PLSC 204 Test #2 2020 Study Notes

Question 1a: Ryegrass Endophyte and Grazing Management

• Endophyte and Staggers:

  • The graph (Figure 1) illustrates the concentration of Lolitrem B, a neurotoxin produced by the ‘wild’ endophyte (Acremonium lolii) in perennial ryegrass, across different parts of the plant.

  • Lolitrem B concentrations vary within the plant: basal leaf sheaths > upper leaf sheaths > leaf blades. Bulk herbage represents an average.

  • Animals grazing ryegrass with high levels of ‘wild’ endophyte, particularly when predominantly consuming basal leaf sheaths (lower plant parts), are likely to suffer from staggers due to the neurotoxic effects of Lolitrem B.

• Grazing Management to Avoid Animal Health Problems:

  • Avoid close grazing: Prevent animals from selectively grazing the base of the plant where Lolitrem B concentration is highest.

  • Rotational grazing: Implement a rotational grazing system with appropriate rest periods to allow for leaf growth, thereby diluting the Lolitrem B concentration in the overall herbage.

  • Dilution: Graze with other forages to dilute the overall concentration of Lolitrem B consumed.

• Alternatives to ‘Wild’ Endophyte Ryegrass:

  • Novel Endophyte Ryegrass: Ryegrass cultivars containing 'novel' or 'safe' endophytes produce alkaloids beneficial for plant protection against pests but do not produce Lolitrem B or other toxins harmful to livestock. Justification: Improves animal health and productivity compared to wild-type endophytes.

  • Endophyte-Free Ryegrass: Ryegrass cultivars without any endophyte. Justification: Eliminates the risk of endophyte-related toxicities. However, these grasses may be more susceptible to pests and environmental stresses.

  • Other Forage Species: Utilize alternative forage species such as white clover, chicory, or plantain in pasture mixes. Justification: Reduces the proportion of ryegrass in the diet, thereby lowering the risk of endophyte-related issues and potentially improving overall pasture quality and nutritional diversity.

Question 1b: Pasture Response to Grazing

• Pasture Production Similarity in Set Stocked and Rotational Grazing:

  • Pasture production can be similar in set stocked and rotational grazing systems due to compensatory growth responses.

  • Set Stocking: Continuous grazing pressure can stimulate tillering and maintain plants in a vegetative state, promoting consistent growth, but can lead to selective grazing and overgrazing of preferred species.

  • Rotational Grazing: Rest periods allow plants to recover and replenish carbohydrate reserves, leading to a flush of growth after grazing. However, undergrazing can lead to a build-up of stem and dead material, reducing pasture quality.

• Impact of Decreasing Stocking Rate:

  • Set Stocked:

    • 7.5 steers/hectare: High grazing pressure, potentially leading to overgrazing, reduced pasture mass, and lower plant vigor. Pasture quality may decline due to selective grazing of preferred species.

    • 5 steers/hectare: Moderate grazing pressure, allowing for improved pasture recovery and higher pasture mass compared to 7.5 steers/hectare. Pasture quality may improve due to reduced selective grazing.

    • 2.5 steers/hectare: Low grazing pressure, leading to undergrazing, increased pasture mass, and accumulation of dead material. Pasture quality may decline due to increased stem content and reduced leaf-to-stem ratio.

  • Rotational Grazing:

    • 7.5 steers/hectare: High grazing pressure during grazing periods, followed by short rest periods. Pasture recovery may be limited, potentially reducing overall pasture production in the long term. Pasture quality may be maintained if rotation is fast enough to prevent plants from becoming overly mature.

    • 5 steers/hectare: Moderate grazing pressure during grazing periods, allowing for adequate pasture recovery during rest periods. This stocking rate is likely to optimize pasture production and quality.

    • 2.5 steers/hectare: Low grazing pressure during grazing periods, leading to undergrazing and extended rest periods. Pasture may become overly mature and stemmy, reducing pasture quality. Pasture production may be lower than optimal due to inefficient utilization of available forage.

Question 2a: Measuring Pasture Mass and Quality - Visual Assessment

• Quantitative Measurement of Pasture Mass:

  • Visual Estimation: Trained assessors estimate pasture mass (DM kg/ha) based on visual cues such as pasture height, density, and stage of growth.

  • Calibration: Calibrate visual estimates with actual measurements (e.g., cutting and weighing pasture samples) to improve accuracy.

  • Reference Standards: Use reference photographs or pasture

Question 1a: Ryegrass Endophyte and Grazing Management

• Endophyte and Staggers:

  • The graph (Figure 1) illustrates the concentration of Lolitrem B, a neurotoxin produced by the ‘wild’ endophyte (Acremonium lolii) in perennial ryegrass, across different parts of the plant. The concentration of Lolitrem B significantly impacts animal health, especially in livestock grazing on infected pastures.

  • Lolitrem B concentrations vary within the plant: basal leaf sheaths > upper leaf sheaths > leaf blades. Bulk herbage represents an average. This variation means animals grazing closer to the ground ingest higher concentrations.

  • Animals grazing ryegrass with high levels of ‘wild’ endophyte, particularly when predominantly consuming basal leaf sheaths (lower plant parts), are likely to suffer from staggers due to the neurotoxic effects of Lolitrem B. Staggers can lead to reduced weight gain, impaired reproductive performance, and even death in severe cases.

• Grazing Management to Avoid Animal Health Problems:

  • Avoid close grazing: Prevent animals from selectively grazing the base of the plant where Lolitrem B concentration is highest. Implement strategies to maintain a taller pasture sward.

  • Rotational grazing: Implement a rotational grazing system with appropriate rest periods to allow for leaf growth, thereby diluting the Lolitrem B concentration in the overall herbage. This also promotes healthier plant growth and reduces stress on individual plants.

  • Dilution: Graze with other forages to dilute the overall concentration of Lolitrem B consumed. Introduce alternative feed sources or pasture mixes to reduce the proportion of toxic ryegrass in the animals' diet.

• Alternatives to ‘Wild’ Endophyte Ryegrass:

  • Novel Endophyte Ryegrass: Ryegrass cultivars containing 'novel' or 'safe' endophytes produce alkaloids beneficial for plant protection against pests but do not produce Lolitrem B or other toxins harmful to livestock. Justification: Improves animal health and productivity compared to wild-type endophytes. These endophytes enhance plant stress tolerance without the negative effects on animal health.

  • Endophyte-Free Ryegrass: Ryegrass cultivars without any endophyte. Justification: Eliminates the risk of endophyte-related toxicities. However, these grasses may be more susceptible to pests and environmental stresses. This can lead to decreased pasture persistence and productivity.

  • Other Forage Species: Utilize alternative forage species such as white clover, chicory, or plantain in pasture mixes. Justification: Reduces the proportion of ryegrass in the diet, thereby lowering the risk of endophyte-related issues and potentially improving overall pasture quality and nutritional diversity. These species offer different nutritional profiles and can enhance overall animal performance.

Question 1b: Pasture Response to Grazing

• Pasture Production Similarity in Set Stocked and Rotational Grazing:

  • Pasture production can be similar in set stocked and rotational grazing systems due to compensatory growth responses. Understanding these responses is crucial for effective pasture management.

  • Set Stocking: Continuous grazing pressure can stimulate tillering and maintain plants in a vegetative state, promoting consistent growth, but can lead to selective grazing and overgrazing of preferred species. This can result in a decline in pasture composition over time.

  • Rotational Grazing: Rest periods allow plants to recover and replenish carbohydrate reserves, leading to a flush of growth after grazing. However, undergrazing can lead to a build-up of stem and dead material, reducing pasture quality. Effective implementation requires careful monitoring of pasture growth and timely adjustments to grazing schedules.

• Impact of Decreasing Stocking Rate:

  • Set Stocked:

    • 7.5 steers/hectare: High grazing pressure, potentially leading to overgrazing, reduced pasture mass, and lower plant vigor. Pasture quality may decline due to selective grazing of preferred species. High stocking rates can deplete soil nutrients and increase erosion risk.

    • 5 steers/hectare: Moderate grazing pressure, allowing for improved pasture recovery and higher pasture mass compared to 7.5 steers/hectare. Pasture quality may improve due to reduced selective grazing. This stocking rate balances animal productivity with pasture sustainability.

    • 2.5 steers/hectare: Low grazing pressure, leading to undergrazing, increased pasture mass, and accumulation of dead material. Pasture quality may decline due to increased stem content and reduced leaf-to-stem ratio. Low stocking rates can result in inefficient use of available forage resources.

  • • 7.5 steers/hectare: High grazing pressure during grazing periods, followed by short rest periods. Pasture recovery may be limited, potentially reducing overall pasture production in the long term. Pasture quality may be maintained if rotation is fast enough to prevent plants from becoming overly mature. This requires intensive monitoring and management.

    • 5 steers/hectare: Moderate grazing pressure during grazing periods, allowing for adequate pasture recovery during rest periods. This stocking rate is likely to optimize pasture production and quality. It allows for a balance between forage utilization and plant health.

    • 2.5 steers/hectare: Low grazing pressure during grazing periods, leading to undergrazing and extended rest periods. Pasture may become overly mature and stemmy, reducing pasture quality. Pasture production may be lower than optimal due to inefficient utilization of available forage. This can also lead to weed encroachment and reduced species diversity.

Question 2a: Measuring Pasture Mass and Quality - Visual Assessment

• Quantitative Measurement of Pasture Mass:

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The aspects mentioned play critical roles in white clover seed production:

(i) Apical buds at the stolon tips: Apical buds are essential for the vegetative propagation of white clover. The growth and development of these buds determine the spread and establishment of clover plants, directly influencing the density of the clover stand. A dense, well-established stand is necessary for maximum seed yield. For example, cultivars with vigorous stolon growth and active apical buds can quickly cover bare ground, increasing the number of potential flowering shoots.

(ii) Soil fertility: Soil fertility significantly affects the growth and seed production of white clover. Adequate levels of nutrients, particularly phosphorus and potassium, are crucial for flower development and seed fill. Phosphorus promotes root development and flowering, while potassium enhances overall plant vigor and seed quality. For instance, a deficiency in phosphorus can lead to reduced flower numbers and poor seed set, whereas sufficient potassium ensures the production of plump, viable seeds. Soil pH also influences nutrient availability; a slightly acidic to neutral pH (6.0-7.0) is generally optimal for white clover.

(iii) Leaf size: Leaf size impacts the plant's ability to capture sunlight for photosynthesis, which drives overall growth and seed production. Cultivars with larger leaves can typically capture more sunlight, leading to greater biomass accumulation and potentially higher seed yields. However, excessively large leaves can also increase shading and reduce air circulation within the canopy, creating a microclimate that favors disease development. Therefore, a balance is needed. For example, medium-leaf cultivars might be preferred in dense stands to ensure adequate light penetration and air movement.

(iv) Bare ground between drill rows: Bare ground between drill rows is important for several reasons such as allows sunlight to penetrate the lower parts of the canopy, promoting more uniform flowering and seed development. Also, it reduces humidity and improves air circulation, minimizing the risk of fungal diseases that can damage flowers and seeds. And lastly, bare ground facilitates mechanical harvesting of seeds, reducing the amount of plant material that needs to be processed. For instance, wider row spacing can create more bare ground, improving light penetration and reducing disease incidence, resulting in higher seed yields.

Increasing row spacing can benefit larger leaf cultivars in high soil fertility situations because cultivars with larger leaves tend to produce a denser canopy, particularly when soil fertility is high. High fertility promotes vigorous vegetative growth, which can lead to excessive shading and reduced air circulation within the canopy. By increasing row spacing, sunlight penetration and air movement are improved, which supports more uniform flowering, reduces disease pressure, and enhances seed set. This is because the increased space allows individual plants to maximize their photosynthetic capacity without overly shading neighboring plants, leading to higher overall seed production.