Diet selection by calves facing pairs of nutritionally complementary foods
Introduction
Experimental evidence is consistent with the hypothesis that farm animals are capable of choosing a nutritionally balanced diet (i.e. a diet appropriate to their metabolic needs) when faced with imbalanced but complementary foods (Illius and Gordon, 1993, Forbes, 1995). Sheep (Kyriazakis and Oldham, 1993, Cooper et al., 1995), goats (Goetsch et al., 2003), cattle (Atwood et al., 2001), and pigs and poultry (Rose and Kyriazakis, 1991) were able to select a balanced diet when foods, varying in energy and protein content, were offered at choice. It has also been observed that sheep prefer foods high in energy after ingesting foods high in protein, and vice versa, leading to balanced diets (e.g., Villalba and Provenza, 1999a).
Learning of the associations between sensory cues and nutrient yield and the development of transient food aversions appear to underlie the process of diet selection (Provenza, 1996, Provenza et al., 2003). The first mechanism (sensory cues–nutrient yield associations) may allow animals to discriminate foods by nutritional value, whereas the second mechanism (transient food aversions) may allow animals to regulate nutrient intake according to their metabolic needs. During this process, several nutrients may be regulated in order to avoid excesses or deficits in the diet. In this way, diet selection could be seen as a multiple nutrient optimization problem (Forbes, 2003, Simpson et al., 2004).
A particularly powerful means to determine the ability of animals to select a nutritionally balanced diet when they are fed two or more imbalanced but complementary foods, is to expose two or more treatment groups to food mixtures composed by different proportions of the same complementary foods (Emmans, 1991, Simpson and Raubenheimer, 1995). If animals regulate food ingestion to approximate as near as possible the nutrient combination required by them, then this confirms that animals are capable of selecting a balanced diet when offered two or more complementary foods (Simpson and Raubenheimer, 1995, Raubenheimer and Simpson, 1997). This experimental design has been used in diet selection studies with invertebrates (Simpson et al., 2004), chickens (Raubenheimer and Simpson, 1997), rats (Simpson and Raubenheimer, 1997) and humans (Simpson et al., 2003).
The objective of our study was to determine and analyse diet selection of calves when they are fed mixtures made with different proportions of the same complementary foods (alfalfa-grass hay and maize grain). To our knowledge the experimental framework of this study has not been used in nutritional regulation research on domestic ruminants.
Section snippets
Animals and housing
The experiment took place on the Research Station of the Buenos Aires Agriculture Ministry located in Pasman (37° 11′ S; 62° 08′ W), Argentina. Five Angus × Hereford calves (236 kg ± 6 kg) were kept in individual pens (24 m2), with two contiguous plastic feed bunks (20 l) under shelter. Animals had free access to fresh water and a mineral-salt mix. Immediately before penning, calves grazed on a barley pasture (onset of stem elongation) for 10 consecutive days. Then they were introduced into
Food analyses
Table 1 shows the chemical composition and energy concentration of foods A and B offered in each treatment. The total amount of hay and grain (food A + food B) fed to calves was the same for all treatments, but the differences between foods A and B in terms of protein:energy ratio increased from T63:37 to T100:0.
Food selection
There was a significant effect of treatment on food A intake (AI, the food with more hay content) (Table 2). AI showed a significant (P = 0.01) quadratic trend across treatments. It
Discussion
Our results suggest that calves do not select a balanced diet when faced to free-choice situations involving high grain foods. As the differences between foods A and B increased from T50:50 to T100:0, calves tended to select higher levels of food B (the food with more grain content) and lower levels of food A (the food with more hay content). This selective behaviour led to a decreasing tendency in the P:E of the diet (Table 2) and to CPI levels below those predicted for a balanced diet (Fig. 1
Conclusions
Calves fed pairs of imbalanced but complementary foods did not converge into a macronutrient balanced diet. As the difference between complementary foods increased, calves tended to select a diet of lower protein: energy ratio. Evolved mechanisms regulating macronutrient intake under natural conditions may fail to adequately operate in artificial environments involving free-choice feeding of high-energy density foods, leading to selection of unbalanced diets.
Acknowledgements
This research was funded by Research Grant N° PIP-02029 from the Consejo Nacional de Investigaciones Científicas y Técnicas de la República Argentina (CONICET). We thank D. Raubenheimer and J.J. Villalba for valuable comments on the manuscript. Nilda Didoné helped us with the protein and fibre analyses. Technical assistance from C. Ibarra is acknowledged.
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