CONICET | Buscador de Institutos y Recursos Humanos

Experimental evidence suggests that ruminant animals are capable of selecting a nutritionally balanced diet from complementary foods. Hence, we hypothesized that when calves are offered different combinations of complementary foods, diet selection would converge into a balanced one. The objective was to determine and analyse diet selection of calves when they are offered different combinations of the same complementary foods. Five calves were fed different combinations of alfalfa-grass hay and maize grain in two separate feed bunks. In Treatment 1, food A and food B had the same composition, 50% hay and 50% grain (T50:50). In the rest of the treatments the combination of hay and grain in food A and food B respectively were as follow: 62.5 37.5% and 37.5 62.5% (T63:37); 75.0 25.0% and 25.0 75.0% (T75:25); 87.5 12.5% and 12.5 87.5% (T88:12); and 100.0 0.0% and 0.0 100.0% (T100:0). Animals were exposed to the five treatments in a 5 x 5 Latin square design. Intake data were analysed according to a complete Latin square design with repeated measures. Crude protein intake was similar for all treatments (P=0.28), but a significant increasing intake of food B (i.e. the food with higher grain content) across treatments (P=0.01) leaded into a significant linear tendency (P<0.01) towards decreasing the protein: energy ratio of the diet from T50:50 to T100:0. Calves selected a consistent diet when they were in the same treatments, but they did not converge into a nutritionally balanced one across treatments. Grain consumption across treatments was higher than expected leading to protein levels in the diet below than those predicted for a balanced diet (19.38 vs. 16.67 (P=0.004), 20.53 vs. 16.10 (P=0.001) and 20.23 vs. 16.79 (P=0.015) g kgMW-1 d-1 were the predicted vs. observed values for T75:25, T88:12 and T100:0 respectively). Even though choice alternatives allowed calves to integrate a balanced diet, when they were offer higher grain foods on choice (more than 62.5 % of maize grain), observed failures in diet composition appears. Evolved mechanisms to control energy intake under natural conditions could fail to operate in artificial environments involving high energy-density foods (e.g. maize grain).