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An evaluation of initial rates of digestion in a strain of Alfalfa selected to prevent bloat in grazing ruminants Berg, Bjorn Peter

Abstract

The practice of grazing alfalfa (Medicago sativa L.) is limited by frothy bloat. Experiments were conducted to verify that AC Grazeland Br (ACG), an alfalfa selected for a low initial rate of digestion (LIRD), has an effect on digestion and bloat that does not differ from a parental standard, Beaver (BVR). The rate of maturity of ACG, measured morphologically (mean stage by stem count, MSC), was greater (P< .05) than BVR in the field and in a controlled environment. ACG stems matured at a rate of 0.09 MSC d"1 compared to 0.06 MSC D ⁻¹ for BVR. Kinetic models of in vivo dry matter disappearance (DMD) on fresh stem tips in the rumens of steers showed that DMD rates declined with advancing maturity. The average hourly rate of DMD did not differ (P>.05) between cultivars. The postulated requirement to prevent bloat was a 25% reduction in DMD from that of a conventional standard alfalfa. Immature ACG had a DMD 4.9% lower than immature BVR. Mature ACG had a DMD 22.2% less than the same standard. Gas production (GP) from in vitro incubations of ACG leaves in rumen fluid, decreased with advancing maturity (P.05) with advancing maturity. Maximum rates of GP from fresh leaves did not differ between cultivars. Late bud ACG stems produced less gas (P^.05) than any other stem class including those of BVR. A predecessor of ACG, LIRD-3, was fed to wethers. Its neutral detergent fibre (NDF) was less than BVR (?.05, 42.8 and 44.1%, respectively) when the maturities were equivalent. Digestibility of the LIRD-3 cell wall was similar (P>.05) to BVR. When immature ACG and BVR were fed, bloat frequencies were similar (P>.05). Bloat incidence in grazing steers was reduced (P≤.05) in only one autumn trial of six trials conducted over two years. Bloat distensions in that trial occurred 29.6% less often (P≤.05) and incidents of severe froth and pressure were 55.2% lower (P≤.05) in cattle grazing ACG. Selection for LIRD produced ACG, an early maturing cultivar, with a digestion profile otherwise similar to a standard, BVR. Maturity is more crucial than the cultivar for preventing bloat.

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