• Atrazine and fertilizer effects on Sandhills subirrigated meadow

      Brejda, J. J.; Moser, L. E.; Waller, S. S.; Lowry, S. R.; Reece, P. E.; Nichols, J. T. (Society for Range Management, 1989-03-01)
      Many Nebraska Sandhills subirrigated meadows have shifted to predominantly cool-season grasses. Meadows are often cut in July when forage quality of cool-season is lower than that of warm-season species. The objective of this research was to evaluate a one-time application of atrazine [6-chloro-N-ethyl-N'-(1-methylethyl)-1,3,5-triazine-2,4-diamine] in restoring dominance of warm-season grasses and to determine if nitrogen (N) with and without phosphorus (P) would enhance or prolong the atrazine effect on species composition, yield, crude protein, and in vitro dry matter digestibility (IVDMD). Atrazine was applied once at 0, 2.2, and 3.3 kg/ha in spring or fall 1983 and 1984 to a Gannett fine sandy loam (coarse loamy mixed mesic Typic Haplaquoll). The year after atrazine application one-half of each spring-treated plot was fertilized with N (50 kg/ha). One-half of the fertilized area received P (18 kg/ha). Both spring and fall applied atrazine decreased cool-season grass species composition and yield. Spring-applied atrazine reduced first-year yields, but yields recovered by the end of the second year. Conversion of plots to warm-season grasses increased crude protein in mid-July for 2 growing seasons. Percentage IVDMD was increased the year of atrazine application on both sets of plots and also the year following application on the 1983-treated plots. A single fertilizer application did not enhance or prolong the effect of atrazine on forage quality. Cool-season grasses regenerated in atrazine-treated plots after 2 years so changes in yield and quality were only temporary, making atrazine use in subirrigated meadows uneconomical.
    • Experimental evaluation of the grazing optimization hypothesis

      Williamson, S. C.; Detling, J. K.; Dodd, J. L.; Dyer, M. I. (Society for Range Management, 1989-03-01)
      The herbivore grazing optimization hypothesis predicts an increase in aboveground net primary productivity (ANPP) at a moderate grazing intensity. The hypothesis was tested by grazing controlled densities (0 to 145 individuals/m2) of big-headed grasshoppers (Aulocara elliotti Thomas) for short time spans (7 to 13 days) on enclosed swards (0.7 m2) of blue grama [(Bouteloua gracilis) (Willd. ex H.B.K.) Lag. ex Griffiths]. ANPP of each of 257 experimental enclosures was estimated following regrowth by using a standing crop index (the product of mean total blade length per tiller and percent basal cover) after the grazing period and clipping after the regrowth period. ANPP was not significantly reduced by grazing in any of the 5 short-duration grazing experiments. In 2 of the 5 experiments, ANPP increased significantly with grazing. In 1 of the other 3 experiments there was evidence for the grazing optimization hypothesis.
    • Observations on biomass dynamics of a crested wheatgrass and native shortgrass ecosystem in southern Wyoming

      Redente, E. F.; Biondini, M. E.; Moore, J. C. (Society for Range Management, 1989-03-01)
      Above- and belowground net primary production (ANPP and BNPP) were compared between a 30-year-old crested wheatgrass site and an adjacent native shortgrass prairie. ANPP was estimated using successive harvests in May, June, July, and October 1985. BNPP was estimated using soil cores to a depth of 100 cm at the same time that aboveground harvests were made. ANPP was significantly greater in the crested wheatgrass site compared to the native site, but belowground and total net primary production were not different. The native shortgrass system, however, had greater live root biomass early in the growing season. The crested wheatgrass system had a high accumulation of aboveground dead material at the start of the growing season, which was followed by a significant decline in June and an increase in July and October. The native shortgrass system, however, had significantly lower accumulations of aboveground dead material. Approximately 92% of the fixed carbon in the native site was allocated belowground, while crested wheatgrass allocated about 85% of its fixed carbon belowground.
    • Silicon in C-3 grasses: effects on forage quality and sheep preference

      Shewmaker, G. E.; Mayland, H. F.; Rosenau, R. C.; Asay, K. H. (Society for Range Management, 1989-03-01)
      Silicon in forage reduces dry matter digestibility and may reduce grazing preference. Two studies were conducted with the following objectives: (1) to evaluate a method of determining grazing preference, and (2) to characterize the distribution and solubility of silicon in 31 accessions of C-3 grasses and relate these traits to grazing preference and estimated forage digestibility. Forage samples were clipped at the beginning of each 7 to 10-day grazing period corresponding to 6 phenological stages of the Agropyron sp. Samples were washed and analyzed for acid detergent fiber (ADF), neutral detergent fiber (NDF), and silicon in ADF and NDF residues. Leaf silicon concentrations increased from the vegetative to seed-ripe stage. Genera were aligned into 3 groups based on the increase in leaf silicon concentration with advancing phenological age. Silicon concentrations in leaves of Agropyron, Pseudoroegneria, and Thinopyrum increased at nearly twice the rate of those in Critesion, Hordeum, Leymus and Psathyrostachys. Elymus leaves contained higher concentrations of silicon at the vegetative stage than the other groups, but the accumulation rate was intermediate. About 32% of total leaf silicon remained in NDF and 76% in ADF residues at the vegetative stage. These insoluble portions of silicon increased with aging. Preference was positively related to estimated dry matter digestibility at boot and anthesis, but was not related to fiber or silicon measurements. Leaf harshness was negatively related to preference at seed-ripe stage. Further progress in characterizing the role of silicon in C-3 forage grasses should be possible by studying a representative species from each group.
    • Some effects of a rotational grazing treatment on cattle preference for plant communities

      Walker, J. W.; Heitschmidt, R. K.; Dowhower, S. L. (Society for Range Management, 1989-03-01)
      Rotational grazing is commonly assumed to improve livestock distribution compared to continuous grazing, but little evidence supports this contention. Research was conducted on the effects of rotational grazing (RG) compared to continuous grazing (CG) on the preference of cattle for plant communities. Different livestock densities in the RG treatments were created by varying the size of paddocks in a 465-ha, 16-paddock, cell designed RG treatment stocked at a rate of 3.6 ha/cow/yr. Paddock sizes of 30 and 10-ha were used to simulate RG with 14 (RG-14) and 42-paddocks (RG-42), respectively. The CG treatment consisted of a 248-ha pasture stocked at 5.9 ha/cow/yr. Data consisted of hourly daylight observations of cattle location and activity during 8 seasonal trials lasting 6-15 days. These data were expressed as a percent of the time cattle were observed in each of 4 plant communities and the area surrounding permanent water. Relative electivity (RE), a preference index, and a selectivity index (SI) that measures departures from random distribution were calculated from these data. Relative electivity (i.e., preference) for plant communities was not affected by grazing treatment. However, cattle were less selective for plant communities as livestock density decreased from the RG-42 to the CG treatment. In the RG-14 treatment, the cattle were either unaffected or less selective on the last day than on the first day in a paddock. We hypothesize that grazing systems influence cattle preference for plant communities by affecting the availability of forage biomass per unit land area rather than by their effect on grazing pressure.