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Received: 1 April 2025

Accepted: 9 September 2025

DOI: 10.1002/csc2.70179

Crop Science

ORIGINAL ARTICLE S e e d P h y s i o l o g y, P r o d u c t i o n & Te c h n o l o g y

Plant growth regulator effects on seed yield of timothy Surendra Bhattarai1

Hari P. Poudel2

Nicole P. Anderson3

Calvin L. Yoder4,5

1 Department of Plant Sciences, College of

Agriculture, Life Sciences and Natural Resources, University of Wyoming, Laramie, Wyoming, USA 2 Lethbridge Research and Development

Centre, Agriculture and Agri-Food Canada, Lethbridge, Alberta, Canada 3 Division of Food Production and Society,

Norwegian Institute of Bioeconomy Research, Ås, Norway 4 SARDA Ag Research, Donnelly, Alberta,

Canada 5 Peace Region Forage Seed Association,

Dawson Creek, British Columbia, Canada Correspondence Calvin L. Yoder, SARDA Ag Research, Donnelly, T0H 1G0 AB, Canada. Email: calvinyoder123@gmail.com. Assigned to Associate Editor Wei Wu. Funding information Agriculture and Agri-Food Canada, Grant/Award Number: J-002132; Peace Region Forage Seed Association

Abstract Timothy (Phleum pratense L.) is a cool-season perennial forage grass species widely utilized for animal feed and fodder in temperate regions of the world. Timothy is one of the main forage grass crops grown for seed in the Peace Region of western Canada. Timothy performs well under well-fertilized and high-moisture soils, but seed production fields are prone to lodging under such conditions. Lodging in grass crops reduces seed production through self-shading, which limits successful pollination, fertilization, and seed fill. In this study, we investigated the effects of two plant growth regulators (PGRs), chlormequat chloride (CCC) and trinexapac-ethyl (TE), on plant height, lodging, seed weight, and seed yield of timothy for 8 site-years. The study encompassed 5 years at one site with crop stands in their first to fifth year of seed production and 3 years at a second site with crop stands in their first to third year of seed production. The PGRs were applied alone (TE and CCC) and in a mix (TE + CCC) at the 2–3 node (Biologische Bundesanstalt, Bundessortenamt, and Chemische Industrie [BBCH] 32–33) and early heading (BBCH 51–52) growth stages. The application of PGRs (TE, CCC, and their mix) at two different growth stages showed a differential decrease in lodging and plant height and an increase in seed yield in all but 1 site year. Among the PGR treatments, CCC applied at the BBCH 32–33 was the most effective in increasing seed yield and economic returns of timothy. Plain Language Summary Utilization of stem-shortening plant growth regulators (PGRs) to prevent lodging is a relatively new management practice for forage seed crops in Canada. Timothy is a winter-hardy forage grass with very low soil nutrient requirements, except for adequate moisture. Thus, timothy seed production fields are prone to lodging when nitrogen fertilizer is applied. This study examined the effects of two PGRs, chlormequat chloride (CCC) and trinexapac-ethyl, on plant height, lodging, seed weight, and seed yield in timothy crops. Results indicated that CCC applied at the 2–3 node

Abbreviations: BBCH, Biologische Bundesanstalt, Bundessortenamt, and Chemische Industrie; CCC, chlormequat chloride; PGR, plant growth regulator; TE, trinexapac-ethyl. This is an open access article under the terms of the Creative Commons Attribution-NonCommercial-NoDerivs License, which permits use and distribution in any medium, provided the original work is properly cited, the use is non-commercial and no modifications or adaptations are made. © 2025 The Author(s). Crop Science published by Wiley Periodicals LLC on behalf of Crop Science Society of America. Crop Science. 2025;65:e70179. https://doi.org/10.1002/csc2.70179

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growth stage (Biologische Bundesanstalt, Bundessortenamt, and Chemische Industrie 32–33) was the most effective in reducing lodging while increasing seed yield and economic return.

INTRODUCTION

Timothy (Phleum pratense L.) is a cool-season, bunchtype, shallow-rooted, perennial grass commonly grown in the higher latitudes of North America, Europe, and East Asia (Ogle et al., 2011; Radkowski et al., 2020). Timothy is one of the most commonly cultivated forage grass seed crops in the Peace River Region of western Canada. In 2019/2020, Canada exported 3.6 million kg of timothy grass seed worth CAD 9.08 million (Wong, 2020). It is characterized as winter hardy, easy to establish, and highly adapted to acidic and compact soils (Ogle et al., 2011; Radkowski et al., 2020). Timothy is commonly used for hay and silage production as livestock feed due to its high palatability, nutritional value, and digestibility (Jing et al., 2014). Cool-season grasses generally produce a large number of inflorescences, but seed production often falls short of its maximum potential because of insufficient pollination, inadequate fertilization, poor seed filling, and seed losses caused by abortion and shattering (Chastain, 2013). Although grass seed production has seen yield improvements in recent years, further increases are still necessary to improve seed production efficiency and boost economic profitability. Grass seed production is constrained not only by genetic factors but also by environmental and agronomic factors, such as lodging, which reduces both seed yield and harvestability (Bitarafan et al., 2019; Macháč, 2013; Szczepanek et al., 2021). When grasses lodge and the reproductive tillers fall to the soil surface, it limits the plant’s ability to capture solar radiation and reduces the supply of photosynthates to the developing seeds (Clemence & Hebblethwaite, 1984). Lodging during reproductive stages also hampers pollination and fertilization (Boelt & Studer, 2010). The impact on seed yield varies based on the severity of lodging, with reductions as high as 95% reported in perennial ryegrass (Chynoweth et al., 2014). Implementing management practices like the application of plant growth regulators (PGRs) and optimized nitrogen fertilization can promote effective pollination and seed development while preventing lodging, ultimately increasing seed yield and profitability. PGRs with growth retarding effects such as trinexapac-ethyl (TE) and chlormequat chloride (CCC), limit internode elongation of stems and reduce lodging by inhibiting gibberellic acid biosynthesis. This, in turn, helps minimize lodging and seed losses in cool-season grass crops (Rademacher, 2015; Rolston et al., 2010).

When there is a high risk of lodging, PGRs have been identified as a potential solution to reduce stem elongation and limit lodging severity in other cool-season grass seed crops (Bhattarai et al., 2024; Chastain et al., 2014; Chynoweth & Moot, 2017; Rolston et al., 2010; Szczepanek et al., 2021). We hypothesized that the application of PGRs with growth retarding effects will reduce lodging and increase seed yield in timothy. This study aims to assess the effect of two PGRs, TE and CCC, applied individually or in combination (TE + CCC), at the 2–3 node (Biologische Bundesanstalt, Bundessortenamt, and Chemische Industrie [BBCH] 32–33) and early heading (BBCH 51–52) growth stages on lodging, plant height, seed weight, and seed yield in timothy crops. The 2–3 node and early heading stages selected in this study represent critical growth phases in grass development. The 2–3 node stage is a vegetative phase characterized by early stem elongation, where PGR application may influence plant architecture and biomass accumulation. In contrast, the early heading stage is a reproductive phase when lodging risk increases, and PGR application can directly impact standability and seed production. By investigating two PGRs and two application timings, the study seeks to identify optimal management practices for maximizing seed yield and economic return in timothy crops.

MATERIALS AND METHODS

Eight PGR experiments were conducted on grower fields in the Peace River region of western Canada. The experimental sites were located at Sunset House (55˚ 12′5.59″ N, 116˚ 50′57.48″ W) and Whitemud (55˚ 29′52.00″ N, 117˚ 34′54.24″ W). Both sites are dominated by various types of luvisolic soils, which are slightly acidic and sandy loam to clay textured with a shallow surface horizon (Lavkulich & Arocena, 2011). The experiments were undertaken for 5 years (2019–2023), totaling 8 site-years of experimentation. At the Sunset House location, studies were conducted for five seed production years (2019–2023), while at Whitemud location, studies were conducted for three seed production years (2020–2022) (Table 1). Weather data were accessed from the Alberta Climate Information Service, Government of Alberta (www.agric.gov. ab.ca), and collected from meteorological stations near the two study sites: Sunset House (High Prairie—Banana Belt; 55˚ 9′46.8″ N, 116˚ 24′54.72″ W) and Whitemud (Whitemud Creek AGCM; 55˚ 24′15.48″ N, 117˚ 10′46.56″ W). Growing

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season weather summaries for each of the experimental sites are presented in Figure 1. The precipitation and air temperature during the 2020 growing season is close to the long-term average, while 2021 and 2022 were drier and hotter than the long-term average at both site locations. In 2019 and 2023, the Sunset House site received more precipitation during the growing season than the long-term average (Figure 1). All agronomic operations and input uses were as per the cooperating farmer’s standard management practices, which generally included zero-tillage seeding with an intensive herbicidal weed control regime and fall or spring fertilization as recommended by a soil test. Studies were placed in areas of the fields where the timothy stand was uniform. Individual plot size was 3 × 10 m. The timothy variety Comtal was grown at the Sunset House, site while Tuukka was grown at the Whitemud site. Timothy variety Tuukka, selected for the cultivation in the area of cold winters, was released in 1993 in Finland and received national registration in Canada in 1997. Tuukka is characterized by a high leaf-to-stem ratio, rapid regrowth, early maturing, and high forage yield. Timothy variety Comtal received national registration in Canada in 1995. Comtal is late-maturing, winter-hardy cultivar exhibiting disease resistance and high forage yield. There are two PGRs registered in Canada: TE (trade name: Moddus, Syngenta) and CCC (trade name: Manipulator, Belchim) (Health Canada, 2020, 2022). There were a total of seven PGR treatments applied at each site, including an untreated control (no PGR), TE (0.150 kg ai ha−1 ), CCC (1.533 kg ai ha−1 ), and TE + CCC (0.067 kg ai ha−1 for TE + 0.372 kg ai ha−1 for CCC) applied at BBCH 32–33 and at BBCH 51–52 (Gustavsson, 2011) (Table 2). The experimental design was a randomized complete block with four replications. The PGRs were applied with a 2-m handheld boom (4 nozzle TeeJet 8001) pressurized by a propane sprayer. The sprayer and walking speed were calibrated to provide 100 L ha−1 of water at a pressure of 270 kPa. Plant height was measured at physiological maturity (BBCH 91–92) by randomly selecting five reproductive tillers from each plot and measuring them from the base of the stem to the tip of the inflorescence. Visual lodging ratings were taken at flowering (BBCH 65–69) and physiological maturity stages (BBCH 91–92). Lodging was scored on a 1–9 scale (1 = erect stand with no lodging, 9 = fully lodged flat condition) (Berry et al., 2003). Plots were swathed using a Zurn

TA B L E 1 Canada.

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Core Ideas ∙ Timothy performs well under well-fertilized and high-moisture soils, but seed production fields are prone to lodging under such conditions. ∙ Lodging during reproductive stages limits seed yield of timothy. ∙ Plant growth regulators reduce lodging and increase seed yield in timothy seed crops grown in western Canada. ∙ Chlormequat chloride applied at the 2–3 node stage increases yield and economic return in timothy crops.

540 High Clearance Tool Carrier (Zurn Harvesting GmbH & Co. KG, Germany) when seed moisture content reached approximately 40%–50%, which was determined by tan coloration in seed head, and left to dry for 7–10 days. Windrows were then combined using a Wintersteiger Delta plot combine (Wintersteiger Ag, 4910 Ried) when seed moisture content reached approximately 12%. Harvested seed was air-dried at room temperature under shade conditions to achieve uniform moisture content and cleaned to marketable standards using a laboratory-size Clipper AGM 224 (A.T. Ferrell Company Inc.), and clean seed yield was calculated. Note that 1000seed weight was measured using seed counter Data Count S 25+ (Data Technologies, Wintersteiger).

COST–BENEFIT ANALYSIS

A cost–benefit analysis was completed using a PGR application cost of CAD 20.00 ha−1 per application and the cost of PGR products: TE (CAD 44.00 L−1 ) and CCC (CAD 27.00 L−1 ), based on 2023 suggested retail prices. The cost of PGR based on their respective rate of application per hectare is CAD 58.52 ha−1 for TE applied at the rate of 1.33 L ha−1 , CAD 66.69 ha−1 for CCC applied at the rate of 2.47 L ha−1 , and CAD 42.29 ha−1 for TE + CCC applied at the rate of 0.593 + 0.600 L ha−1 . The timothy seed price used in the calculation was CAD 2.00 kg−1 .

Plant growth regulator (PGR) treatments and their application rate applied to timothy seed crops in the Peace River region of western

Treatment

Trade name

Active ingredient

Application rate (kg ai ha−1 )

Moddus

Trinexapac-ethyl

0.150

CCC

Manipulator

Chlormequat chloride

1.533

TE + CCC

Moddus + Manipulator

Trinexapac-ethyl + chlormequat chloride

0.067 + 0.372

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T A B L E 2 Site location, year, stand age, treatment application, and operation time for plant growth regulator (PGR) experiments conducted on timothy seed crops in the Peace River region of western Canada.

Year and location

Stand age (year)

Fertilizer (N-P-K-S kg ha−1 )

Crop stage and PGR application date 2–3 Node

Heading

2019 Sunset House

Spring 70-20-20-10

May 30

June 11

Swathed

Combined

August 22

August 30

2020 Whitemud

Fall 80-0-0-0

June 10

June 19

August 17

August 28

2020 Sunset House

Spring 65-20-25-20

June 10

June 19

August 17

August 28

2021 Whitemud

Fall 90-0-0-0

June 4

June 21

August 3

August 10

2021 Sunset House

Spring 65-20-25-20

May 31

June 13

August 3

August 10

2022 Whitemud

Fall 90-0-0-0

June 5

June 21

August 13

August 18

2022 Sunset House

Spring 75-15-15-15

June 5

June 21

August 15

August 23

2023 Sunset House

Fall 80-15-15-15

May 27

June 7

August 1

August 7

F I G U R E 1 Monthly average air temperature (˚C) and total precipitation (mm) at both Peace River region experimental sites and the long-term averages (1991–2020).

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STATISTICAL ANALYSIS

Data were analyzed using the R software package, version 4.2.0 (R Core Team, 2022). Assumptions of analysis of variance were tested before analyzing the data, any outlier observations were removed prior to analysis, and the data that did not meet the criteria for normality were transformed either by using natural log or square root transformations. Homogeneity of variance was tested using Levene’s test using “leveneTest” function in the “car” package. Normality of data were tested using the Shapiro–Wilks test using the “shapiro.test” function. Data analysis was performed after fitting a linear mixed model using the “lmer” function from “lmerTest” package, where treatments and stand age were used as a fixed effect and replication as a random effect factor. Means were separated using the TukeyHSD post hoc test at p < 0.05 using the “emmeans” package. Since the two site locations utilized different timothy varieties, the data for each location was analyzed separately due to the potential effects of genotype on the results.

RESULTS

The effects of TE and CCC, and their mixture (TE + CCC), applied at two growth stages on timothy lodging, plant height, seed weight, and seed yield were evaluated for 5 years and 3 years at Sunset House and Whitemud locations, respectively (Table 3) (Figures 2 and 3).

5.1

Plant height

Plant height was affected by PGR treatment, production year, and the treatment-year interaction (p < 0.001) at both site locations. Figure 2 shows the effect of PGRs on plant height across both site locations and years. All PGR treatments reduced plant height compared to control at both locations and in all production years despite there being lack of statistical significance in some instances. At Sunset House, as compared to the untreated control, plant height was reduced by 14.5% upon application of TE + CCC at heading stage in the first year. In third year, the application of TE alone or in combination with CCC at both stages reduced plant height by at least 11.4%. In fourth year, application of TE at heading stage and TE + CCC at both stages reduced the plant height by more than 10.7% compared to control (Figure 2). Likewise, in the fifth year, all PGR treatments reduced plant height compared to the untreated control (Figure 2). At Whitemud, there was no difference in plant height in the first year but in the 2nd and third years. In second year it was reduced by >18.1% when treated with TE + CCC at any stage and by 20% when treated with TE at 2–3 node stage (Figure 2). Likewise, in the third year, all PGR treatments reduced plant height compared

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to the untreated control, except CCC applied at heading stage (Figure 2).

5.2

Lodging

Lodging scores at both flowering and physiological maturity were affected by PGR treatment (p < 0.001), year (p < 0.001), and the interaction between treatment and year (p < 0.01) at both site locations (Table 3). In general, the PGR applications led to a reduction in lodging that is consistent across both site locations at both growth stages. Application of PGRs reduced lodging at maturity in the first and second years at Sunset House and first year at Whitemud (Table 3). At Sunset House, during first, third, and fifth production years, no significant difference between the treatments was observed for lodging score at flowering stage, but second and fourth production years showed a significant difference between the treatments (Table 3). Likewise, at Whitemud, the lodging score at the flowering stage differed significantly among treatments in the first and third production years, but not in the second year (Table 3). At Sunset House, the lodging score at maturity differed significantly among treatments in all production years except the third and fifth. At Whitemud, significant differences among treatments were observed in the first and third production years (Table 3).

5.3

Seed weight

Timothy seed weight showed variation among PGR treatments at both site locations. At Sunset House, 1000-seed weight varied among treatments (p < 0.001) and production year (<0.001) (Table 3). Likewise, at Whitemud, 1000-seed weight was influenced by all factors that include treatment (p < 0.001), production year (p < 0.001), and their interaction (p < 0.001) (Table 3). In both locations, the largest 1000-seed weight was observed when TE was applied at the 2–3 node stage in the third production year at Sunset House and second production year at Whitemud (Table 3). At Sunset House, 1000-seed weight ranged from 0.439 to 0.487 g, while at Whitemud it ranged from 0.392 to 0.463 g (Table 3). When observing each production year separately, no significant differences among the seven treatments were found in any of the five production years at Sunset House. In contrast, at Whitemud, significant differences among treatments were observed in the second and third production years (Table 3).

5.4

Seed yield

At the Sunset House location, seed yield was affected by PGR treatments (p < 0.01) and varied across production years

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T A B L E 3 Analysis of variance and mean value of plant growth regulators (PGRs) trinexapac-ethyl (TE), chlormequat chloride (CCC), and their combination (TE + CCC), on lodging (1 = standing, 9 = flat), and 1000-seed weight of timothy crops grown in the Peace River region of western Canada.

Treatment

Lodging score at flowering

Lodging score at maturity

1000-seed weight (g)

Sunset

Untreated control

1.0c

3.3a

0.444bcd

House

CCC (2–3 node stage)

1.0c

2.5ab

0.455a–d

CCC (heading stage)

1.0c

1.8bc

0.444cd

TE (2–3 node stage)

1.0c

1.0d

0.465a–d

TE (heading stage)

1.0c

1.0d

0.451bcd

TE + CCC (2–3 node stage)

1.0c

1.0d

0.461a–d

TE + CCC (heading stage)

1.0c

1.0d

0.459a–d

Untreated control

1.8ab

3.4a

0.439d

CCC (2–3 node stage)

1.0c

1.0d

0.455a–d

CCC (heading stage)

1.6b

1.8bc

0.45bcd

TE (2–3 node stage)

1.0c

1.0d

0.453bcd

TE (heading stage)

1.4bc

1.5cd

0.439cd

TE + CCC (2–3 node stage)

1.0c

1.0d

0.462a–d

TE + CCC (heading stage)

1.4bc

1.3cd

0.451bcd

Untreated control

1.0c

1.0d

0.455a–d

CCC (2–3 node stage)

1.0c

1.0d

0.46a–d

CCC (heading stage)

1.0c

1.0d

0.472abc

TE (2–3 node stage)

1.0c

1.0d

0.487a

TE (heading stage)

1.0c

1.0d

0.465a–d

TE + CCC (2–3 node stage)

1.0c

1.0d

0.477ab

TE + CCC (heading stage)

1.0c

1.0d

0.47a–d

Untreated control

2.3a

3.0a

0.453bcd

CCC (2–3 node stage)

1.0c

1.0d

0.457a–d

CCC (heading stage)

1.0c

1.0d

0.45bcd

TE (2–3 node stage)

1.0c

1.0d

0.461a–d

TE (heading stage)

1.0

0.451bcd

TE + CCC (2–3 node stage)

1.0c

1.0d

0.456a–d

TE + CCC (heading stage)

1.0c

1.0d

0.452bcd

Untreated control

1.0c

1.0d

0.466a–d

CCC (2–3 node stage)

1.0

0.459a–d

CCC (heading stage)

1.0c

1.0d

0.457a–d

TE (2–3 node stage)

1.0c

1.0d

0.464a–d

TE (heading stage)

1.0c

1.0d

0.453bcd

TE + CCC (2–3 node stage)

1.0c

1.0d

0.461a–d

TE + CCC (heading stage)

1.0c

1.0d

0.462a–d

Treatment

<0.001

Year

<0.001

Treatment:year

<0.001

0.53

Untreated control

1.5b

5.4a

0.427cd

CCC (2–3 node stage)

1.0c

2.4bcd

0.442abc

CCC (heading stage)

1.5b

4.4ab

0.430c

TE (2–3 node stage)

1.0c

2.5bcd

0.434c

Location

Production year

Whitemud

(Continues)

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TA B L E 3

Location

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(Continued) Production year

Treatment

Lodging score at flowering

Lodging score at maturity

1000-seed weight (g)

TE (heading stage)

1.1c

2.5bc

0.442abc

TE + CCC (2–3 node stage)

1.0c

1.9cd

0.436bc

TE + CCC (heading stage)

1.1

abc

3.1

0.443abc

Untreated control

1.0c

1.0d

0.422cde

CCC (2–3 node stage)

1.0c

1.0d

0.435c

CCC (heading stage)

1.0c

1.0d

0.420c–f

TE (2–3 node stage)

1.0

0.463a

TE (heading stage)

1.0c

1.0d

0.435c

TE + CCC (2–3 node stage)

1.0c

1.0d

0.460ab

TE + CCC (heading stage)

1.0c

1.0d

0.427cd

Untreated control

bcd

2.0

2.4

0.392g

CCC (2–3 node stage)

1.0c

1.0d

0.399efg

CCC (heading stage)

1.0c

1.0d

0.397fg

TE (2–3 node stage)

1.0c

1.0d

0.404def

TE (heading stage)

1.0

0.399efg

TE + CCC (2–3 node stage)

1.0c

1.0d

0.402efg

TE + CCC (heading stage)

1.0c

1.0d

0.405def

Treatment

<0.001

Year

<0.001

Treatment:year

<0.001

<0.01

<0.001

Note: Means with same letters within the column for each trait are not significantly different at p = 0.05.

F I G U R E 2 Effect of plant growth regulators (PGRs) trinexapac-ethyl (TE), chlormequat chloride (CCC), and their combination on plant height of timothy crops over five production years in the Peace River region of western Canada. Letters followed by PGR name represent growth stages: A represents 2–3 node stage and B represents early heading stage. Dots represent individual data points beyond the whiskers, which extend to 1.5× the interquartile range from the box.

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F I G U R E 3 Effect of plant growth regulators (PGRs) trinexapac-ethyl (TE), chlormequat chloride (CCC), and their combination on seed yield of timothy crops over five production years grown in the Peace River region of western Canada. Letters followed by PGR name represent growth stages: A represents 2–3 node stage and B represents heading stage. Dots represent individual data points beyond the whiskers, which extend to 1.5× the interquartile range from the box. T A B L E 4 Mean seed yield (kg ha−1 ) of timothy crops under plant growth regulators (PGRs) treatments—trinexapac-ethyl (TE), chlormequat chloride (CCC), and their combination (TE + CCC) treatments—at two locations in the Peace River region of Western Canada.

Treatment

Sunset House (5-year mean)

Whitemud (3-year mean)

Untreated control

493

595

CCC (2–3 node stage)

542***

659*

CCC (heading stage)

510

638

TE (2–3 node stage)

521*

669**

TE (heading stage)

485

644*

TE + CCC (2–3 node stage) 519

659*

TE + CCC (heading stage)

489

635

SEM

16.4

29.4

Note: One-tailed test PGR treatments compared with untreated control. Abbreviation: SEM, standard error of the mean. Significant at *p < 0.05, **p < 0.01, and ***p < 0.001.

(p < 0.001). There was no interaction between PGR treatment and year (p = 0.37). The highest seed yields were recorded at Sunset House during the second year. CCC applied at the 2– 3 node stage produced 22% higher yield than the untreated control (Table 4; Figure 3). Among PGRs at Sunset House, CCC applied at 2–3 node stage and TE applied at 2–3 node stage showed significantly higher seed yield as compared to untreated check (Table 4). The increase in seed yield with

application of PGRs as compared to the untreated control was observed at second and fourth years of production (Figure 3). At the Whitemud location, seed yield was not affected by PGR treatments (p = 0.41) but varied across production years (p < 0.001). In contrast, when data were averaged across all production years, PGR application showed a consistent increase in seed yield at the Whitemud location (Table 4). Among PGRs at Whitemud, CCC (2–3 node stage), TE (2– 3 node stage and heading stage), and TE + CCC (2–3 node stage) showed significantly higher seed yield as compared to untreated check (Table 4). When PGRs were applied at 2–3 node stage, the seed yield was increased by >10% compared to control (Figure 3). The variability in seed yield over production years is evident from Figure 3, which shows higher seed yields in Whitemud in the first year compared to Sunset House. Below-average precipitation in 2021 resulted in a low seed yield in the second year of production at Whitemud and third year of production at Sunset House.

5.5

Cost–benefit analysis

The average seed yield from both site locations increased when PGRs were applied, with the highest seed yield being achieved with CCC applied at 2–3 node stage. The cost– benefit analysis of timothy seed yields as impacted by PGRs is presented in Table 5. The net economic benefit analysis showed that there was profit when PGRs were applied at the

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T A B L E 5 Cost and benefit analysis of timothy seed yield under different plant growth regulators (PGRs) trinexapac-ethyl (TE), chlormequat chloride (CCC), and their combination (TE + CCC) application (average seed yield data from both locations). Cost of treatment applicationa (CAD ha−1 )

Increased gross revenue profitb (CAD ha−1 )

Net benefitc (CAD ha−1 )

Treatment

Seed yield (kg ha−1 )

Yield difference (kg ha−1 )

Untreated control

531b

CCC (2–3 node stage)

586a

+55

86.69

110.00

23.31

CCC (heading stage)

558ab

+27

86.69

54.00

−32.69

TE (2–3 node stage)

576ab

+45

78.52

90.00

11.48

TE (heading stage)

544

+13

78.52

26.00

−52.52

TE + CCC (2–3 node stage)

571ab

+40

62.29

80.00

17.71

TE + CCC (heading stage)

544b

+13

62.29

26.00

−36.29

Cost of PGR and application cost. Values in CAD; USD conversion at 1 CAD = 0.727 USD (Bank of Canada, August 8, 2025). Seed price at CAD 2.00 kg−1. c Increased gross revenue profit minus cost of treatment application. a

2–3 node growth stage, but loss was observed when PGRs were applied at the heading stage (Table 5). Similarly, the highest net benefit of CAD 23.31 ha−1 was obtained when CCC was applied at the 2–3 node growth stage (Table 5).

DISCUSSION

There is increasing advocacy for the adoption of perennial crops to improve agricultural sustainability (Chapman et al., 2022). Perennial crops, which produce over multiple years, possess deeper root systems that improve nutrient and water uptake efficiency and facilitate the storage of organic carbon in the soil. This transition toward perennial crops may offer a more sustainable approach to meeting global food and forage demands while promoting environmental benefits. This multi-year and multi-location study examines the impact of TE and CCC PGRs on lodging, plant height, seed weight, and seed yield of timothy seed crops, thereby contributing to our understanding of how management practices can enhance the productivity of perennial cropping systems. This study found that TE and CCC were both effective in reducing plant height and lodging, thereby increasing the seed yield of timothy crops. The average seed yield from both locations showed that CCC applied at 2–3 node stage showed the highest seed yield as compared to other PGR treatments and the untreated control. This multi-year, multi-location trial aimed to capture the diverse climate and soil types of the Peace River region of western Canada. However, in the normal precipitation years 2020 and 2022, Whitemud showed higher seed yield than Sunset House, suggesting there could be some genotype or location effect but not in a drought year (2021) (Table 4; Figure 3). Timothy forage production does well in fertile land with irrigation or annual precipitation of at least 450 mm (Ogle et al., 2011). Timothy is one of the major forage

seed crops in the Peace River region of western Canada, which receives average annual precipitation of 350–600 mm (ESWG, 1996), representing multiple ecoregions with the diversity of climate. Timothy has low soil requirements except for adequate moisture; therefore, the area suitable for timothy seed crop production will generally show a positive seed yield response from PGR application. This study also showed that application of PGRs with growth retarding effects was effective under good moisture conditions, as suggested by Bhattarai et al. (2024) in a recent creeping red fescue study. In the year 2021, the third production year in Sunset House and second production year in Whitemud were limited by drought conditions and showed the lowest seed yield, suggesting that seed production of timothy is not dependent on the production year but on soil moisture conditions. Furthermore, the largest 1000-seed weight was observed with TE applied at 2–3 node stage in third production year (Sunset House) and second production year (Whitemud) (Table 3), suggesting role of TE in increasing seed size under drought conditions; however, seed yields were affected by many other factors. In addition, the variability in lodging score and impact of lodging resistance by PGR application were distinctly observed in 2019 and 2020 at Sunset House and in 2020 at Whitemud, which correlates with precipitation and soil moisture conditions where 2020 was close to the long-term average and 2019 received more precipitation than long-term average (Figure 1). However, in this study each field received varying amounts of fertilizer according to the soil test, and no specific trend was observed between the amount of fertilizer applied and the lodging score. The Whitemud location, which received more nitrogen fertilizer than the Sunset House location (Table 2), exhibited higher average seed yield across all years (Table 4). Nitrogen fertilizer is required for crop growth, but high nitrogen fertilizer will cause excess growth and increase the risk of lodging. Further research is needed to

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better understand the interaction of PGRs, soil moisture, and nitrogen fertilizer on timothy seed production. Grass seed yields are negatively affected by lodging in two main ways: first, self-shading from the lodged plants reduces seed filling, and second, lodging decreases the total number of seeds produced. Negative correlation between seed yield and lodging was observed in several previous studies on cool-season perennial grasses (Bhattarai et al., 2024; Bitarafan et al., 2019). Minimizing and delaying lodging is one of the crop management strategies used to increase seed yield. Application of PGRs with growth retarding effect were proven effective in reducing lodging and increasing seed yield in red fescue (Bhattarai et al., 2024; Bitarafan et al., 2019; Szczepanek et al., 2021). Previous research has shown that reduction in lodging has a secondary advantage of improving harvestability, which reduces the cost of production. In this study, the cost–benefit analysis did not consider the economic aspect of improving harvestability; however, timothy seed production averaged across two locations over multiple years showed that PGRs (CC, TE, and TE + CCC) applied at 2–3 node stage had a net economic benefit, while PGRs applied at heading stage had no economic advantage (Table 5). This cost–benefit analysis is crucial for the adoption of this practice by timothy seed producers. The lower cost per unit of CCC and higher seed yield when CCC is applied at 2–3 node growth stage suggest that CCC applications for timothy seed production will have higher seed yield and economic return. AU T H O R C O N T R I B U T I O N S Surendra Bhattarai: Data curation; formal analysis; investigation; software; validation; visualization; writing—original draft. Hari P. Poudel: Formal analysis; validation; writing— review and editing. Nicole P. Anderson: Conceptualization; methodology; writing—review and editing. Calvin L. Yoder: Conceptualization; data curation; formal analysis; funding acquisition; investigation; methodology; project administration; resources; software; supervision; validation; visualization; writing—review and editing. AC K N OW L E D G M E N T S This project was jointly funded by the Agriculture and AgriFood Canada and the Peace Region Forage Seed Association under the Canadian Agricultural Partnership program— AgriScience Project J-002132. The authors are grateful to grower cooperators who allowed use of their fields to conduct this work. The authors would further like to thank Talon Gauthier, manager of the Peace Region Forage Seed Association, and SARDA Ag Research staff for their support. We also acknowledge many summer employees from SARDA Ag Research who assisted with this project. C O N F L I C T O F I N T E R E ST STAT E M E N T The authors declare no conflicts of interest.

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ORCID Surendra Bhattarai https://orcid.org/0000-0003-49245810 Hari P. Poudel https://orcid.org/0000-0002-9427-0153 Nicole P. Anderson https://orcid.org/0009-0002-63066288 REFERENCES Berry, P. M., Spink, J., Sterling, M., & Pickett, A. A. (2003). Methods for rapidly measuring the lodging resistance of wheat cultivars. Journal of Agronomy and Crop Science, 189(6), 390–401. https://doi.org/10. 1046/j.0931-2250.2003.00062.x Bhattarai, S., Khanal, N., Anderson, N. P., & Yoder, C. L. (2024). Plant growth regulator effects on red fescue seed crops in diverse production environments. Crop Science, 64, 3608–3618. https://doi.org/10.1002/ csc2.21386 Bitarafan, Z., Rasmussen, J., Westergaard, J. C., & Andreasen, C. (2019). Seed yield and lodging assessment in red fescue (Festuca rubra L.) sprayed with trinexapac-ethyl. Agronomy, 9, 617. https://doi.org/10. 3390/agronomy9100617 Boelt, B., & Studer, B. (2010). Breeding for grass seed yield. In B. Boller, U. K. Posselt, & F. Veronesi (Eds.), Fodder crops and amenity grasses. Handbook of plant breeding (Vol. 5, pp. 161–174). Springer. Chapman, E. A., Thomsen, H. C., Tulloch, S., Correia, P. M. P., Luo, G., Najafi, J., DeHaan, L. R., Crews, T. E., Olsson, L., Lundquist, P. O., Westerbergh, A., Pedas, P. R., Knudsen, S., & Palmgren, M. (2022). Perennials as future grain crops: Opportunities and challenges. Frontiers in Plant Science, 13, 898769. https://doi.org/10.3389/fpls.2022. 898769 Chastain, T. G. (2013). Seed production: Pollination and seed yield in grass seed crops. https://blogs.oregonstate.edu/seedproduction/2013/ 09/14/pollination-and-seed-yield-in-grass-seed-crops/ Chastain, T. G., Garbacik, C. J., & Young, W. C., III. (2014). Spring-applied nitrogen and trinexapac-ethyl effects on seed yield in perennial ryegrass and tall fescue. Agronomy Journal, 106, 628–633. https://doi.org/10.2134/agronj2013.0343 Chynoweth, R. J., & Moot, D. J. (2017). Seed growth of three perennial ryegrass cultivars sown on two dates and treated with trinexapac-ethyl straw shortener. Grass and Forage Science, 72, 271–280. https://doi. org/10.1111/gfs.12236 Chynoweth, R. J., Trethewey, J. A. K., Rolston, M. P., & McCloy, B. L. (2014). Reduced stem length increases perennial ryegrass seed yield. Agronomy New Zealand, 44, 61–77. Clemence, T. G. A., & Hebblethwaite, P. D. (1984). An appraisal of ear, leaf, and stem 14 CO2 assimilation, 14 C-assimilate distribution and growth in a reproductive seed crop of amenity Lolium perenne L. Annals of Applied Biology, 105, 319–327. https://doi.org/10.1111/j. 1744-7348.1984.tb03056.x ESWG. (1996). A national ecological framework for Canada. Agriculture and Agri-Food Canada and Environment Canada, Research Branch, Centre for Land and Biological Resources Research, and Environment Canada, State of the Environment Directorate, Ecozone Analysis Branch. http://sis.agr.gc.ca/cansis/publications/ ecostrat/cad_report.pdf Gustavsson, A. M. (2011). A developmental scale for perennial forage grasses based on the decimal code framework. Grass and Forage Science, 66(1), 93–108. https://doi.org/10.1111/j.1365-2494.2010. 00767.x

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How to cite this article: Bhattarai, S., Poudel, H. P., Anderson, N. P., & Yoder, C. L. (2025). Plant growth regulator effects on seed yield of timothy. Crop Science, 65, e70179. https://doi.org/10.1002/csc2.70179

BHATTARAI ET AL.