Irrigated Lucerne Under a Water Allocation Cut
A 60 ha lucerne stand on a Riverina mixed farm has historically received 7.5 ML/ha per season from a 450 ML general security allocation. This season the allocation is cut to 60%, giving 270 ML, and the irrigation season is shortened by six weeks. The stand is in year 3, plant density is still high, and the grower wants to keep the stand for at least two more years while running 400 Merino ewes on the stubble. Soil is a self-mulching clay with a root zone holding about 180 mm of plant-available water. Evaporation in the coming summer is forecast to be 10% above average. The grower has three options: reduce irrigated area, reduce frequency of watering across the whole stand, or dryland the stand and accept a yield penalty. Last season's hay yield was 14 t/ha over four cuts, and hay prices are currently strong at $280/t on farm.
1. Calculate the water available per hectare if the whole 60 ha is kept irrigated, compare this with the historical rate, and explain the agronomic consequences for a perennial lucerne stand of spreading limited water across the full area rather than concentrating it.
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270 ML ÷ 60 ha = 4.5 ML/ha, which is 60% of the historical 7.5 ML/ha, or 3.0 ML/ha less than the stand is used to. At roughly 1 ML/ha per 100 mm of applied water, that is about 300 mm less applied water across the season, on top of a shortened season and higher evaporative demand.
Lucerne is a deep-rooted perennial that persists by maintaining carbohydrate reserves in the crown and taproot. Spreading water thinly across the whole area keeps every plant alive but none vigorous: each plant gets a partial profile refill, so the interval between stress events shortens, plants run down reserves, and the stand thins slowly rather than failing outright. The result is often a low-yielding, patchy stand that is expensive to keep and hard to renovate.
Concentrating water on part of the area (say 36 ha at 7.5 ML/ha) keeps those plants productive and lets the remaining area be spelled, cut for hay early, or used as dryland pasture. The judgement is that persistence of a productive core is worth more than uniform survival of a marginal stand, especially because the grower needs the stand for two more years.
2. The grower must choose between reducing area, reducing frequency, and full drylanding. Recommend a strategy and justify it using the yield, price, and flock information given.
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Recommended strategy: concentrate water on about 36 ha at 7.5 ML/ha, cut the remaining 24 ha for hay early in the season, then use it as dryland pasture and stubble for the ewes.
Reasoning: at 4.5 ML/ha across 60 ha, expect a yield penalty of perhaps 30-40%, so hay might fall to 8-9 t/ha. That is 60 ha × 8.5 t/ha ≈ 510 t, worth about $143,000. Concentrating on 36 ha at full water should hold yield near 13-14 t/ha, giving about 36 × 13.5 ≈ 486 t, worth about $136,000, plus hay from the first cut of the spelled area and better stand persistence.
The two options look similar in the hay year, so the deciding factors are persistence and the sheep. Reducing frequency across the whole area stresses every plant and risks losing the stand before year 5, which is the expensive outcome. Drylanding the whole stand avoids water cost but sacrifices most of the hay income and still leaves the stand vulnerable.
Concentrating water protects the productive core, allows a clean hay cut from the spelled area, and provides dry feed for 400 ewes. The grower should also monitor soil moisture and be prepared to sell surplus hay if the season turns wet, since hay at $280/t is a strong price.
3. Identify two risks in this plan and state how the grower should monitor or mitigate each.
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Risk 1: the concentrated area is overwatered or waterlogged, causing root disease and lost yield. Mitigation: schedule irrigation on soil moisture monitoring in the root zone, not on a calendar, and check drainage at the low end of each bay. A self-mulching clay can hold a lot of water, so apply enough to refill the profile but not so much that water sits on the surface.
Risk 2: the spelled 24 ha dries out and the stand dies, leaving a bare, eroded paddock. Mitigation: cut hay early while there is still moisture, retain stubble cover, and monitor plant density and crown health. If the stand is clearly failing, plan a renovation with a cereal or annual pasture rather than pouring water into a dying stand.
A third risk worth noting is that the forecast 10% higher evaporation means the 7.5 ML/ha figure may be insufficient; the grower should build in a buffer by keeping some allocation in reserve and reviewing the plan at mid-season.