A study was conducted from May 1986 to June 1987 with white-tailed deer (Odocoileus virginianus) to determine seasonal nutritive value and nutrient digestibilities of guajillo (Acuciu berlandieri) and a pelleted diet. In vivo dry matter digestibility (DMD) of guajillo varied seasonally from 35.2 to 48.1% and was inversely correlated to levels of condensed tannins in the forage. Apparent protein digestibility varied seasonally from 13.7 to 45.8% and was a highly dependent function (R* = 0.97) of the amount of neutral detergent fiber nitrogen (NDFN) digested and the negative impact of condensed tannins. Cellulose and hemicellulose digestibilities also varied seasonally (0.6 to 13.5% and 52.3 to 71.1%, respectively). Nutrient digestibilities of the pelleted diet did not vary by season, sex, or age. Dry matter digestibility of the pelleted diet was 75.6$$0 f 0.9 and true protein digestibility was 95.0 f 0.04. Results suggest summer is a stressful period for south Texas deer due to low protein and energy digestibility and high levels of condensed tannins.Varner and Blankenship 1987, Barnes et al. 1990). Answers to these questions would allow a more adequate determination of rangeland nutritional carrying capacity for white-tailed deer. Because many ranchers in south Texas feed deer a supplement year round, we also investigated in vivo nutrient digestibilities of a pelleted ration, thus determining the optimum period for supplementation. MethodsThe dietary importance of forage species studied was determined from published studies (Hughes 1982, Varner and Blankenship 1987) and examination of rumen contents of sacrificed deer from Dimmit, LaSalle, Maverick, Uvalde, and Zavala counties, Texas (Varner unpubl. data). Guajillo (Acacia berlandieri) is eaten by deer and cattle during all seasons and may constitute up to 37% of the diet (Hughes 1982, Varner and Blankenship 1987). . Serve110 provided helpful comments on earlier drafts of this manuscript. This is contribution TA239900 of the Texas Agricultural Experiment Station. the pelleted diet was not conducted during the fall because this is not a nutritionally stressful period for deer. Experimental animals were born in captivity, hand-reared on evaporated milk and calf starter, and thereafter maintained on a nutritionally complete pelleted ration supplemented with browse, alfalfa, or native hay, and field corn. A mixture of adult (>2.5 years of age) male and female animals, that were well acclimated to metabolism crates, were used in trials with guajillo because there were no differences in pelleted diet nutrient digestibilities attributed to sex.Deer (N = 6) were confined in 1.2 X 1.2 X 1.2 m metabolism crates that were housed in a climate-controlled room. Temperatures closely approximated outdoor conditions except in summer,
Responses of woody plant communities on native rangelands in the western South Texas Plains to fire are not clearly understood. Our objective was to compare woody plant cover, density, and diversity on burned and nontreated rangelands. Five rangeland sites that received 2 dormant-season burns, 5 rangeland sites that received a combination of 1 dormant-season and 1 growing-season burn, and 5 sites of nontreated rangeland were selected on the Chaparral Wildlife Management Area, Dimmit and La Salle Counties, Tex. Woody plant cover was estimated using the line intercept method, and stem density was estimated in 25-x 1.5-m plots. Species richness did not differ among treatments. Percent woody plant cover was reduced by 50 and 41 % on winter and winter-summer combination burned sites, respectively. Honey mesquite (Prosopis glandulosa Torr.), twisted acacia (Acacia schaffneri S. Wats.), Texas persimmon (Diospyros texana Scheele), lotebush [Ziziphus obtusifolia (Hook.) T. & G.], wolfberry (Lycium berlandieri Dunal), and tasajillo (Opuntia leptocaulis Cand.) canopy cover was greatest on nontreated sites. Woody plant density declined by 29 and 23% on winter and winter-summer combination burned sites, respectively. Density of guayacan (Guajacum angustifolium Engelm.), wolfberry, and tasajillo was less on all burning treatments. Percent cover of spiny hackberry (Celtis pallida Torr.) and density of Texas pricklypear (Opuntia engelmannii Salm-Reif.-Dyck) declined on winter burned sites. Inclusion of summer fire into the burning regime did not increase declines in woody plants. Fire created a post-fire environment which resulted in the decline of many woody plant species. It is unclear to what degree other environmental factors such as herbivory and competition between woody plants and among woody and herbaceous vegetation may have interacted with fire in producing woody plant declines. Fire may be a useful tool in managing woody vegetation on native south Texas rangelands, while maintaining woody plant diversity.
Since the 1950s, many south Texas rangelands have been seeded with buffelgrass, a perennial C4 bunchgrass native to Africa that is believed to contribute to reductions in biodiversity. Forb species represent a critical habitat component throughout the breeding period for many wildlife species as seed (summer to fall), as green vegetative material (spring to summer), and as habitat for arthropods (spring to summer). Reductions in richness and diversity of crucial ecosystem components such as forbs and arthropods have large implications for grassland birds and other wildlife. We sampled annual and perennial forbs within 1-m2 quadrats on 15 study plots (1 ha; n = 20 quadrats/plot) at Chaparral Wildlife Management Area, in LaSalle and Dimmit counties, Texas, during 2005 and 2006. Study plots were divided into five light-buffelgrass plots (0 to 5% buffelgrass canopy coverage), five moderate-buffelgrass plots (5 to 25% buffelgrass canopy coverage), and five heavy-buffelgrass plots (> 25% buffelgrass canopy coverage). Buffelgrass in study plots was composed of naturalized plants, and was not deliberately planted. During 2005 we observed that plots with > 25% buffelgrass had a 73% reduction in forb canopy of native species, a 64% reduction in native forb species richness, and a 77% reduction in native forb stem density compared to plots with 0 to 5% buffelgrass. These trends in native forb reduction (−79% native forb canopy, −65% forb species richness, −80% forb stem density) were nearly identical in 2006, even with greatly reduced rainfall. Simple linear regression revealed negative relationships between buffelgrass cover, total exotic grass cover (buffelgrass and Lehmann lovegrass), and total grass cover and the richness, coverage, and density of forbs/m2. Reductions in diversity may have larger implications regarding ecosystem function and available useable space and densities of desired bird species such as northern bobwhite.
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