Environmental impacts on water resources from summer crops in rainfed and irrigated systems.

Irrigation is an intensification technology to increase productivity in agricultural systems, but the impacts of irrigation on the environmental performance of crops are not well understood. We evaluated impacts on water use and quality of rainfed and irrigated systems for corn and soybean production in temperate South America using nonparametric ANOVA tests for small sample sizes. We modeled blue water footprint, ecotoxicity, N and P balance, and eutrophication potential for six farms producing corn and soybean in rainfed and irrigated systems in Uruguay. Crop yields were 5948 and 7862 kg ha-1 for corn and 2482 and 3423 kg ha-1 for soybean, under rainfed and irrigation, respectively. The average blue water footprint for irrigated systems was 264 m3 ton-1 and zero for rainfed systems, with no difference between corn and soybean. The ecotoxicity was greater for soybean than for corn (1679 vs 325 CTUe kg-1) but there were no statistically significant differences in ecotoxicity between rainfed and irrigated systems. Based on Usetox methodology, insecticides had a greater ecotoxic effect (3.2 × 106 CTUe ha-1) than herbicides (7.3 × 104 CTUe ha-1), despite the lower doses applied (insecticides: 0.51 kg ha-1; herbicides: 6.83 kg ha-1). The aquatic eutrophication potential (based on Impact 2002 + methodology) among rainfed and irrigated systems presented no differences (29 vs 24 kgPO4-eq ha-1 for corn and 19 vs 27 kgPO4-eq ha-1 for soybean). The standardized environmental impacts for corn calculated per ha were similar than those per kg of grain when comparing rainfed vs irrigated systems. For soybean, however, standardized environmental impacts per ha were greater in the irrigated than in the rainfed systems, but were similar per kg of grain (except for water footprint). In summary, irrigation resulted in higher productivity and increased blue water footprint than rainfed, but in the set of farms analyzed it did not significantly increase inputs use, so no differences were detected in nutrient balance, eutrophication potential, or ecotoxicity. Soybeans had greater environmental impacts than corn in ecotoxicity and N excess per unit of area, but no statistically significant difference was found in the other indicators. These indicators may be useful as a predictive tool for resource management. Decision makers should consider the trade-offs between productivity, water use, and water quality when using irrigation for intensification of crop production.

Modeling plant interspecific interactions from experiments with perennial crop mixtures to predict optimal combinations.

The contribution of plant species richness to productivity and ecosystem functioning is a longstanding issue in ecology, with relevant implications for both conservation and agriculture. Both experiments and quantitative modeling are fundamental to the design of sustainable agroecosystems and the optimization of crop production. We modeled communities of perennial crop mixtures by using a generalized Lotka-Volterra model, i.e., a model such that the interspecific interactions are more general than purely competitive. We estimated model parameters -carrying capacities and interaction coefficients- from, respectively, the observed biomass of monocultures and bicultures measured in a large diversity experiment of seven perennial forage species in Iowa, United States. The sign and absolute value of the interaction coefficients showed that the biological interactions between species pairs included amensalism, competition, and parasitism (asymmetric positive-negative interaction), with various degrees of intensity. We tested the model fit by simulating the combinations of more than two species and comparing them with the polycultures experimental data. Overall, theoretical predictions are in good agreement with the experiments. Using this model, we also simulated species combinations that were not sown. From all possible mixtures (sown and not sown) we identified which are the most productive species combinations. Our results demonstrate that a combination of experiments and modeling can contribute to the design of sustainable agricultural systems in general and to the optimization of crop production in particular.

Discussion: Prioritize perennial grain development for sustainable food production and environmental benefits.

Perennial grains have potential to contribute to ecological intensification of food production by enabling the direct harvest of human-edible crops without requiring annual cycles of disturbance and replanting. Studies of prototype perennial grains and other herbaceous perennials point to the ability of agroecosystems including these crops to protect water quality, enhance wildlife habitat, build soil quality, and sequester soil carbon. However, genetic improvement of perennial grain candidates has been hindered by limited investment due to uncertainty about whether the approach is viable. As efforts to develop perennial grain crops have expanded in past decades, critiques of the approach have arisen. With a recent report of perennial rice producing yields equivalent to those of annual rice over eight consecutive harvests, many theoretical concerns have been alleviated. Some valid questions remain over the timeline for new crop development, but we argue these may be mitigated by implementation of recent technological advances in crop breeding and genetics such as low-cost genotyping, genomic selection, and genome editing. With aggressive research investment in the development of new perennial grain crops, they can be developed and deployed to provide atmospheric greenhouse gas reductions.

Seasonal Effect on Feed Intake and Methane Emissions of Cow-Calf Systems on Native Grassland with Variable Herbage Allowance.

The aim of this study was to measure methane emissions (CH4) and herbage intake, and, on the basis of these results, obtain the methane yield (MY, methane yield as g CH4/kg dry matter intake (DMI) and Ym, methane yield as a percentage of Gross Energy intake), from beef cows grazing on native grasslands. We used forty pregnant heifers, with two treatments of herbage allowance (HA) adjusted seasonally (8 and 5 kg dry matter (DM)/kg cattle live weight (LW), on average), during autumn, winter and spring. Methane emissions (207 g CH4/d), organic matter intake (OMI, 7.7 kg organic matter (OM)/d), MY (23.6 g CH4/kg DMI) and Ym (7.4%), were similar between treatments. On the other hand, all variables had a marked increase in spring (10.8 kg OM/d and 312 g CH4/d), except for Ym. The methane emission factor from Intergovernmental Panel on Climate Change (IPCC) Tier 2 estimated with these results was 78 kg CH4/head/year. The results show that methane emissions and intake were influenced by the season, but not by the HA analyzed in this study. This information for cow-calf systems in native grasslands in Uruguay can be used in National greenhouse gases (GHG) inventories, representing a relevant contribution to global GHG inventories.

Sustainability of meat production beyond carbon footprint: a synthesis of case studies from grazing systems in Uruguay.

Livestock production has been challenged as a large contributor to climate change, and carbon footprint has become a widely used measure of cattle environmental impact. This analysis of fifteen beef grazing systems in Uruguay quantifies the range of variation of carbon footprint, and the trade-offs with other relevant environmental variables, using a partial life cycle assessment (LCA) methodology. Using carbon footprint as the primary environmental indicator has several limitations: different metrics (GWP vs. GTP) may lead to different conclusions, carbon sequestration from soils may drastically affect the results, and systems with lower carbon footprint may have higher energy use, soil erosion, nutrient imbalance, pesticide ecotoxicity, and impact on biodiversity. A multidimensional assessment of sustainability of meat production is therefore needed to inform decision makers. There is great potential to improve grazing livestock systems productivity while reducing carbon footprint and other environmental impacts, and conserving biodiversity.

Methane Emission and Milk Production of Dairy Cows Grazing Pastures Rich in Legumes or Rich in Grasses in Uruguay.

Understanding the impact of changing pasture composition on reducing emissions of GHGs in dairy grazing systems is an important issue to mitigate climate change. The aim of this study was to estimate daily CH4 emissions of dairy cows grazing two mixed pastures with contrasting composition of grasses and legumes: L pasture with 60% legumes on Dry Matter (DM) basis and G pasture with 75% grasses on DM basis. Milk production and CH4 emissions were compared over two periods of two weeks during spring using eight lactating Holstein cows in a 2 × 2 Latin square design. Herbage organic matter intake (HOMI) was estimated by chromic oxide dilution and herbage organic matter digestibility (OMD) was estimated by faecal index. Methane emission was estimated by using the sulfur hexafluoride (SF6) tracer technique adapted to collect breath samples over 5-day periods. OMD (0.71) and HOMI (15.7 kg OM) were not affected by pasture composition. Milk production (20.3 kg/d), milk fat yield (742 g/d) and milk protein yield (667 g/d) were similar for both pastures. This may be explained by the high herbage allowance (30 kg DM above 5 cm/cow) which allowed the cows to graze selectively, in particular in grass sward. Similarly, methane emission expressed as absolute value (368 g/d or 516 L/d) or expressed as methane yield (6.6% of Gross Energy Intake (GEI)) was not affected by treatments. In conclusion, at high herbage allowance, the quality of the diet selected by grazing cows did not differ between pastures rich in legumes or rich in grasses, and therefore there was no effect on milk or methane production.