Fire and clipping effect on purple threeawn (Aristida purpurea) during three phenological stages
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This study was conducted to develop strategies to improve cattle production on purple threeawn (Aristida purpurea) infested grasslands using different defoliations practices at specific phenological stages. Specific objectives included: 1) to determine prescribed fire and clipping effect on purple threeawn survival and change in basal area during three phenological stages.- 2) to evaluate the effect of burning and clipping on forage quality of purple threeawn at three phenological stages, and 3) to generate a specific equation to predict purple threeawn biomass production during three phenological stages. This study was carried out at the Texas Tech University, Native Rangeland located in Lubbock, Texas, during 2010 and 2011. In the 2010 growing season, during vegetative, reproductive and post-reproductive phenological stages, four hundred fifty purple threeawn plants were selected randomly regardless plant size, to be treated with one of the three defoliations types. During each phenological stage one hundred-fifty plants were treated; fifty plants were clipped simulating 90% utilization and fifty plants more were left untreated used as control plants. Phenological stages were: vegetative (June), reproductive (July) and post-reproductive (October). Two, four, and six months after defoliations, I collected forage samples from both treated and control plants to determine forage quality. Forage quality in this study was composed of two variables: crude protein content (CP) and in vitro dry matter digestibility (IVDMD). Basal crown area of defoliated and control plants was measured just after applied treatments. At the end of 2010 growing season we measured basal crown area again in our plants. Percent change in basal crown area was calculated. In addition, mortality was evaluated in defoliated and control plants at the beginning of the next growing season after defoliations. Finally, during each phenological stage 65 plants were randomly selected to measure variables related to biomass production, clipped and weighed to develop models to predict biomass. Experimental treatments to analyze forage quality (CP and IVDMD) were composed of three factors; 1) phenological stage (S); vegetative (V), reproductive (R), and post-reproductive (PR); 2) defoliation types (D); burning (B), clipping (CL), and control (CT); and 3) sampling time after defoliation (T) ; 2, 4, and 6 months after defoliations. Additionally, treatments for mortality and change in basal area analyses were composed of two factors: 1) phenological stage (PS); vegetative (V), reproductive (R), and post-reproductive (PR); and defoliation types (D); burning (B), clipping (CL), and control (CT). We used multiple linear regression to predict biomass production at each phenological stage. Analysis of variance, test of normality and heterogeneous variances were performed on forage quality, basal area change and mortality data. Significant differences between treatments were concluded using Fisher’s LSD test at P<0.05 level. Mortality and change in basal area results might be affected by an above and uniform rain distribution during the 2010 growing season. Threeawn plant mortality and basal area change were significantly affected by phenological stage. This suggests that threeawn mortality in response to severe defoliations depends on plants phenology. Burning during reproductive stage was the treatment combination that promotes the highest plants mortality as well as negative basal area change. Threeawn plants exhibited fire resistant during vegetative and post-reproductive stage at least in a in a short term study. Forage quality of unmanaged threeawn plants is low for cattle production during most of the growing season, during the vegetative stage forage quality can be classified as median levels; once it changes to reproductive it is classified as poor. As result, there is a necessity to increase threeawn forage quality using management tools such as clipping and fire. CP content and IVDMD were significantly affected by an interaction between defoliation type and sampling time at each phenological stage. Clipping and burning treatments proved efficient for increasing threeawn forage quality during each phenological stage, particularity 2 and 4 months after defoliations. Clipping and control treatments stimulated regrowth which in most cases possessed similar forage quality. However, burning plants tend to promote higher forage quality levels. Our forage quality data suggest that fire applied during post-reproductive stage, and deferment during the winter with grazing started at the beginning of the next growing season is the best choice to generate the highest forage quality for cattle in threeawn grasslands, although this advantage is lost in a short period of time, usually no more than 4 months. We generated efficient models to predict threeawn biomass at each phenological stage. These models might be used to calculate fine fuel densities to applied prescribed fires.