As you say, it's extremely difficult to know whether it is the wool or the sheep that are being referred to, because the previous sentence refers to the wool. If you opt for this translation you could continue the sentence as follows:

Its adaptation to the rural environment means that large flocks can be run; hence he maintained that this breed was the only one able to give, under intensive management, the highest yields per hectare ...

Sus cualidades de rusticidad, la hacen apta para manejarla en altas dotaciones, por lo que sostuvo que era la única capaz de dar, en manejo intensivo, los más altos rendimientos por Há; para certificar lo cual exponía sus propios resultados.
http://www.idealuruguay.com/historia.html

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Note added at 4 hrs (2008-06-23 06:55:33 GMT)
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Perhaps "cualidades de rusticidad" would be understood better as "thriftiness" (I know this is not the term asked, but it helps situate it in context). This means that even if the grazing is poor, the flock can grow well on it, in other words it doesn't require a lot of supplementation. Examples of the use of "thriftiness" to describe a flock:

http://www.google.es/search?hl=es&q=thriftiness flock&meta=