In 99 studies of mule deer diets, some 788 species of plants were eaten by mule deer, and their diets vary greatly depending on the season, geographic region, year, and elevation.
The studies  gave these data for Rocky Mountain mule deer diets:
The diets of mule deer are very similar to those of whitetail deer in areas where they coexist.
Mule deer are intermediate feeders rather than pure browsers or grazers; they predominantly browse, but also eat forb vegetation, small amounts of grass, and where available, tree or shrub fruits such as beans, pods, nuts (including acorns, and berries).

Mule deer readily adapt to agricultural products and landscape plantings.
In the Sierra Nevada range, mule deer depend on the lichen Bryoria fremontii as a winter food source.

The most common plant species consumed by mule deer are:
Among trees and shrubs: Artemisia tridentata (big sagebrush), Cercocarpus ledifolius (curlleaf mountain mahogany), Cercocarpus montanus (true mountain mahogany), Cowania mexicana (Mexican cliffrose), Populus tremuloides (quaking aspen), Purshia tridentata (antelope bitterbrush), Quercus gambelii (Gambel oak), and Rhus trilobata (skunkbush sumac).

Among forbs: Achillea millefolium (western yarrow), Antennaria sp. (
pussytoes), Artemisia frigida (fringed sagebrush), Artemisia ludoviciana (Louisiana sagewort), Aster spp.,
Astragalus sp. (
milkvetch), Balsamorhiza sagittata (arrowleaf balsamroot), Cirsium sp. (
thistle), Erigeron spp. (
fleabane), Geranium sp.,
Lactuca serriola (prickly lettuce), Lupinus spp. (
lupine), Medicago sativa (alfalfa), Penstemon spp.,
Phlox spp.,
Polygonum sp. (
knotweed/smartweed), Potentilla spp. (
cinquefoil), Taraxacum officinale (dandelion), Tragopogon dubius (western salsify), Trifolium sp. (
clover), and Vicia americana (American vetch).

Among grasses and grasslike species: Agropyron, Elymus (wheatgrasses), Elytrigia, Pascopyrum sp. (
wheatgrasses), Pseudoroegneria spicatum (bluebunch wheatgrass), Bromus tectorum (cheatgrass), Carex spp. (
sedge), Festuca idahoensis (Idaho fescue), Poa fendleriana (muttongrass), Poa pratensis (Kentucky bluegrass), and other Poa spp. (
bluegrass).

Mule deer have also been known to eat ricegrass, gramagrass, bromegrass, and needlegrass, as well as antelope brush, bearberry, bitter cherry, bitterbrush, black oak, California buckeye, ceanothus, cedar, cliffrose, cottonwood, creek dogwood, creeping barberry, dogwood, Douglas fir, elderberry, fendlera, goldeneye, holly-leaf buckthrorn, jack pine, knotweed, kohleria, manzanita, mesquite, oak, pine, rabbitbrush, ragweed, redberry, scrub oak, serviceberry (including Pacific serviceberry), Sierra juniper, silktassel, snowberry, stonecrop, sunflower, tesota, thimbleberry, turbinella oak, velvet elder, western chokecherry, wild cherry, and wild oats.
Where available, mule deer also eat a variety of wild mushrooms, which are most abundant in late summer and fall in the southern Rocky Mountains; mushrooms provide moisture, protein, phosphorus, and potassium.

Humans sometimes engage in supplemental feeding efforts in severe winters in an attempt to avoid mule deer starvation.
 Wildlife agencies discourage most such efforts, which may cause harm to mule deer populations by spreading disease (such as tuberculosis and chronic wasting disease) when deer congregate for feed, disrupting migratory patterns, and causing overpopulation of local mule deer populations and overbrowsing of shrubs and forbs.
Supplemental feeding efforts are appropriate when carefully conducted under limited circumstances, but to be successful, the feeding must begin early in the severe winter, before poor range conditions and severe weather cause malnourishment or starvation, and must be continued until range conditions can support the herd.

Mule deer are variably gregarious, with a large proportion of solitary individuals (35 to 64%) and small groups (groups with ≤5 deer, 50 to 78%).
Reported mean group size measurements are three to five and typical group size (i.e. crowding) is about seven.
In 99 studies of mule deer diets, some 788 species of plants were eaten by mule deer, and their diets vary greatly depending on the season, geographic region, year, and elevation. The studies  gave these data for Rocky Mountain mule deer diets:
The diets of mule deer are very similar to those of whitetail deer in areas where they coexist. Mule deer are intermediate feeders rather than pure browsers or grazers; they predominantly browse, but also eat forb vegetation, small amounts of grass, and where available, tree or shrub fruits such as beans, pods, nuts (including acorns, and berries).
Mule deer readily adapt to agricultural products and landscape plantings. In the Sierra Nevada range, mule deer depend on the lichen Bryoria fremontii as a winter food source.
The most common plant species consumed by mule deer are:
Among trees and shrubs: Artemisia tridentata (big sagebrush), Cercocarpus ledifolius (curlleaf mountain mahogany), Cercocarpus montanus (true mountain mahogany), Cowania mexicana (Mexican cliffrose), Populus tremuloides (quaking aspen), Purshia tridentata (antelope bitterbrush), Quercus gambelii (Gambel oak), and Rhus trilobata (skunkbush sumac).
Among forbs: Achillea millefolium (western yarrow), Antennaria sp. (pussytoes), Artemisia frigida (fringed sagebrush), Artemisia ludoviciana (Louisiana sagewort), Aster spp., Astragalus sp. (milkvetch), Balsamorhiza sagittata (arrowleaf balsamroot), Cirsium sp. (thistle), Erigeron spp. (fleabane), Geranium sp., Lactuca serriola (prickly lettuce), Lupinus spp. (lupine), Medicago sativa (alfalfa), Penstemon spp., Phlox spp., Polygonum sp. (knotweed/smartweed), Potentilla spp. (cinquefoil), Taraxacum officinale (dandelion), Tragopogon dubius (western salsify), Trifolium sp. (clover), and Vicia americana (American vetch).
Among grasses and grasslike species: Agropyron, Elymus (wheatgrasses), Elytrigia, Pascopyrum sp. (wheatgrasses), Pseudoroegneria spicatum (bluebunch wheatgrass), Bromus tectorum (cheatgrass), Carex spp. (sedge), Festuca idahoensis (Idaho fescue), Poa fendleriana (muttongrass), Poa pratensis (Kentucky bluegrass), and other Poa spp. (bluegrass).
Mule deer have also been known to eat ricegrass, gramagrass, bromegrass, and needlegrass, as well as antelope brush, bearberry, bitter cherry, bitterbrush, black oak, California buckeye, ceanothus, cedar, cliffrose, cottonwood, creek dogwood, creeping barberry, dogwood, Douglas fir, elderberry, fendlera, goldeneye, holly-leaf buckthrorn, jack pine, knotweed, kohleria, manzanita, mesquite, oak, pine, rabbitbrush, ragweed, redberry, scrub oak, serviceberry (including Pacific serviceberry), Sierra juniper, silktassel, snowberry, stonecrop, sunflower, tesota, thimbleberry, turbinella oak, velvet elder, western chokecherry, wild cherry, and wild oats. Where available, mule deer also eat a variety of wild mushrooms, which are most abundant in late summer and fall in the southern Rocky Mountains; mushrooms provide moisture, protein, phosphorus, and potassium.
Humans sometimes engage in supplemental feeding efforts in severe winters in an attempt to avoid mule deer starvation.  Wildlife agencies discourage most such efforts, which may cause harm to mule deer populations by spreading disease (such as tuberculosis and chronic wasting disease) when deer congregate for feed, disrupting migratory patterns, and causing overpopulation of local mule deer populations and overbrowsing of shrubs and forbs. Supplemental feeding efforts are appropriate when carefully conducted under limited circumstances, but to be successful, the feeding must begin early in the severe winter, before poor range conditions and severe weather cause malnourishment or starvation, and must be continued until range conditions can support the herd.
Mule deer are variably gregarious, with a large proportion of solitary individuals (35 to 64%) and small groups (groups with ≤5 deer, 50 to 78%). Reported mean group size measurements are three to five and typical group size (i.e. crowding) is about seven.
Mule deer