We know that water is a limited resource, and we want our crops to have free access to it. But that doesn’t always happen because weeds continuously compete for a limited quantity of water and use it for their growth and development. We rarely account for that water loss because most crop-weed competition studies are focused only on crop-yield losses. However, net profit from a farm is not only determined by crop yield, but also the cost-to-benefit ratio. Uncontrolled weeds can add more than $20 per acre in direct irrigation cost (Norris et al. 1996). Therefore, we must consider weeds as significant consumer of water to get a comprehensive assessment of agroecosystems. It will help for estimating cost of weeds to agriculture, the actual benefits and the economics of weed management.

What Do We Know?

More than a century ago, scientists defined the term “water requirement” as the ratio of the quantity of water used by plants to the quantity of dry matter produced during its growth (Briggs and Shantz 1913). This term is used interchangeably with “Transpiration ratio” or “Transpiration coefficient”. Shantz and Piemeisel (1927) and Dillman (1931) conducted pot studies and determined the water requirements of different plant species. In addition to crops, they also studied some weed species, which makes their work quite important for weed science. The water requirement estimates of these weed species are given in Table 1.

In a simplistic approach, we can use these estimates of water requirement to estimate the water expense of weeds. Let’s say, we have two fields with weed biomass of 2,000 lbs/acre in each field. Suppose one field is dominated by common lambsquarters (C₃) and another by redroot pigweed (C₄). Based on estimates by Shantz and Piemeisel (1927), we can calculate that common lambsquarters will use 158,000 gals (2,000 lbs*79 gals/lb) of water, while redroot pigweed will use 74,000 gals (2,000 lbs* 37 gals/lb) to produce the given biomass. This will translate to the water consumption of 5.8 acre-inches for common lambsquarters and 2.7 acre-inches for redroot pigweed (one acre-inch equals 27,154 gals).

Considering the irrigation cost of $5 per acre-inch, it will cost $29/acre in direct irrigation cost for common lambsquarters and about $14/acre for redroot pigweed infestation, given that we replaced all the water used by weed species through irrigation.

The aim behind this basic math is to highlight the fact that the water costs of weeds can be significant, especially under irrigated agriculture. In rainfed systems, water deficit will translate into crop water stress, biomass reduction and potentially yield reduction. However, calculating the accurate water cost of weeds is very complex in cropping systems because many factors, such as cropping system, irrigation cost, weather, weed density and composition, yield goal, etc. govern these interactions.

The bottom line is that water expenses of weeds should be considered in farm economics; however, our understanding on this topic is very limited. The reasons are:

1. Water use of weeds is not a priority because historically, producers have been more interested in economics associated with managing water for their crops.
2. The standard models, principles and techniques for quantifying water use from vegetation are developed and parameterized for homogenous, well-managed and uniform systems such as agricultural systems. These standardized frameworks and techniques cannot be directly used for heterogeneous class such as weeds whose density, growth, ground coverage, and thus overall interactions with wind and energy are not easy to predict.
3. Weeds co-exist and compete with crops, and show complex trade-offs for resources (light, nutrients and water). Additionally, producers are implementing weed suppression and control measures, and are managing irrigation and nutrients for their crops, which further complicates the understanding of crop-weed interactions. Therefore, complexity of the agricultural systems make it challenging to quantify the water use of weeds within cropping systems. 

What Did We Find?

We recently conducted a systemic review on the water use of weeds to compile quantitative estimates of the water use of weed species on a global scale (Singh et al. 2022). We found 23 relevant peer-reviewed published studies with 226 water use measurements of 34 weed species. The authors primarily quantified water use (WU) of weeds using eight water use metrics: (a) water use efficiency, (b) depth-based WU, (c) mass-based WU, (d) volume-based WU, (e) mass-based transpiration flux, (f) molar-based transpiration flux, (g) stomatal conductance, and (h) stomatal conductance (Figure 1).

These estimates have bimodal or multimodal distribution with wide variability in magnitude, which is possibly due to the diverse range of weed species and experimental conditions in these studies. The use of such a wide range of WU metrics itself indicates that there are some pitfalls in quantifying WU of weeds, with the two major shortcomings being:

1. Scientists often do not seem to adhere to the best practices and standardized protocols of measuring, estimating, reporting and communicating water use research.
2. They don’t always report all the essential metadata. Metadata is additional relevant data such as soil properties and environmental characteristics that help in giving a complete picture to effectively interpret water use estimates.

Secondly, throughout the literature, there was significant heterogeneity in water use metrics/definitions/terminologies. If we want to synthesize robust water use estimates, there should be a strong consensus on:

1. Which variables can sufficiently represent weed water use;
2. What definitions/formulations should be used to quantify these variables; and
3. What vegetation footprint and temporal resolution are desirable for effective application.

Overall, there is a need to standardize methods, practices and protocols for quantifying water use of weeds. This will increase the confidence and representativeness of estimates and will make it easier for stakeholders to compare, interpret and utilize water use estimates effectively from a practical standpoint.

How to Best Quantify the Water Use of Weeds?

If we want to characterize the water use of weeds, it should be reported in physical quantities that are consistent with those used for agronomic crops. Weed evapotranspiration (ET) or weed-crop system ET — which is represented as the depth of water during a period, e.g. mm/day — is an appropriate option. It is important to report it at sufficient temporal resolution (daily, weekly) during the whole season to account for any trends or variability within the growing season. It gives additional benefit, from a producers’ standpoint, because they are familiar with terms, and they can compare weed and crop water use and visualize the water penalty of weeds.

In addition, we should report all the essential metadata for effective communication and interpretation, efficient use and facilitate application of this data to the global ecosystems. A good set of metadata variables should include:

• Relevant soil properties such as field capacity, infiltration rate, particle size distribution, permanent wilting point, saturation point, residue cover, water holding capacity, etc., for a complete understanding of soil-plant-atmospheric relations.
• Evaporative demand information to account for how “thirsty” the environment was where water use was measured. The “thirstiness” of the environment drives the water consumption of plants. For example, Shantz and Piemeisel (1927) reported a 37 gals/lb water requirement of redroot pigweed in a normal year (1914). However, it decreased by 10 to 27 gals/lb in an unusually damp and cool year (1915) and increased by four to 41 gals/lb in an unusually hot and dry year (1916).
• Soil moisture conditions at incremental depths in the root zone to communicate whether data were taken during water-stressed or sufficient conditions.
• Weed growth in terms of phenological stages or cumulative heat accumulation to help transfer this data across time and space, because weed growth dictates water consumption.

What are the Directions to Consider for Future Research?

Based on the potential usefulness of water use-related information on weeds for stakeholders, we propose the following suggestions:

1. Develop “weed coefficients” for at least major weed species. By this, we can transfer water use information across environmental conditions. Since 2019, research has been conducted at UNL to estimate evapotranspiration of Palmer amaranth and volunteer corn in corn, soybean and sorghum under center pivot and subsurface drip irrigation systems (Mausbach 2021; Singh et al. 2023). This research will help establish baseline data of water use of economically important weeds in multiple crops under diverse irrigation conditions.
2. Quantify relative rates of weed and crop water use to separate these two quantities.
3. Understand how weed evapotranspiration modifies the microclimate and surface energy balance of agronomic crops.
4. Understand how multiple weed species affect the water use dynamics of agronomic crops at different densities and proximities.
5. Merge water use impacts of weeds with yield penalties to understand their overall impact on crop water use efficiency.

Take-home Messages

• Limited research is available on the water use of weeds with sporadic estimates. What we hope is to generate a season-long dataset that is more accurate, robust and transferable at the same time.
• Researchers don’t seem to follow fundamental requirements, standard protocols and best practices for measuring and reporting water use research. This might be due to disconnect among researchers in weed science and irrigation science. Thus, we encourage interdisciplinary collaboration for the transfer of these standardized water use concepts, metrics, approaches and applications for cash crops to weeds.
• Measured data on water use of weeds hold much significance for comprehensive understanding of hydrologic states in agronomic systems. Therefore, we need to adopt standard definitions, practices and protocols to accurately report this data and increase the reliability and reuse of this data.

For More Information on This Topic, Read This Reference:

• Singh M, Kukal MS, Irmak S, Jhala AJ (2022) Water use characteristics of weeds: A global review, best practices, and future directions. Front Plant Sci 7;12:794090

References

• Briggs LJ and Shantz HL (1913) The water requirement of Plants. I. Investigations in the Great Plains in 1910 and 1911. U.S. Department of Agriculture. Bureau of Plant Industry – Bulletin No. 284. Washington, DC, 7 p
• Dillman AC (1931) The water requirement of certain crop plants and weeds in the Northern Great Plains. J Agric Res 42:187-238
• Mausbach J (2021) Evaluating evapotranspiration and management of glyphosate-resistant Palmer amaranth (Amaranthus palmeri S. Watson). M.Sc Thesis. Lincoln, NE: University of Nebraska-Lincoln. 44p
• Shantz HL, Piemeisel LN (1927) The water requirement of plants at Akron, Colo. J Agric Res 34: 1093-1190
• Singh M, Kukal MS, Irmak S, Jhala AJ (2022) Water use characteristics of weeds: A global review, best practices, and future directions. Front Plant Sci 7;12:794090
• Singh M, Lindquist J, Knezevic S, Irmak S, Kumar V, Jhala AJ (2023) Evapotranspiration of volunteer corn in corn, soybean, and sorghum cropping systems. Page 33 in Proceedings of Weed Science Society of America-Northeastern Weed Science Society Joint Meeting. Arlington, VA: Weed Science Society of America