What is the dry matter content of autumn sown ‘Moata’/Italian ryegrass and does it vary between irrigated and unirrigated pastures?
Short answer: DM% can’t reliably be estimated so you will need to measure it. Unirrigated, stressed pastures/crops are highly likely to have a significantly higher DM% than the irrigated.
Longer explanation: Dry matter percentage varies throughout the year (and throughout the day) so the only accurate method is to measure it directly.
In round figures an irrigated actively growing Italian ryegrass pasture in summer could be 10% DM. In contrast, in winter an Italian ryegrass pastures could be 20% DM. This is a 100% difference in your total yield estimates from the same mass of fresh herbage. Because of this farmers frequently overestimate their Italian ryegrass yields and under feed stock at certain times of the year.
In summer/autumn the irrigated pasture is likely to have a lower DM% than the unirrigated crop. Depending on the level of water stress that could be 10% DM in the irrigated pastures versus 15% DM in the water stressed pasture. This would translate to 50% yield difference in yield estimates made from the same mass of fresh herbage.
Irrigated plants produce leaves with large plant cells and expand these to maximize light interception (larger cells = larger leaves = more light interception = more energy and area to photosynthesize and produce more dry matter). When nitrogen deficiency, or temperature or water stress, kicks in then leaf extension/expansion is usually the first thing to be affected. These stressed crops look shorter and individual leaves are smaller. However, the large leaves on irrigated plants are full of water and their vacuoles (storage organs) are empty. In contrast, on dryland or winter sown pastures, where leaf extension is restricted, leaves may be shorter but photosynthesis is proportionately less affected. This means the plant makes sugar but has less leaf area to allocate it to. The stressed plants then store the sugar in those vacuoles as starch.
Dry matter is essentially a measure of the amount of that sugar that is stored per until leaf area. So our highest quality pastures, those with high sugar levels, are usually those grown over winter months. They are short and leafy with a high sugar (DM) content.
Equally, standing pasture at the early stages of a drought will also be high quality because the leaf expansion will be reduced proportionately more than photosynthesis. So again there is a build-up of sugar in the leaves and the ME is often higher than initially thought – until it starts to die but that’s a different question. So one bite of a pasture for a lamb/cow or deer is not the same as another one – it depends on the DM content of the feed. Low leaves often require more bites to achieve the same level of DM intake.