A deep dive into drenching

Beef finishers should ease up on the drenching to avoid the likelihood of drench resistant parasites, writes Ben Allott.

Beef finishing or dairy heifer grazing systems present a number of challenges around parasite control. The high stocking rate of a single stock class under rotational grazing leads to a rapid rise in parasite challenge if drench intervals are not kept tight and regular. Meeting high liveweight gain targets is usually necessary to meet dairy heifer target weights or to maintain a profitable finishing model. Because of these factors it has become common practice to drench young cattle monthly through large parts of the year.

My blunt, honest assessment of an intensive cattle system that finds itself needing to drench all young cattle once monthly is that your system is unsustainable. This high level of chemical input is creating drench resistant parasites in a big way. If you want to be able to continue finishing young cattle or rearing dairy heifers, you need to find a way to reduce the amount of drench you use. You need to develop a plan to incorporate refugia into your parasite management plan. Refugia involves keeping a population of susceptible worms on the farm which can slow the build-up of parasites resistant to drenches.

In the same breath, you need to do this while maintaining a high level of performance. Simply drenching less frequently will usually not help you achieve this and the production cost can be large.

One idea you should explore with a trusted animal health advisor is developing a targeted selective treatment (TST) programme. An example TST programme could run like the outline below. However, it is important to get good farm specific advice prior to implementing a programme on your farm:

• The decision on whether to drench an animal or not is made based on the individual average daily gain (ADG) since the last weigh event. Animals that have achieved a high ADG are left un-drenched. Animals with a low ADG are drenched.

You continue to bring all young cattle into the yards at set, regular intervals for a weigh event. At each weigh event some animals will receive a parasite drench. For very young cattle in a highly intensive system these weigh events are set at 4-weekly intervals. For older finishing cattle or in less intensive systems I have set longer intervals.

• Prior to each yarding you set an average daily gain target for the mob. Setting robust ADG targets is an important part of the program and these should factor in cattle breed, cattle age, season, feed quality, and pasture allocations. If you set your target too high you will drench more animals and reduce the provision of refugia. If you set your target too low you will under-drench the mob. This could lead to high parasite burdens, lower growth rates, higher faecal egg counts, and the contamination of pasture with parasite larvae.

• My approach is to run 20 head through the scales back into the mob. The average daily gain of these 20 gives me a mob ADG which I use to set three ADG thresholds which will drive my drench decisions for the day:
a) Low growth group = ADG is <80% of mob average (multiply mob ADG by 0.8)
b) Average growth group = ADG is 80%-120% of mob average
c) High growth group = ADG is >120% of mob average (multiply mob ADG by 1.2)

How do you know who to drench?

a) If the mob ADG (average of 20) is solid compared to the pre-set target ADG, then only drench the lower growth group (those with ADG <80% of mob average)

b) If mob ADG is disappointing compared to target ADG, then drench the lower growth and the average growth groups. The higher growth group should be left un-drenched.

c) If mob ADG is very poor i.e. even the higher growth threshold is disappointing compared to target ADG, then drench all cattle but consider animal disease, trace element deficiency, or unrealistic expectations and collect samples to improve the model e.g. feed test to check pasture quality, or bloods for trace element levels.

For example: Late Spring – yearling Hereford steers on good native pasture, allocations not limited. The scenario I run through Q-graze puts pasture quality at 10.5-10.8MJME and sets a mob ADG expectation of 1.2kg/day. I run 20 head through first to measure my mob ADG and the average is 0.9kg/day = disappointed compared to expectation. My three groups are:
Low growth = 0.9kg/day x 0.8 (80%) = cattle with ADG less than 0.72kg/day
Average growth = cattle with ADG of 0.72 – 1.08kg/day
High growth = 0.9 x 1.2 = cattle with ADG greater than 1.08kg/day.

“My blunt, honest assessment of an intensive cattle system that finds itself needing to drench all young cattle once monthly is that your system is unsustainable.” 

In this example, drench any cattle coming through the crush with ADG of less than 1.1kg/day – the lower growth and the average growth groups.

New Zealand research to date has shown that with a well-run TST programme you can expect to reduce the amount of drench you use by 50-70% while experiencing reductions in ADG of less than 5%. With dairy calves, some advisors will book a whole mob in to be drenched after several rounds of TST.

The fantastic part of the TST is that with a well-run approach backed by good advice there is no cost to the system and minimal disruption to your current farm practice. No new cultivation to introduce new forage types, you yard cattle every four weeks just like you did before, there is no new product cost, and there should be very little impact on production. It takes some thought to set the programme up but once up and running it becomes simple to manage.
Food for thought.

• Ben Allott is a North Canterbury veterinarian.