Jacques and Barb have a problem with their three organic heifers - they've just become infested with ticks. They write: "We have read that cooking oil can be effective. Do you know which type is most effective and how is it best applied? Is there any organic treatment that can be internally given? We are BioGro certified so need to be very aware of what we are applying."
The short answer is, that ticks are tough. If you're aiming for total eradication there are probably no organic options. It's true that they don't like oil, and applying cooking oil (or almond or olive oil) to a tick is a good way to get it to withdraw its mouthparts and harmlessly drop off its host. (Pulling the ticks off by hand runs the risk of leaving their front ends embedded in the animal, with the consequent risk of infection.) You could use it on ticks as you see them, but rubbing cooking oil over a cattle beast to the extent of saturating its entire coat is not really an option - you'd have to cover every inch, including the insides of the ears and up into the groin, which are areas which often carry the heaviest tick populations. In any case, the ticks are not killed, and they can live without a feed of blood for up to a year. So they will be quite happy to wait for the first shower of rain to wash the oil off, then re-infest your cattle.
One oil that does seem to have been used effectively to control ticks is neem oil. This has known insecticidal properties and can be applied more sparingly. Bio-Gro NZ, however, regards it with caution, and you would need to get their written approval before using it or any of its extracts. If you do want to go down this path, one recipe is to mix 30ml of oil in five litres of water, with a few drops of mild soap to emulsify the oil, then spray it on the animals every two weeks, or as needed. Neem oil can be bought at garden centres (where it's sold as an organic insect spray), and there are websites where it can be ordered online.
In warmer parts of the country, ticks can be present on the animals at any time of year except mid-winter, while in cooler areas such as Horowhenua or the Volcanic Plateau (ticks have been recorded through most of the North Island, and in Golden Bay) the livestock may be tick-free for up to six months.
Tea tree (Melaleuca) oil has also been shown to have mite-killing properties and is used in a number of commercial pet shampoos - though most of the promotional brochures for these say they "control" fleas and "aid in the control" of ticks. It's hard to find much specific information on how it could be used in a farm situation, but one website explicitly says that to kill the ticks you need to apply pure tea tree oil directly, then remove the tick manually, so it doesn't sound like it's much of an improvement over other, cheaper oils.
As with all organic measures, dealing with ticks requires an integrated approach to farm management. The so-called New Zealand cattle tick Haemaphysalis longicornis (probably from Japan originally) spends most of its time off the animal living in a pasture, so managing your paddocks to make them less desirable as tick habitat is important for getting on top of any tick problem. Specifically, ticks like it damp, so make sure your drainage is up to scratch (or do your bit for local biodiversity, retire those boggy bits of pasture and convert them to wetlands!). Using your cattle to eat out almost all the grass on small areas of your property in rotation can help cut tick numbers appreciably. Don't let your grass get too long anywhere - top any grass that's starting to get away, and clear out any rushes.
One thing we should be thankful for in this country is that we don't have the deer ticks that infest much of North America. These nasty little animals carry Lyme disease, which sounds thoroughly evil. Unpleasant as 'our' ticks are, at least they aren't known to carry any diseases. Americans, not surprisingly, take ticks very seriously and have worked out several clever ways to control them. Not all are applicable to the New Zealand species, but one that might be worth a try is to have a couple of dozen guinea fowl on the property. These quaint birds are apparently very fond of ticks and will pick them out of the grass as they forage. They're good eating too, so tick control isn't the only reason you might want to have a few around.