Particle size alterations during neutral detergent fiber (NDF) determination and in situ rumen incubation were analyzed by dry sieving and image analysis. The in situ procedure for estimation of NDF degradation parameters and indigestible NDF concentration used for feed evaluation was thereby evaluated in terms of particle size for a broad range of feedstuffs which typically serve as NDF sources in dairy cow rations. Early and late cut grass silages, corn silage, alfalfa silage, rapeseed meal and dried distillers grains were examined. Treatments were I) drying and grinding of forage samples and grinding of concentrates, II) neutral detergent soluble (NDS) extraction, III) machine-washing and NDS extraction, IV) 24 h rumen incubation, machine-washing and NDS extraction, and V) 288 h rumen incubation, machine-washing and NDS extraction. Degradation profiles for potentially degradable NDF were determined and image analysis was used to estimate particle size profiles and thereby the risk for particle loss. Particle dimensions changed during NDF determination and in situ rumen incubation and variations depended on feedstuff and treatment. Late cut grass silage varied most in particle area among feedstuffs with a decrease of 74 % between 24 h and 288 h in situ rumen incubation. Together with the highest mass proportion (20 %) of particles in the critical zone for escape (smaller than 0.005 mm2 in area) for late cut grass silage after 288 h in situ rumen incubation, this imposes a risk for particle loss during incubation in the bags. Particle area decreased linearly with increasing incubation time. Particle loss during in situ rumen incubation could therefore not be excluded and was likely to vary among feedstuffs.
Journal of Dairy Science, 2013, Vol 96, Issue 7, p. 4601-4614