Phytomass dynamics and deer and cattle nutrition under different grazing practices in the Texas Coastal Bend

A two-year study was conducted at the Welder Wildlife Refuge in the Texas Coastal Bend to determine the nutritive quality of deer and cattle diets and phytomass dynamics under two grazing systems, (1) continuous yearlong (CYG) and (2) short duration grazing (SDG) at two stocking rates, (1) heavy (HE) (1 AU/2.4 ha/yr) and (2) moderate (MO) (1 AU/4.9 ha/yr). Also, nutrient content of plant species consumed by deer was determined. Deer and cattle diets differed (P<0.05) between grazing systems in crude protein (CP) and digestibility (IVDOM). Dietary CP and IVDOM coefficients were slightly, but significantly, greater under CYG than those under SDG. Values estimated for deer were similar under both stocking rates, while cattle diets were greater (P<0.05) under HE than MO in CP and IVDOM. Also, differences (P<0.05) among seasons were found for dietary CP and IVDOM for both cattle and deer.

Dietary CP levels met maintenance requirements (7%) for deer. Also, CP levels were high enough to meet low- to midgestation requirements (8 to 13% CP). White-tailed deer dietary protein requirements for growth and lactation, however, were never met regardless of grazing strategy. Although protein content of cattle diets was relatively low, these values satisfied cattle maintenance needs. Nursing cows, however, would not have met their requirements in any season sampled regardless of grazing system or stocking rate. Neither were the requirements for cows in their last third of pregnancy met in any grazing treatment.

Grazing systems did not affect (P>0.05) total or forb phytomass during the study. Under CYG, phytomass of desirable grasses and desirable grass-like plants was 13% and 56% greater (P<0.05) than under SDG. Stocking rate affected (P<0.05) total phytomass, desirable grasses, desirable grasslike, and litter. Total phytomass was 14% greater (P<0.05) under MO than under HE stocking rate. All forage classes varied (P<0.05) seasonally. Stocking rates, particularly HE, had a more significant impact on vegetation biomass than grazing system.

Plant groups varied (P<0.05) across the study period for both CP and IVDOM. A grass-like species was highest in CP values; grasses were lowest. Forb and shrub species had similar CP values and were intermediate between grass-like and grass species. Shrub species were 50% less digestible than the grass-like plant. Overall, differences (P<0.05) in digestibility were observed between forbs and grasses. Forb species of particularly high quality were clay violet, Mexican hat, and western ragweed. These plants should be considered as key species when considering a management plan.

The results of this study suggest that when range condition is in a high serai stage, a combination of CYG and HE stocking rate was the strategy that produced the best short-term results in terms of deer nutrition. One option for a management plan in the mesquite mixedgrass habitat could involve one year of continuous yearlong grazing with a heavy stocking rate followed by rapid rotational grazing with moderate stocking.