Information on greenhouse gas (GHG) emissions in organic crop production using sheep (Ovis aries L.) grazing to control weeds is lacking. We examined GHG emissions from May 2013 to April 2016 under wheat (Triticum aestivum L.)-based sequences in organic and conventional crop productions in the northern Great Plains. Organic crop production included sheep grazing to control weeds without N application (OCP) and conventional crop production included herbicides, pesticides, and N applications (CCP). Cropping sequences in a 5-yr rotation of safflower (Carthamus tinctorius L.)/sweetclover cover crop [Melilotus officinalis (L.) Lam.]-sweetclover cover crop-winter wheat-lentil (Lens culinaris L.)-winter wheat were lentil after winter wheat (L-W), winter wheat after sweetclover cover crop (W-C), and winter wheat after lentil (W-L). The CO 2 and N 2 O fluxes peaked immediately after tillage, planting, fertilization, intense precipitation, and snowmelt, but CH 4 uptake increased in the autumn. Cumulative CO 2 flux was lower with OCP than CCP for W-C in 2014-2015 and for W-L in 2015-2016. Cumulative N 2 O flux was also lower with OCP than CCP for W-C in 2014-2015, but was greater with OCP than CCP for L-W in 2015-2016. Treatments did not affect cumulative CH 4 flux. The global warming potential (GWP) was lower with OCP than CCP for W-C in 2014-2015 and for W-L in 2015-2016. Organic crop production using sheep grazing with no chemical input reduced GHG emissions under winter wheat following clover cover crop or lentil compared with conventional crop production using chemical inputs under lentil following winter wheat.Abbreviations: CCP, conventional crop production with herbicide, pesticides, and nitrogen applications; GHG, greenhouse gas; GWP, global warming potential; L-W, lentil after winter wheat; OCP, organic crop production with sheep grazing to control weeds but no herbicides, pesticides, and nitrogen applications; W-C, winter wheat after sweetclover cover crop; W-L, winter wheat after lentil.
Cheatgrass (Bromus tectorum L.) is one of the most problematic weeds in western United States rangelands and sagebrush steppe. It responds positively to different forms of disturbance, and its management has proven difficult. Herbicide or targeted grazing alone often fail to provide adequate long-term control. Integrating both may afford better control by providing multiple stressors to the weed. We assessed herbicide application, targeted sheep grazing and integrated herbicide and grazing on B. tectorum and the plant community in rangeland in southwestern Montana from 2015 until 2017. Herbicide treatments included spring-applied (May 2015 and 2016) glyphosate, fall-applied (October 2015) glyphosate, imazapic and rimsulfuron, and spring-applied glyphosate plus fall-applied imazapic. Targeted grazing, consisting of four sheep/0.01 ha for a day in 5 m × 20 m plots (all vegetation removed to the ground surface), occurred twice (May 2015 and 2016). While no treatments reduced B. tectorum biomass or seed production, grazing integrated with fall-applied imazapic or rimsulfuron reduced B. tectorum cover from approximately 26% to 14% in 2016 and from 33% to 16% in 2017, compared to ungrazed control plots, and by an even greater amount compared to these herbicides applied without grazing. By 2017, all treatments except spring-applied glyphosate increased total plant cover (excluding B. tectorum) by 8%–12% compared to the control plots, and forbs were generally responsible for this increase. Bromus tectorum management is difficult and our results point to a potential management paradox: Integrating grazing and fall-applied herbicide decreased B. tectorum cover but did not increase native grass cover, while some herbicides without grazing increased native grass cover, but failed to control B. tectorum. Additional research is necessary to determine grazing strategies that will complement herbicide control of B. tectorum while also stimulating native grass recovery, but this initial study demonstrates the potential of integrated management of B. tectorum compared to grazing or herbicide alone.
Sixty mature ewes (non-pregnant, non-lactating) were used in a completely randomized design to determine if feeding method of pea-barley forage (swath grazing or hay in confinement) had an effect on individual ewe mineral consumption and variation in supplement intake. Thirty ewes were randomly allocated to 3 confinement pens and 30 ewes were randomly allocated to 3 grazing plots. The study was conducted September 25 to October 15, 2010 and September 6 to 19, 2011. Targhee ewes (65.4 ±5.84 kg BW) were used in 2010. Rambouillet ewes (61.9 ±6.28 kg BW) were used in 2011. Ewes had ad libitum access to food, water, and a mineral supplement containing 11 to 12.5% salt with 2% titanium dioxide added as an external marker to estimate individual mineral intake. Forage intake was calculated using estimates of fecal output obtained by dosing gelatin capsules containing 2 g chromic oxide every day for 14 d, and in vitro 48-h DM indigestibility. Fecal grab samples were collected from each individual ewe for a period of 7 d and composited by ewe. Forage and mineral intakes were analyzed using individual ewe as the experimental unit. A year × treatment interaction (P < 0.01) existed for forage DMI and mineral DMI. Ewes in confinement consumed more forage than grazing ewes in 2010 (2.60 vs. 1.86 kg/d, respectively), but less than grazing ewes in 2011 (1.99 vs. 2.49 kg/d, respectively). Mean mineral intake was highest (P < 0.01) by grazing ewes in 2011 and 2010 (average 69 g/d), intermediate by ewes in confinement in 2010 (57 g/d), and lowest by ewes in confinement in 2011 (31 g/d). A year × treatment interaction (P = 0.05) existed for mineral DMI CV. Mineral DMI CV was higher (P = 0.04) for the confinement treatment than the grazing treatment in 2011 (67.2 vs. 33.7%), but similar for confinement and grazing treatments in 2010 (55.4 vs. 46.5%, respectively). In this study, both swath grazing ewes and ewes in confinement consumed more mineral than recommended by the mineral manufacturer and the NRC indicating that more research is needed to develop a better understanding of the factors that regulate and impact mineral intake.
Sheep (Ovis aries L.) grazing on weeds and crop residue during the fallow period may enhance soil carbon (C) and nitrogen (N) through urine and faeces returned to the soil. We compared sheep grazing, tillage, and herbicide application as weed management practices on soil total C (STC), total N (STN), ammonium (NH4+)-N, and nitrate (NO3–)-N contents in a dryland 5-year crop rotation from 2012 to 2015 in the northern Great Plains, USA. The treatments were sheep grazing with no chemical input in organic crop production (GO), minimum tillage with chemical inputs (MT), and conventional tillage with no chemical input in organic crop production (TO). The 5-year crop rotation was safflower (Carthamus tinctorius L.)/sweet clover (Melilotus officinalis L.) cover crop–sweet clover cover crop–winter wheat (Triticum aestivum L.)–lentil (Lens culinaris L.)–winter wheat. At the 0–1.20 m depth, STC was 14–20 Mg C ha–1 greater in GO than MT and TO, but STN was 2.1–2.2 Mg N ha–1 greater in TO than GO and MT. The NH4+-N and NO3–-N contents were 5–21 kg N ha–1 greater in MT than GO and TO. While STC and STN tended to increase with year for all treatments, NH4+-N and NO3–-N contents varied with treatments and years. Sheep grazing enhanced soil C storage, but had a variable effect on N storage and residual N compared to tillage and herbicide application for weed control.
A five-week research project was designed as part of a summer internship for high school students, and could also be used with educators or in introductory undergraduate research courses. This is a guided-inquiry-based project, framed within the significant issue of supplementing fertilizer use in agriculture with nitrogen-fixing microorganisms. This experience exposes students to how scientists are studying real-world problems; it teaches them basic research techniques, and promotes inquiry-based learning in a real research environment. It also fills a current gap in K-12 education that lacks enough microbiology emphasis. Research interns collect soil samples from various fields and use culture-dependent and culture-independent techniques to test whether there are nitrogen-fixing microorganisms that can be isolated and identified in each soil sample. Students work in a research laboratory making nitrogen-free media; culturing, isolating, and identifying microorganisms; extracting soil DNA; and amplifying the 16S rRNA and nifH genes. We administer a pre-test and a post-test, and students present their research both in a short talk and with a poster. By hosting high school students in a research laboratory and immersing them in laboratory science, we hope to inspire them to pursue a STEM-related career.
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