"Buckwheat" and # 4 are both Larrea tridentata
#5 is a Chamaesyce sp.
#6 looks like Croton (Eremocarpus" setigera.

I agree wtih nonmember, #6 looks like turkey mullein. http://www.ipm.ucdavis.edu/PMG/WEEDS/turkey_mullein.html

I've found the UC Davis site helpful in identifying weeds. http://www.ipm.ucdavis.edu/PMG/WEEDS/broad_preview.html

#5 & #6 are weeds and not natives
Check the link below to check out the natives that occur in your region and to see their images.
Buckwheat is definitely the Larrea species.
#1 could be a Senecio or an Artemis or ???
#2 a Purshia ???
#3 is Equisetum arvense and SHOULD BE REMOVED as could be toxic to your horses.
#4 looks like a Rhus but probably something else but not Larrea as it is old enough to be flowering as the Buckwheat.

Here is a link that might be useful: High Desert Gardens

terrestrial_man, what would you say #6 is then? Sure looks like http://www.ipm.ucdavis.edu/PMG/WEEDS/turkey_mullein.html