Understanding the concept of “hatching rain” is useful if you are preparing low worm-risk paddocks.
In southern Australia, summer and early autumn is the time to prepare low worm-risk winter weaner paddocks and lambing paddocks.
In the northern sheep production areas, low worm-risk spring lambing paddock preparations are currently underway.
Hatching rain describes a rainfall event that enables worm eggs to develop to infective larvae on the paddock. Technically, most sheep worms already hatch from the eggs in the dung, but then a suitable amount of moisture (what I call “hatching rain”) and warmth is needed for them to successfully develop to infective larvae.
These infective larvae are what sheep eat to become infected. They mature inside the sheep in about 21 days and can then reproduce and lay eggs.
Back at the egg stage, about 10–15 mm of “hatching rain” is needed for development to occur, but that total can fall over a period of about 4 days; flooded or waterlogged ground can also allow development.
But worm eggs don’t live forever. During dry weather, the worm eggs deposited onto the pasture live only a short time waiting for suitable conditions; up to 5 days for barber’s pole worm and 2–3 weeks for scour worms. This explains why barber’s pole worms are easier to control with grazing management than scour worms.
With the extremely dry autumn in the New England and other regions, almost everyone’s paddocks were being “prepared” as low worm-risk for a spring lambing, courtesy of Mother Nature. However, recent rainfall events may have undone this good work.
In the moderate to high summer rainfall areas where temperatures do not exceed about 35 degrees for extended periods, 6 months is needed to prepare a spring lambing paddock through the winter months.
During this time, you must prevent further contamination of the lambing paddock with worm larvae by one of these methods: spell paddocks, graze with cattle or graze with sheep in the period after an effective drench when very few worm eggs will be shed in faeces (up to 21 days for a short-acting drench; longer for mid and long acting products).
In the hotter areas, about 5 months preparation is needed to achieve the same reduction in worm larvae (at least 90% reduction), so that the paddock can be considered as low worm-risk.
Despite the recent rain, it may not be too late to continue preparing some paddocks for summer rainfall regions.
Paddocks with wormy sheep in them up to 5 days before and 1 day after a hatching rain period will end up with some successful development of barber’s pole worm larvae. Although if soils become wet, this is enough to extend the period after the rain period when development of worm larvae can occur.
However, if you have paddocks that did NOT have wormy sheep in them during the rainfall event, these paddocks could be very helpful to you at lambing.
Provided they have not been contaminated by a rainfall event in the last month or two (back to the start of the 5 or 6 month period before lambing is planned), then you should consider actively managing these paddocks to continue to keep them free of further contamination.
If you are in the particularly cold summer-rainfall area of the New England region of NSW, you have more flexibility during your 6-month preparation period. When daily maximum temperatures are consistently (say 4 days in a row) below 18°C, you can graze wormy sheep on the paddocks that are being prepared because almost no barber’s pole worms will develop at that temperature, even if there is hatching rain.
In areas where scour worms predominate, maximum temperatures must be much cooler to stop most development. Brown stomach worm successfully develop down to about 8°C, the more harmful variety of black scour worm, Trichostrongylus vitrinus down to 12°C, and the common black scour worm, Trichostrongylus colubriformis develops down to 15° C. See Factors contributing to paddock contamination with worms.
However, don’t confuse eggs with larvae. While eggs die quite quickly if suitable conditions don’t occur, larvae are incredibly tough. They live for many months waiting to be eaten by a sheep. In dry weather, no new development is occurring, but the larvae that developed in past months are still there to infect sheep.
For more information on preparing low worm-risk paddocks go to Your Program on WormBoss.