Interpretation of rumen degradability of concentrate feeds with a Gompertzmodel

An investigation of the kinetics and synchronicity of rumen crude protein a nd starch degradability was performed for maize, flaked maize, ensiled maiz e cob, barley, flaked barley, wheat, oat, sorghum and triticale grain, usin g the in situ polyester bag technique. Kinetics of rumen degradability were corrected for particle losses from the bag before degradation. Washing los ses were measured by shaking feed samples in polyester bags in tap water at 20 degrees C for Ih and recovering the particle losses in fibre glass filt er. Mean washing losses of dry matter were 442 g/kg DM (soluble fraction 17 %); mean washing losses of nitrogen were 446 g/kg nitrogen, with 52% being water soluble. Starch escaped to a higher extant from the bags in the washi ng machine (average washing losses were 581 g/kg starch, with a soluble fra ction of 7%). Degradability data for dry matter, nitrogen and starch measur ed at each time of incubation were corrected for the respective particle lo sses and fitted with both first order and Gompertz (sigmoidal) models. The difference between the estimated parameters obtained with the two models wa s negligible, although differences occurred for the immediately soluble fra ction (a) of dry matter and starch and for total degradable fractions of dr y matter, nitrogen and starch. No differences were observed between effecti ve degradabilities, independent of rumen outflow rate (0.04 and 0.08/h), ap art for the mean effective degradability of nitrogen at 0.08/h, which was h igher for the first order model (577 versus 564 g/kg, P < 0.001). The advan tage of using the Gompertz model to interpret the kinetics of rumen degrada bility of cereals was due to the possibility of studying synchronicity of n itrogen and starch released into the rumen and ranking feeds according to t heir degradability pattern, which required the calculation of the first der ivatives of the Gompertz model for nitrogen and starch and weighted for the ir respective amounts in the feeds. The difference of these ratios from the optimum value of 30 mg N per 1 g starch gives the instantaneous synchronic ity of the cereals. Maize, flaked maize, maize cob and sorghum had negative and almost constant values during the first 8 h of incubation. Barley, fla ked barley, wheat, triticale and oat changed from an initially negative val ue to a positive value from about 4 h onwards; oats had a similar pattern, bur a positive value was observed only after 6 fi. (C) 1999 Elsevier Scienc e B.V. All rights reserved.