Acid-base status and serum l-lactate in growing/finishing bull calves fed different high-grain diets

This study evaluated the influence of diet grain mix on the serum acid-base balance and productive performance of calves maintained on high-grain diets in a commercial feedlot system, monitoring progress over the entire 140-day productive cycle (i.e. both the growing and finishing periods). Thirty 14-week-old Belgian Blue bull calves were randomly allotted in equal numbers to one of three experimental groups defined by whether the cereal grain in their diet was predominantly corn (group C), predominantly barley (group B), or predominantly a mixture of corn and barley in approximately equal proportions (group CB). Blood pH, pCO₂, HCO₃ ⁻, base excess and serum l-lactate were determined, as were several productive performance variables. The three groups differed significantly with respect to their weights at the end of the growing period (CB>C>B), but not in final weight; group B gained significantly more weight and had a lower daily intake than either group C or group CB during the finishing period. At no time during the study period was altered ruminal function suggested by either clinical signs or blood parameters, possibly because of the relatively high crude protein (CP) contents of the feeds and the use of barley straw as forage. Nevertheless, HCO₃ ⁻ and base excess were significantly higher in group C than in the other groups during the growing period, suggesting that supplementation of a high-CP corn-based diet with bicarbonate could lead to alkalotic blood base values (group C was the only group to receive bicarbonate supplement in this study). Also, in group CB HCO₃ ⁻ and base excess were generally lower than in the other groups during both the growing and finishing periods, much of the time exhibiting a falling trend compatible with the use of blood bases to counteract overproduction of ruminal acids; this behaviour, which may have been due to the lower dietary fibre content of the CB diet, does not support claims that diets with equal proportions of rapidly and slowly digested starch sources are more beneficial than those including only one of these starch source types. Serum l-lactate levels were almost invariably higher in group B than in group C, with group CB in between, in both the growing and the finishing periods; and a significant timexgroup interaction during the growing period suggests that the time course of serum l-lactate was determined mainly by the influence of diet on microbial growth rates in the rumen. Negative correlation between blood pH and pCO₂ around the switch between growing and finishing regimens suggests that diet may possibly modulate the influence of breed on pCO₂, but further is required to examine this hypothesis.