Tough break to the season - Grazing cereal crops to fill the gap
Serina Hancock, Murdoch University, WA
Corresponding author: firstname.lastname@example.org
Another tough start to the year and a late break has seen little feed on offer for pregnant ewes. In late pregnancy, ewes have high nutritional demands and may require supplementary feeding.
A strategic and tactical option available to producers where crops and livestock coexist is the grazing of vegetative wheat, barley and oats (both dual purpose and traditional spring varieties).
The high winter growth rates of these crops offer the opportunity to fill the winter feed gap.
The crops are an excellent source of metabolisable energy (approximately 12 MJ/kg DM) and protein (crude protein 15 to 37%), making them well suited to meet the requirements of reproducing ewes. Grazing crops also allows deferment of grazing on pastures.
Importantly, a lower biomass or feed on offer (FOO) of young vegetative crops is required for liveweight gain (less than 500kg DM/ha) compared to conventional pastures.
This was shown in recent research, led by Murdoch University in collaboration with NSW Department of Primary Industries and Charles Sturt University and funded by Meat & Livestock Australia (MLA). In these studies, pregnant ewes gained condition score (3-3.2) with FOO ranging from 200 to 3400kg DM/ha. This is a much lower FOO than is required when grazing conventional pastures.
The time to graze crops is the Z21 stage (main shoot and one tiller, approximately six-eight weeks from plant emergence depending on the variety) as any earlier and the crop may not be anchored and grazing will result in loss of the plant and hence yield from the crop.
It is recommended sheep be removed from crops before growth stage Z31 (first node formed at base of main tiller); grazing beyond this time may reduce grain yield.
In addition, grazing pregnant ewes for an extended period of time (longer than 21 days) may increase the risk of metabolic disease such as hypocalcaemia due to the mineral imbalance in cereal crops.
Lower risk of mineral deficiencies in Western Australia
Analysis of mineral composition in over 9500 crop samples from locations across Australia indicated wheat may be the highest risk crop whilst oats and barley have a lower incidence of concentrations of sodium, calcium and magnesium that are less than requirements for sheep.
For reasons that are not entirely clear, Western Australia (WA) was identified as having a lower risk of mineral deficiency compared to the eastern states (Figure 1). This may be related to lower potassium levels in the soil and the tendency to graze oats and barley over wheat in WA.
Materials and Methods
More recent research from the same groups of researchers investigated the mineral status of pregnant ewes grazing cereal crops and tested supplements that aimed to minimise ewe mortality and ill health in New South Wales (NSW) and WA.
The research was conducted over two years; year one determined the risk and year two tested mineral supplementation.
In the first year the mineral status of both forage and of reproducing ewes grazing wheat, oats or barley was monitored on 18 farms in WA (six farms) southern NSW (seven farms) and central NSW (five farms). The average period of grazing was 20 days.
In the second year the effectiveness of two mineral supplements was assessed on six farms over three weeks. On each farm 90 twin bearing ewes in late gestation (day 115 to day 129 of gestation) were divided into three treatment groups (n=30/treatment). The control group was given no supplement, a second group was provided with the industry standard (40% causmag, 40% limestone and 20% salt) and the third group was provided with a new formulated supplement containing the same cations but as magnesium chloride, gypsum and salt.
In both experiments samples of blood plasma and urine were collected from ewes pre and post grazing and samples of crop and soil were also collected for mineral analysis for calcium, sodium, magnesium and potassium.
Results supported the earlier crop analysis, confirming that crops grown in WA were lower risk and usually contained sufficient magnesium, calcium and sodium.
Mineral supplements increased calcium concentrations in urine, blood plasma and calcium fractional excretion (Figure 2), a reliable indicator of calcium level based on both blood and urine measures, on all but one of the six farms included in the study (Farm 2 was excluded).
Take home messages
- WA crops are usually suitable for grazing as there is a lower risk of mineral deficiencies and pregnant ewes gain condition on as little as 200 kg DM/ha.
- High soil potassium was associated with low sodium, magnesium and calcium in forage and an increased risk of metabolic disorders.
- Mineral supplements will increase calcium status and to a lesser extent magnesium status. It remains unclear if the supplements can overcome the hormonal disturbances which can result in hypocalcaemia during short periods of calcium stress. Supplements require testing on a commercial scale.
- Mineral supplements should be used in conjunction with crop grazing of pregnant ewes and any mineral supplement used should contain calcium, magnesium and sodium.