|Cattle and sheep in mixed grazing. Image: Susan Schoenian|
Domesticated herbivores consume plants differently. Cattle predominantly eat grass, with some weeds and bush/shrub leaves. Sheep eat more weeds, and can be successfully kept on shorter, less mature grass than cows. Goats prefer to browse on leaves. This means that different species can be grazed in the same area (either all at once or at different times in the year) as they will not eat the same plants. This can reduce the chance of damaging the pasture by over-grazing. However farmers have to consider the potential for disease/parasite transmission between species. More info: www.sheep201.info/.
Plants differ in isotope composition because of differences in their metabolism. The most important factors are their photosynthetic mechanism (C3/C4/CAM), nitrogen fixing ability (root microbe symbiosis type) and water-use efficiency (transpiration rate). This last can also differ strongly between different parts of the same plant, and between seasons as humidity and temperature change. Therefore, different domesticated herbivores grazing on the same pasture may consume diets which are different isotopically.