By Charlotte Westwood, Veterinary Nutritionist
With last months release of the National Environment Standards (NES) regulatory requirements around Intensive Winter Grazing (IWG), it’s great to see so much focus on planning for 2021 winter crop paddocks to minimise risk to the environment.
Animal welfare remains another key aspect to think through when selecting winter crop paddocks. We need crop and/or stand off areas where there’s good shelter from wind for stock during adverse winter weather.
Trees can make awesome wind breaks!
Shelter for stock during rough weather (to reduce wind chill) greatly reduces stress for all stock classes, especially pregnant stock. Crop paddocks and / or sheltered paddocks near tree lines and forestry plantations are great options to reduce wind chill for stock, making life much more pleasant mid-winter. For pregnant cows, certain tree species bring an increased risk of abortion if cattle eat both green and dry parts of some tree types.
Some tree species put pregnant cows at risk
Many NZ forestry and wildling tree species contain compounds that may cause abortions in cattle, typically during the 3rd trimester of pregnancy.
Isocupressic acid (ICA) is found in cypress, pines and other tree types. Once eaten, ICA is broken down into other compounds that cause abortion in late pregnant cattle.
Levels of ICA are highest in Cupressus macrocarpa (“macrocarpa”, sometimes called Monterey Cypress). Pine trees also contain ICA but usually at relatively lower levels than in macrocarpa. Pinus contorta contains moderate levels of ICA. Pinus radiata and douglas fir contains relatively low levels of ICA but both have still been occasionally linked to abortions in cattle. Other species that contain ICA include firs (Abies spp), Juniperus spp, spruces (Picea spp), all Pinus spp, and cedars. We can post a couple of screen shots from a good paper which lists which trees are risky (just ask in the comments section if that would be useful for you).
ICA is found in all parts of cypress and pine trees including dry and green pine needles, green foliage of macrocarpas, branch tips and tree bark – parts of the tree most often nibbled on by cattle.
How does ICA damage a pregnancy?
Once eaten by the cow, ICA is broken down to a range of metabolites that restrict blood supply across the placenta from Mum to her calf. The unborn calf misses out on oxygen and nutrients ... causing either death of the calf (stillbirth) or the birth of a premature calf who’s not at all well. The premmy calf will often need a lot of nursing to survive. Cows are unlikely to ‘bag up’ with colostrum or milk, depending on how close to her due date she is.
Abortion of still born or live, premature calves are seen as quickly as one day after cows access trees - or up to three weeks after cattle first eat macrocarpa or pine needles. Cows usually hold onto their membranes/cleanings and end up with metritis, often getting very sick.
NOTE that cows can abort their calves for a wide range of other reasons. If you've experiencing abortion challenges in your cows always get your veterinarian involved to help find out what's going on.
The amount of ICA needed to cause abortion depends on
- Concentration of ICA in tree parts and
- How much pine needles/macrocarpa is eaten.
Researchers found cows need to eat 2-3 kg of pine needles / cow /day to cause abortion – so… quite a bit, not just a single one-off mouthful of foliage or needles.
What factors increase risk of abortion for pregnant cows eating pine or macrocarpa foliage?
- Trees or branches down following wind or snow are the biggest risk to pregnant cattle, with wilted green material and needles especially tasty/attractive to cattle, especially in calf R2's who tend to be more curious and interested in eating things they shouldn't!
- Cattle grazing under pines and macrocarpas can eat dried needles/foliage by mistake as they graze grass. Note that dried needles are just as risky as green material for abortion risk, though cattle are more likely to seek out and eat green (wilted) needles than dry needles.
- Hungry cows are more likely to seek out tree material than fully fed cattle so are much more at risk of eating pine and macrocarpa.
- Rumen acidosis in crop-fed cattle may increase risk of cows eating pine needles & macrocarpas. Cattle fed high rates of brassica or fodder beet crop-fed and not enough baleage or hay may crave fibre and actively eat tree parts (needles, branch tips, bark all of which contain ICA) if they have clinical or sub-clinical rumen acidosis.
- Cows fully fed on silage, baleage and with a well balanced diet are at lesser risk of ICA-induced abortion than hungry cattle fed winter forage crops as a high percentage of their diet.
- Pregnant cows in lighter body condition score are at greater risk of ICA abortion than better conditioned cows (seems that some of the ICA compounds can be absorbed into body fat, slightly reducing abortion risk in better conditioned cows).
- Almost all abortions are seen in the last third of pregnancy, embryo or foetal loss earlier in pregnancy appears to be relatively less risky. For cows calving 1st August this means keeping cows well away from trees from mid March onwards and most certainly for the whole of time wintering on winter crops.
- Dietary additives such as magnesium, calcium and sodium bentonite don’t make any difference to risk of ICA abortion.
- Though cattle seem to somewhat adapt over time to eating ICA it’s just too risky to rely on an adjustment period to ICA access – keep pregnant cattle well away from these trees at all times.
- Sheep are unlikely to abort but may give birth to greater numbers of dead, full term lambs after eating macrocarpa or pine needles.
- For non-pregnant stock, ICA (and other compounds in cypress and pine trees) might sometimes cause toxicity, including kidney damage and upset guts. Abortion by pregnant cattle remains by far the more important risk for stock.
Keep late pregnant cows well away from cypress and pine trees
Mid winter shelter means trees will definitely play a role for wind chill protection for next years crop paddocks. But, keep in mind risks from macrocarpa and pine trees to in-calf cows.
Restricting access by any pregnant cattle to forestry or wind break areas planted with cypress or pines is the best approach to minimise risk of abortions. If storms drop branches or trees into crop paddocks, remove or fence off access by cattle to branches. Keep cattle fully fed to reduce risk of hungry cattle eating tree parts.
Happy winter crop planning!