Whole wheat grain feeding of lambs. III. Rumen metabolic responses and animal physiological adjustment to mineral buffer supplements

Abstract

An intensive 17-week study was made of nine rumen-cannulated crossbred lambs off pasture when given mineral buffer supplements as they were changed from lucerne chaff diets to diets of whole wheat grain plus 2 % of ground limestone. Diets were given ad libitum. Buffers were supplied during transition (period 11) and after transition (period III). The buffers were a 1/1/1/1 mixture of powdered disodium hydrogen phosphate, sodium bicarbonate, potassium bicarbonate, and calciuns hydrogen phosphate given as 2% of the grain offered (group B) and powdered alumirn~~m hydroxide (Al(OH)3) given as 1 % by weight of the grain ofiered (group C). The wheat gram diet was low in sodium (0 008%) and contained 1.50% nitrogen. Production responses to both mineral buffer treatments resulted. Soth buffer treatments increased the voluntary intake (P < 0.01) of wheat above that of the control animals. The rate of eating (g/hr) and pattern of eating did not differ significantly between groups, or between periods I (lucerne chaff) and 111 (post-transition). Groups B and C increased in liveweight to a greater extent than did control animals (P < 0 01) and had heavier carcasses (P <0.01). Results with the mineral mixture given to group B (above) confirm our earlier report. Ingestion of aluminium hydroxide was associated with abomasal ulceration. No significant differences were found between groups in any period for total volatile fatty acids in rumen liquor. Towards the end of period 111, animals given buffer had lower proportions of acetic and higher proportions of propionic acid than did control animals (P < 0.01). Hexanoic acid was detected in all rumen liquor samples obtained in period 111, sometimes in amounts equal to 17 % of total volatile fatty acids. Lactic acid was not detected in rumen liquor in period I, but was present in large amounts in all lambs during period 11, and in lesser but substantial amounts in all lambs during period III. Levels of lactic acid in the rumen liquor in animals receiving mineral buffer supplements fell at a faster rate during period I11 ?hall did levels in control animals. There was a significant decline in rumen pH values for all groups from period I to period III (P < 0.01). Significant rumen pH differences attributable to mineral buffer treatment were apparent (P < 0 05). Buffers were associated with a less acid rumen condition than found in control animals. Relative to control animals, animals given mineral buffers showed increases in rumen buffering capacity and salivary flow rates, and an increased buffering index of the total mixed saliva. Period effects were apparent. The saliva flow rates of control lambs fed on wheat were significantly increased compared with their flow rates on the roughage diet of lucerne chaff. In general, supplementation with aluminium hydroxide and with the mixed buffer respectively yielded similar patterns of response. It is concluded that responses were not primarily due to the sodium level of the diet per se, and that alterations in the animals' electrolyte and acid-base physiology induced by buffer treatment might be contributory factors. Various possible physiological mechanisms are discussed.