The Influence of Cow Age on Botanical Composition of Diets in Mountain Riparian Areas in Eastern Oregon of the United States

Abstract

<p>The objective of this study was to evaluate the effect of cow age on botanical composition of diets in mountain riparian areas. Treatments consisted of 30 first-calf heifers, and 30 mature cows randomly assigned to four pastures (2 pastures/treatment, average 21.5 ha) in a 2-year study with a cross-over design. Botanical composition of diets was determined in fecal samples obtained from 10 animals in each treatment (5 per pasture) on the fourth week of 35 to 42-days grazing periods using the microhistological technique. Crude protein (CP) content and <em>in vitro</em> dry matter digestibility (IVDMD) were determined, and correction factors were calculated for 22 major plant species. In digested samples, grasses were overestimated, whereas, all forbs but northern bedstraw (<em>Galium boreale </em>L.), were underestimated, and all shrub species were overestimated except common snowberry (<em>Symphoricarpos albus </em>L.). Ponderosa pine (<em>Pinus ponderosa </em>Dougl.) was highly overestimated after digestion. There was no difference between cattle age class in the total number of plant species found in the diets (<em>p</em> > 0.10) averaging in 41 species. Most individual grass and grasslike species made up more than 5% of the diets, while all individual forb, shrub, and tree species were minor components, not exceeding 5%, except ponderosa pine. Western wheatgrass (<em>Agropyron smithii </em>Rybd.) and tufted hairgrass (<em>Deschampsia caespitosa </em>(L.) Beauv.) accounted for over 10% of the diets. Heifers consumed more (<em>p</em> < 0.05) grasses and fewer (<em>p </em>< 0.10) shrubs and trees compared to mature cows. The diet of heifers contained more western wheatgrass, Baltic rush (<em>Juncus balticus </em>Willd.), and pinegrass (<em>Calamagrostis rubescens </em>Buckl.) (<em>p</em> < 0.10), but less Kentucky bluegrass (<em>Poa pratensis </em>L.) (<em>p</em> < 0.01), than the diet of mature cows. The proportion of ponderosa pine needles was higher in the diet of mature cows (<em>p</em> < 0.10), than in the diet of first calf heifers. In summary, mature cows appeared to have selected a diet that contained less grasses and more shrubs and trees compared to younger cows.</p>