What do cattle eat in tropical rangelands? - Implications for animal performance and grazing management

Summary

Cattle are predominantly bulk feeders and in grassy environments like the tropical rangelands that means grass makes up most of their diet. Despite the general dominance of grass in these environments, cattle have an extremely wide diet choice available to them which can have an impact on diet quality and animal production. Non-grass plants can make up a significant proportion of the diet and this can have a large effect on animal performance. This initial exploration of DNA approaches to detecting plant species consumed by ruminants has proved promising. Fairly coarse universal primers were successful in differentiating a wide range of plant species typically found in north-east Queensland rangelands using standard genetic techniques i.e. polymerase chain reaction (PCR), which allows small genetic fragments to be amplified large enough to be detected visually in electrophoresis gels.
​Further refinement of primers used should allow nearly all species in these complex rangeland environments to be differentiated. Initial testing of dung samples from the same paddocks where plant species were collected indicated that plant DNA fragments could be recovered and identified to species level. On the basis of these initial encouraging results a pen study was undertaken where diets of differing numbers of component species were fed to cattle and the dung analysed to determine whether known dietary components could be detected. In the more complex diets, the majority of species (70%) could be detected. Given that this exploratory study used fairly coarse universal primers the fact that not all species could be detected is not a major concern. This study also examined whether the relative amounts of species could be detected in the dung. The relative amount of DNA material in the dung was not at all consistent with that in the diet. These differences could not be explained by differential digestibility of species as it would be expected that digestibility of the species used would be in the range of 45%-65%. It is likely that the universal primers used in this study were differentially amplifying the DNA extracted from dung.