2.1 Experimental site and design
Field and greenhouse studies were conducted from 2015 to 2019 at the University of Wyoming Laramie Research and Extension Center (LREC), Laramie, WY. The study comprised 12 weed removal or addition timing (growing degree days [GDD]) treatments in 2015 and 2016 and nine treatments in 2018 and 2019 (Table 1). The study was also conducted in 2017, but severe weather events and an irrigation system failure led to uneven emergence and heavy losses of experimental units, so the 2017 data were excluded from the analysis.
The large-pail design used by Green-Tracewicz, Page, and Swanton (2011) and modified by (Schambow et al., 2019) was used in these studies (Figure 1). This approach prevents any direct resource competition, making it possible to assess the sole effect of reflected light on sugar beet growth. Briefly, 19 L black plastic pails were filled with a potting mix (Berger, BM Custom Blend, Saint-Modeste, QC Canada) leaving about 7.5 cm head-space. A cardboard tube (10 × 122 cm Staples Kraft Heavy-Duty Mailing Tube: Staples Inc., Framingham, MA) of height 7.5 cm and 10 cm diameter was taped (Black Gorilla Tape: Gorilla Glue Co., Cincinnati, Ohio, USA) to a 2-mL white plastic bag and laid on top of the potting mix such that the cardboard tube was in the middle of the pail and flush with the rim of the pail. The plastic prevented roots of the weed from interacting with sugar beet roots (Figure 1). Potting mix was then added onto the plastic and into the cardboard tube.
Three sugar beet seeds (cultivar “BTS60RR27” in 2015 and 2016 and “RR014GEM50” in 2018 and 2019) were planted per pail in the center of the cardboard tube on July 3, 2015; May 31, 2016; June 1, 2018; and June 3, 2019. Emerged seedlings were thinned to one seedling per pail immediately after emergence. Sod of Kentucky bluegrass (Poa pratensis L.), was used to simulate low-growing weeds and planted around the cardboard tube in the weedy treatments (Figure 1).
The treatments were grouped into early-season and late-season treatment series (Table 1). The early-season series included treatments with weeds present at sugar beet emergence and then removed later in the growing season. The late-season series included treatments in which weeds were added at various times during the season and allowed to remain until sugar beet harvest. Treatments were arranged in a randomized complete block with 15 blocks in 2015 and 2016 and 27 blocks in 2018 and 2019.
Growing degree days (GDD) were estimated from daily minimum and maximum temperature [Eqn. 1], where Tmax is the daily maximum temperature (°C), Tmin is the daily minimum temperature (°C), and Tbase is the base temperature of 1.1 °C (Holen & Dexter, 1997; NDAWN Center, 2020). The first weed removal (early-season series) and first weed addition (late-season series) timings corresponded to the sugar beets reaching the two true-leaf stage, which occurred between 297 to 365 GDD after planting, depending on the year (Figure 2). Final harvest occurred between 1181 and 1240 GDD after sugar beet planting.
\(GDD=\left(\frac{T_{\max}-T_{\min}}{2}\right)-T_{\text{base}}\)[1]
Sugar beet was drip irrigated daily until maturity to ensure no yield limitation due to moisture stress. The model weed was also drip-irrigated separately to ensure green growth and optimal reflection of FR light. Sugar beet was fertilized with 85 g pail-1 of 14:14:14:5.5% (N:P:K:S) polymer-coated fertilizer (FlorikoteTM NPK, Florikan E.S.A.-LLC, Sarasota, FL) at planting to ensure slow and continuous release of nutrients throughout the growing season. The model weed was clipped regularly to prevent direct competition for sunlight.
Using the same experimental system, a greenhouse study was conducted from Nov 11, 2018, to Feb 5, 2019. Day and night temperatures were maintained at 22 and 20 °C, respectively. No supplemental light was used, and plants were overhead irrigated twice a day throughout the experiments. The greenhouse study did not include the addition or removal treatments and comprised only two treatments: sugar beet surrounded by grass or no grass (weed-free), to enable detailed leaf measurements under controlled conditions.