A new liquid marker, cobalt-ethylenediamine tetraacetic acid (EDTA), and two solid markers, chromium (Cr) and cerium (Ce) mordanted plant cell walls, were investigated. Synthesis and methods of analysis are described for the markers. The Cr-and Ce-cell wall complexes were tested for stability to EDTA, hydrochloric acid and rumen microorganisms. Plant cell walls were rendered indigestible by mordanting with Cr and 9 8 9 i of the marker remained on the fibre after a simulated sequence (ill vitro) of digestion. Ce-mordanted cell walls were 35% digestible in vitvo using a rumen culture, and 567; of the marker could be washed off the remaining fibre. Treatment with EDTA removed all Ce and 159; of the Cr. Hydrochloric acid (0.01~) had a negligible effect on the removal of Cr from the cell walls, whereas 0 . 1~ acid removed, on average, lo:/, of the marker. Losses of Cr from the mordant may be related to the quality of the preparation. Co-EDTA was found to be comparable to Cr-E:DTA. The urinary excretion of Cr and Co was 2-37; in most animals except in rabbits, which excreted as much as 30% of the liquid markers in the urine. 42

No abstract

Effect of maintenance and ad libitum intakes on digesta kinetics was studied with six ruminally fistulated cows and six ruminally fistulated wethers to validate the use of sheep as a model of cattle. Complete diets were made up of ratios of alfalfa:cracked corn and soybean meal of 80:20, 55:45, and 30:70. The rate of passage of Cr-mordanted alfalfa and soybean meal in the reticulorumen was negatively related to percentage of concentrate in the diet in both species at low intakes. Passage values of particulate and liquid markers were faster at high than at low intakes in both species for all diets. Rumen liquid volume increased with intake only in the cows on the low and intermediate concentrate diets. No substantial differences were found in particulate passage values between sheep and cattle. However, liquid passage rates from the rumen and the differentials between liquid and particulate dilution rates were higher in cows than in sheep for all diets at both intakes. These results together with those for digestibility data reported in a previous communication suggest that caution should be exercised when extrapolating results from one species to the other.

We tested whether cropping or digestion by Thomson’s gazelles (Gazella thomsoni) constrains daily energy intake under sward conditions normally encountered during the growing season. Distinguishing between these alternatives is important in understanding grass–grazer interactions and modeling grazer energetics. Grazing trials on artificial swards showed that gazelles had a monotonically saturating functional response, but that relationships between grazing rate and forage density changed with grass height. Grazing rate was positively related to biomass on short swards, yet there was no significant relationship for tall swards. Bite mass and bite rate also differed in their relationship to biomass across sward heights, with the strongest relationships being found on short swards. Bite rate and bite mass were inversely related, as predicted by current theory for dense grass swards. Voluntary energy intake on a daily basis was a positive function of the digestible energy content of forage, but a negative function of sward biomass. Therefore, our results indicate that daily energy intake is constrained by digestive processes on swards with biomass >25 g/m2, whereas intake is constrained by cropping processes at lower sward biomass. Our data additionally suggest that variation in bite rate and bite mass with sward height could permit a small ruminant to select high‐quality grass, thereby achieving high energy gain on immature swards.

Effect of maintenance and ad libitum intakes on digestibility of different feed fractions was studied with six ruminally fistulated cows and six ruminally fistulated wethers to validate the use of sheep as a model for cattle. Complete diets were made up of ratios of alfalfa:cracked corn and soybean meal of 80:20, 55:45, and 30:70. The regression coefficient of the line relating organic matter digestibility with proportion of concentrate in the diet was smaller for the cows at ad libitum intake than for the other groups. Increasing the intake caused a decrease in digestibility of different fractions. The depression in digestibility was greater for the 30:70 forage:concentrate diet than for the others. At high intake, digestion values in the cows were less than those in the sheep for all diets. An increase in intake depressed the digestion of cell wall fractions and cell solubles including starch in cows, whereas in sheep, an increase in intake reduced cell wall digestion and to a lesser extent cell solubles, without affecting starch digestion. The digestive physiology of these species is sufficiently different to preclude the use of sheep data in formulating nutrient requirements for cows.