Diet selection by calves facing pairs of nutritionally complementary foods

Experimental evidence suggests that ruminant animals are capable of selecting a nutritionally balanced diet (i.e. a diet appropriate to their metabolic needs) from imbalanced but complementary foods. We tested this hypothesis by offering calves different combinations of the same complementary foods. A 5 x 5 Latin square design was used to assess the effect on diet selection of different combinations (termed as food A and food B) of two complementary foods (alfalfa-grass hay and maize grain) offered in separate feed bunks. In Treatment 1, food A and food B had the same composition, 50.0% hay and 50.0% grain (T50:50). In the rest of the treatments, hay-grain combinations in foods A and B were, respectively: 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). Daily intake data were analysed through repeated measures analysis. Crude protein intake was similar across treatments (P=0.28). An increasing tendency (P=0.01) in food B intake (the food with more grain content) from T50:50 to T100:0, however, led into a decreasing tendency (P < 0.001) in the protein: energy ratio of the diet across treatments. Grain consumption in treatments T75:25, T88:12 and T100:0 was higher than expected, leading to protein levels in the diet below those required for a balanced diet (predicted vs. observed: 19.38 vs. 16.67, P=0.004; 20.53 vs. 16.10, P=0.001; and 20.23 vs. 16.79 g kgMW(-1) d(-1), P=0.015, for T75:25, T88:12 and T100:0, respectively). Although calves were potentially able to select a balanced diet in all feeding treatments, they failed when offered high grain foods at choice (75% of maize grain or more). Evolved mechanisms to regulate macronutrient intake under natural conditions may fail to adequately operate in artificial environments involving high-energy density foods (e.g. maize grain). (c) 2008 Elsevier B.V. All rights reserved.