Pasture Production Principles

This lecture is about how pastures and crops grow, particularly focusing on the different weather factors that affect pasture production in New Zealand.
It covers how the time of year and where you are in New Zealand can impact how much pasture grows.
Important readings include:
- "Pasture Supplements for Grazing Animals" (Chapter 2)
- "New Zealand Pasture and Crop Science" (Chapter 3)

Light (Photosynthetically Active Radiation - PAR):
- Light is essential for photosynthesis, which is how plants make their food.
- The PAR includes light wavelengths from 400 to 700 nanometers, with red and blue light being the most useful for plants.
- Different types of plants use light to create dry matter at different rates:
- C3 plants (like ryegrass) produce around $2.5$ grams of dry matter per megajoule (MJ) of light.
- C4 plants (such as maize) are more efficient and produce about $3.8$ grams per MJ.
- In Canterbury, during summer, there's about $11$ MJ of light available per square meter daily, which drops to $2.1$ MJ in winter.
Temperature:
- Temperature affects photosynthesis and respiration (the process plants use to turn food into energy).
- Warmer temperatures generally lead to quicker growth and higher rates of respiration.
- Temperature also influences how well soil microorganisms break down nitrogen, which plants need to grow.
- Most temperate plants grow best in temperatures between $15$ and $25$ °C.
Soil Moisture:
- Water is crucial for plants, especially for expanding leaves and performing photosynthesis during dry spells.
- Plants control water loss through small openings called stomata, which they close when under stress.
- In New Zealand, the soil moisture usually gets replenished during winter, providing needed water for plants.

Growth Rates:
- The temperature (above $3$ °C) impacts how quickly plants grow.
- For example, warm autumn weather can help pasture grow more for winter feeding.
- There's a significant difference between pasture growth rates in summer and winter, heavily influenced by temperature and moisture.
Thermal Time Calculation:
- This measures growth potential based on temperature.
- The calculation is:
  $ext{(Max Temp + Min Temp) / 2} - 3$
- Using this, scientists can estimate how much dry matter pastures will produce based on temperature variations.

Pasture recovery (how well it grows back after being eaten by livestock) has different phases:
- Lag Phase: Initially, there is slow growth right after grazing.
- Linear Phase: Growth picks up speed, and plants absorb more light during this phase.
- Asymptotic Phase: Growth levels off as production balances losses.
The height at which animals graze can influence how quickly the pasture recovers, so leaving some grass is important.

Feed Profiling:
- This is about making sure pasture growth meets the food needs of livestock throughout the year.
Strategies to balance feed availability:
- Making hay or silage when there's excess growth.
- Having feeding plans when there's not enough pasture growth.

Different areas in New Zealand have varying growth conditions due to climate:
- Winton: Cold and humid with about $1100$ mm of rain, leading to a short growing season with up to 12 tonnes of biomass.
- Dargaville: Warmer and more humid with $1200$ mm of rainfall and a longer growing season, resulting in higher production.
- Canterbury: Has about $600$ mm of rainfall, needs irrigation for better growth, and typically yields lower biomass without it.
- Poolburn: A semi-arid area where growth is minimal, relying heavily on winter forage and irrigation.

It's crucial to understand how climate and other environmental factors affect pasture growth.
The next lecture will discuss what happens to pastures after grazing and how to manage these systems effectively.
Suggested readings will be about seasonal changes and optimizing production in New Zealand's various environments.