Livestock with heritage genetics may increase chances for simultaneously achieving conservation and agricultural production goals in the American Southwest. Past research shows that compared with conventional Angus x Hereford crossbreds (AH), heritage Raramuri Criollo (RC) typically distribute themselves more widely across Chihuahuan Desert landscapes. Accordingly, RC genetics may help overcome some of the problems associated with localized overgrazing in the region, but a better understanding of why RC range more widely is needed. Hypotheses under investigation pertain to differences in the breeds’ dietary preferences, heat tolerance, and mothering style.
We explored the dietary preferences and heat tolerance of the two breeds during the growing season of 2015. Ten mature cows of each breed grazed two adjacent pastures (1190 ha, 1165 ha) for 28 days in July-August. They were switched between the pastures on the 14th day, and fecal samples were collected on the 14th and 28th days. GPS coordinates recorded at 10-min intervals were overlaid on habitat type maps, and Ivlev’s electivity index (E) was calculated for each habitat type in each pasture. Fecal samples were examined for dietary composition with DNA metabarcoding, and we used ordination and ANOVA to compare the composition of the breeds' samples.
Selectivity of habitat types (E) and dietary composition differed between the two breeds in each pasture; however, habitat selection and dietary composition were surprisingly incongruent. In one pasture, heritage RC showed a stronger preference for shade-free areas of bare ground (ERC = 0.46; EAH = -1.00; P<0.01), but the herbaceous species that grow in those areas constituted a lower proportion of their diets than they did in AH diets. In the other pasture, conventional AH showed a stronger preference for shadier mesquite dune areas (ERC = 0.05; EAH = 0.27; P = 0.01), but they consumed less mesquite than RC did. These results suggest that both breeds were moving more in response to thermal variation than to the availability of particular forage species, and that heat tolerance is an important factor driving differences in the spatial distribution of both breeds, at least in the summer months.