Developmental morphology quantification and biomass response to seasonal defoliation of short, mid, and tall grasses of North America
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Abstract
This study was conducted during the 2015 and 2016 growing seasons (June -November) and consisted of two separate experiments. The first study was conducted during the 2015 growing season under greenhouse conditions where developmental morphology and tiller recruitment were evaluated. The second study was a two-year study performed under field conditions where biomass allocation influenced by clipping intensities were evaluated. Main objective was to determine how developmental morphology stage affects biomass production in short, mid, and tall grasses of North America. Grasses evaluated in this study were the short-grass species blue grama (BG), the mid-grass species sideoats grama (ST), the introduced species WW-B Dahl (WB), and the tall grass species switchgrass, where four cultivars were evaluated: Kanlow (KL), Alamo (AL), cultivar (I), and cultivar (II).
At the beginning of 2015 growing season 90 plants were established in individual 19-L pots and grown under greenhouse conditions. Plants were irrigated at rates to simulate Lubbock annual precipitation and adjusted to the growing season (70% of the total). Developmental morphology was evaluated monthly from June to November using the Nebraska system where mean stage count (MSC) index was calculated. One-way analysis of variance was performed at each evaluation date looking at differences in MSC values among species. Experimental treatments were grass species: 1) AL 2), BG 3), ST 4), KL 5), CI and 6) CII, with 15 replications.
MSC was affected (P<0.05) by grass species at each sampling time. AL most of the time presented significantly higher MSC values in relation to BG and ST at each measurement date, higher MSC values indicates that AL completes its growth cycle faster. However, there was no clear difference between BG and ST. Our results indicated that BG and ST expended the same time to move from growth stage to growth stage and kept a low MSC index even late in the growing season.
The field portion of this study was designed to quantify total biomass allocation to main grass structures and how this was affected by developmental morphology and defoliation intensities. At the beginning of 2015 and 2016 growing season 252 plants were stablished in 19-L pots and grown under field conditions. Grasses were exposed to moderate and heavy utilizations, at thee phenological stages. Total plant biomass was harvested at the end of the growing seasons and separate, into aerial tillers, crowns, and roots. Experimental treatments were composed of three factors and several levels by factor. Factors: A was grass species, with 4 levels (BG, ST, KL, and WB). Factor B was clipping intensities, with 3 levels (0%,50%, and 75%.) Factor C) was phenological stage with 3 levels (vegetative, reproductive, and post-reproductive). In total there were 36 treatments with 7 replications per treatment. Response variables evaluated were aerial tillers biomass, crown biomass, root biomass, total biomass, aboveground to belowground ratio, and water use efficient (WUE). Analysis of variance, including test of normality, and homogeneous variances were performed for each response variable. Tukey’s (HSD) test at P<0.05 were conducted to test differences among treatment means.
Biomass accumulation to the aerial tiller portion was significantly higher than crown and roots. Close to 40% of the belowground biomass was crown structures and 60% roots. Switchgrass was the species with lower defoliation tolerance, followed by ST and WB, while BG showed the higher defoliation tolerance. Heavy utilization during the vegetative stage produced under-compensation biomass values while moderate utilization regardless of plant’s morphological stage produced compensatory values. Clipping, regardless of intensity, reduced roots biomass and favored shoot production. The results of this study illustrate the importance of incorporate developmental morphology as a meaningful variable in the design of grazing schemes due to its significant influence in plant biomass response to defoliations.