein degradation in the rumen of thermal treated and untreated soybean meal postprandial using trichloracetic (TCA) and tungstic acid (TA) as precipitants of N-compounds in the rumen fluid. The hypothesis is that the difference between TA and TCA precipitation represents peptides with small molecular weight. In two experiments, castrated male sheep were used for the determination of digestibility, N-balance, determination of passage rate of the solid phase, ammonia-N determination in the rumen fluid and for the precipitation trial. The animals were assigned to two treatments in experiments 1 and 2 and fed ad libitum twice a day with chopped wheat straw treated with a solution of 4% NaOH and supplemented with soybean meal either thermally treated (149 °C for 4 h) (TT) or untreated (UT). Rumen fluid was sampled 0.5, 2.5, 4.5, 6.5 and 7.5 h after the morning feeding. The precipitation of N-compounds was affected by a significant interaction among thermal treatment, sampling time and precipitant. At 0.5 h postprandial, no differences (P > 0.05) in the precipitation for treatment or precipitant were observed. In the TT treatment, TA precipitated significantly more N-compounds in rumen fluid (in mg N/100 ml/g of N consumed) at 2.5 h onwards. There were no differences (P > 0.05) in precipitation of N-compounds in rumen fluid comparing the different postprandial samplings within treatment. Results of ammonia-N determination were in line with those of N precipitation with TA or TCA. At 2.5 h postprandial, ammonia-N was significantly higher in UT than TT fed animals. In conclusion, the punctual accumulation of small peptides was not observed through the precipitation of N-compounds in rumen fluid. In general, more small peptides were precipitated when TA was used as precipitant and soybean meal was thermally treated.