A meta-analytical evaluation of the regulation of voluntary intake in cattle fed tropical forage-based diets

The objective of this work was to use a meta-analytical approach to evaluate the regulation of voluntary intake in cattle fed tropical forage-based diets using dietary digestibility and bulkiness. Treatment mean values were collected from 53 studies with lactating dairy cows and 126 studies with feed-lot growing and finishing cattle that were published in Brazil from 1990 to 2012. In all studies animals were fed ad libitum with diets based on tropical forages. The dataset was analyzed according to meta-analysis techniques, using linear and nonlinear mixed models. The main independent variables were the D-value and the B-value of the diets. The D-value corresponds to the dietary content of digested OM and is associated with energy content of the diet. The B-value is defined as the undigested fraction of NDF and indicates bulkiness of the diet. The DMI presented significant associations with both D-value and B-value. In the first case, DMI intake responded quadratically to the D-value (P = 0.042). On the other hand, DMI decreased linearly as the B-value of the diet increased (P = 0.004). Two new variables called Di and Bi were created to express the intake in terms of D-value and B-value units. A conceptual explicative model was developed from Di and Bi. This model showed that DMI is simultaneously regulated by both physical constraints and metabolic feedbacks. There were differences in the intake pattern of lactating cows and growing cattle.