>I'm looking for some help in explaining possible dietary differences
>between horses and cattle/sheep. My research area is stable isotope
>analysis & I need to consider possible causes for the carbon differences
>often seen between horses and these other two grazers. Can anybody
>point me in the right direction for literature references which might
>answer questions such as: do horses graze to a different level of a
>grass (i.e. more or less close to ground level) than cattle/sheep? Do
>they have a higher water requirement, so that they might regularly graze
>closer to watercourses? Are they more likely (in a prehistoric context)
>to have been fed regularly with stored feed? Are they more/less likely
>to feed in shady conditions than cattle/sheep? (I'm particularly
>interested in that question).
Horses have a vastly different and in most respects inferior digestive
system compared to ungulates. They require more and better-quality forage
to survive, and spend considerably more of their time grazing. Their
ability to detoxify secondary compounds is also inferior to ungulates which
affect their selection of plants to graze. Generally speaking they tend to
keep to plants that use structural rather than chemical defences (e. g.
grass) and to eat smaller quantities of a larger number of plant species
(this applies also to e. g. tapirs which have similar digestive systems).
And, yes, horses are more likely to have received supplemental feed in a
prehistoric context. Indeed the need for high-quality fodder and the
consequent costliness of maintaining horses has probably been one of the
main reasons for their generally "aristocratic" associations through history.
As for grazing strategies my limited experience is that sheep crop more
closely than horses which in their turn crop closer than cows. The picture
is rather complicated though. Cows greatly prefer young tender plants and
tend to crop some areas quite closely to keep the grass short there while
more or less ignoring areas with high, rank grass while horses can
apparently handle more fibrous grass and crop more evenly.
Also note that ungulates obtain a considerable part of their nutrition "at
second hand" from the micro-organisms in their rumen. This might well
affect the stable isotope ratios.
It might be interesting to compare with geese which are also grazers but
have even less efficient digestive systems. Generally speaking they can
only use the liquid part of the green plant material and consequently
consume surprisingly large quantities of grazing (and produce equally
surprisingly large quantities of droppings).
This subject is treated at some length by Dale Guthrie in "Mosaics,
Allelochemics and Nutrients" in "Quaternary Extinctions a prehistoric
revolution" (P. S. Martin & R. G. Klein (eds), Arizona University Press
1984). There are more references there.
Tommy Tyrberg
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