Abstract
In Chapter 1 extensive documentation exists regarding the general temperature and moisture requirements for various weed species. Nonetheless, evidence suggests that production practices and environmental conditions influence the response of weed species to temperature and moisture. In 2023, laboratory studies were initiated at the University of Idaho Kimberly Research and Extension Center to ascertain the germination temperature and moisture requirements of Italian ryegrass (Lolium multiflorum Lam.) and spring wheat (Triticum aestivum L.) collected from Idaho and Washington. For the germination temperature requirement experiments, weed populations and wheat cultivars were germinated on a thermogradient table with 10 different temperatures ranging from 4°C and 35 °C. In the germination moisture requirement experiments, polyethylene glycol (PEG 8000) was used to achieve 10 different osmotic potentials (0 to -2 MPa) at the temperature of 20°C. No differences were observed in germination speed and maximum germination between Italian ryegrass and spring wheat of 30°C or greater. However, Italian ryegrass exhibited a faster germination speed and greater maximum germination at temperatures below 10°C. Most Italian ryegrass populations achieved approximately 50% germination at 4°C, whereas spring wheat exhibited less than 5% germination. At osmotic potentials of 0 to -0.2 MPa, germination rates ranged from 80% to 100% for both Italian ryegrass and spring wheat. Italian ryegrass germination exceeded that of spring wheat at osmotic potentials of -0.4 to -0.8 MPa. These findings indicate that Italian ryegrass is likely to germinate more rapidly and be more competitive with spring wheat under cooler temperatures and competitive with spring wheat under dry conditions. In Chapter 2 temperature and moisture availability play a crucial role in shaping the ecology and growth patterns of weeds in agricultural systems. The specific temperature and moisture needs of various weed species have been well-documented. However, there is evidence suggesting that production practices and environmental conditions can affect how weed species respond to changes in temperature and moisture. Laboratory studies conducted in 2024-2025 at the University of Idaho Kimberly Research and Extension Center aimed to determine the germination temperature and moisture requirements of cheatgrass (Bromus tectorum L.) collected from Oregon, Washington, and Idaho, as well as winter wheat (Triticum aestivum L.). In these studies, both weed populations and the wheat cultivar were grown on a thermogradient table with 10 temperature settings ranging from 4°C to 35°C. For the moisture requirement study, polyethylene glycol (PEG 8000) was used to create ten different osmotic potentials, from 0 to -2 MPa. At the coldest temperature, winter wheat germinated at approximately 75%, which was higher than some cheatgrass populations. All populations and winter wheat took nearly two weeks to germinate at the coldest temperature. Heat did not impede germination for either cheatgrass or winter wheat, with all populations and wheat achieving germination rates of 86% or higher. Moisture restriction did not significantly reduce germination until after -0.2 MPa, with germination occurring rapidly. At the lowest osmotic potential where seeds germinated (-1.0 MPa), winter wheat outperformed cheatgrass, with 50% of winter wheat seeds germinating compared to a maximum of 30% for cheatgrass. Cheatgrass populations thus can germinate under varying temperatures and moistures and remain competitive with winter wheat.
Finally, in Chapter 3 studies were conducted to determine how climate change has significantly influenced the environmental conditions of dryland and irrigated production systems, particularly by altering moisture availability. As climatic patterns become increasingly unpredictable and moisture becomes limited, the efficacy of pre-emergence herbicides becomes more challenging to forecast. In addition to limited moisture, the depth at which seeds are sown can affect the effectiveness of pre-emergence herbicides, as seeds placed at greater depths may require extended periods to germinate. To determine the impact of restricted moisture and seed depth placement on the efficacy of pre-emergence herbicide treatments, studies were conducted in 2025 at the University of Idaho Kimberly Research and Extension Center in Kimberly, Idaho. The objectives of these studies were to: 1) ascertain the minimum moisture levels and timing required for the activation of pre-emergence herbicide treatments for the control of Italian ryegrass, and 2) evaluate how the placement of Italian ryegrass seeds at varying depths of 0, 0.64, 1.27, 2.54, 5.08, and 10.16 cm influences the efficacy of pyroxasulfone (Zidua SC) and trifluralin (Treflan HFP). The results indicated that pyroxasulfone treatments were more effective in controlling Italian ryegrass across multiple seed depths. In contrast, trifluralin efficacy varied with seed depth, showing reduced control at greater depths; optimal control was achieved at a depth of 0.64 cm, resulting in a 100% reduction in biomass. At the maximum depth of 10.16 cm, emergence did not occur in the nontreated controls, suggesting that Italian ryegrass does not emerge at this depth. Similarly, under conditions of restricted moisture, pyroxasulfone outperformed trifluralin, demonstrating superior efficacy with limited moisture availability. Germination of Italian ryegrass ceased when moisture levels fell below 40% of field capacity or less than 0 MPa water potential. The timing of watering (day of, 3 days after and 6 days after herbicide application) did not significantly affect the efficacy of either herbicide treatment.