An evaluation of n-alkanes as markers to estimate dry matter intake, diet selection and solid digesta passage rates in ruminants
Magadlela, Andrew Mtutuzeli.
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Study I (exp 1, 2, and 3) assessed the use of alkanes for estimating diet composition and intake in lambs. In exp 1, 36 lambs at 30% and 45% of projected mature sizes were used in a factorial design with 2 breeds (Suffolk and Scottish Blackface), two sexes (male and female) and three feed treatments [pelleted lucerne (Medicagosativa) or pelleted ryegrass (Lolium spp) alone or both as a choice] to compare alkane-derived estimates of dry matter intake (DMI) and selected diet with direct measurements. In exp 2, to assess diurnal variation in n-alkane concentration in the gut, 6 restricted and 6 ad libitum fed lambs at 45% mature size were used in a split- plot design to compare the ratios of the amount of dotriacontane (C32) to tritriacontane (C33) in the faeces collected at 4 hourly intervals over a 24 hour period. In exp 3, to evaluate the efficacy of dosing once-daily (cf. twice-daily) with C32, 6 once-daily and 6 twice-daily dosed lambs were used in a split-plot design to compare faecal alkane ratios of C33 to C32- Exp 1 suggested general agreement between measured quantities and those estimated using the alkane technique However, at 30% mature size, for the lambs fed on grass only, dry matter intake was slightly overestimated and for those fed on lucerne and grass as a choice, dry matter intake of lucerne was underestimated. Exp 2 indicated that the ratio of the concentration of C3 3:C32 in the faeces was not affected by sampling time and thus no diurnal variation irrespective of whether the lambs were restricted or ad libitum fed. Experiment 3 suggested no difference in the ratios of the concentrations of C33:C32 between the two dosing strategies until the fifth day of dosing. Study II used the alkane pair of C36 and C35 to compare the voluntary dry matter intake of Brachiaria decumbens by 8 dry cows, averaging 301 kg liveweight and 8 lactating Criollo cows, averaging 320 kg liveweight during the wet season and 8 lactating cows averaging 300 kg liveweight and 7 dry cows, averaging 289 kg liveweight during the dry season of Bolivia. Results suggested that this alkane pair can be used to estimate the dry matter of intake of tropical forages, but if the concentrations of C35 are very low the accuracy o f the estimate may be compromised. Studies III and IV sought to validate a conceptual model which proposed the involvement of buoyancy and size in the rate of passage of particulate digesta from the rumen. Study III used 4 ruminally cannulated Scotttish Blackface wethers, fed on pelleted grass ad libitum and on bailed hay ad libitum, in a cross-over design to investigate the rates of passage [expressed as mean retention time in the rumination turnover pool (M RTi), in rumen escape pool (MRT2) and in the whole digestive tract (M RTt)] of n-alkane-coated fibre particles of two sizes (large and small) and two densities (dense and buoyant), in a factorial fashion, in the gastrointestinal tract. The small particle pool had a shorter (¿><0.001) MRTi and MRTt than the large particle pool. In addition, there was a diet and particle size interaction (p<0.005) for MRTi, suggesting that the difference in MRTi between the two particle pools was more pronounced when the sheep were fed on hay. There was no difference in M RT2 between the large and small particle pools. There was a diet and particle size interaction (p><0.001) in M RT2- large particles were held 5 h longer than small particles when the sheep were fed on hay, compared to 2 h when they were fed on pelleted grass. There was no difference in MRTi between the buoyant and non-buoyant particle pools and no dietary influence on the MRTi of the particles. There was a particle size and buoyancy interaction (p><0.05) such that large particles of the dense fraction had a 5 h longer MRTi than smaller ones whereas large particles of the buoyant pool were retained 1.5 h longer than small ones. The non-buoyant pool remained in the tract 4.7 h longer (p<0.001) than the buoyant pool. Study IV used four sizes of a tropical cattle breed, Criollo and one size o f a mature Criollo X Holstein cross in a split-plot design to compare the rates o f passage o f the same fibre particles used in Study III in the digestive tracts of cattle. Across all cattle groups the large particles had longer MRTj than small particles. For all but one group MRT2 for small particles were shorter than those for large particles. For MRTt, the large particles were retained for over 8.5 h longer (p<0.001) than the small particles. There was no effect o f buoyancy on MRTi and M RT2, but non-buoyant particles displayed a 2 h longer (p<0.05) M RTt than the buoyant ones. There was no apparent effect o f animal size or genotype. The 4 studies showed that n-alkanes could be successfully used to estimate DMI, diet selection and solid digesta passage rates in sheep and cattle. Studies III and IV suggested the involvement of buoyancy and size in particulate digesta retention in or passage out o f the rumen, but full support for the proposed model could not be justifiably claimed