Mating ewes on lucerne? Are there any issues for reproductive success?

The following is reproduced from the animal nutrition Facebook group, “The Rumen Room”

How does lucerne sometimes cause reproductive problems in ewes?

Ewe reproductive performance can sometimes be reduced when ewes flush on lucerne before they go to the ram. Sometimes is the key word here; lucerne will not always cause issues so read on before you choose to completely avoid grazing your ewes on lucerne.

The compound found in lucerne that messes with ewe reproduction is a phyto-oestrogen called coumestrol. Coumestrol alters normal function of the reproductive tract by competing with the ewes own oestrogen which results in normal reproduction outcomes starting to fail. Ovulation rates can be reduced (more dry dries at scanning) or fewer multiples and more singles than usual.

Coumestrol works in a similar way but unrelated way that formononetin (red and sub clovers) also reduces ewe mating outcomes.

What factors influence levels of coumestrol in lucerne?

Lucerne will not necessarily always contain high enough levels of coumestrol to influence ewes mating success. Lots of things change levels and therefore risk of problems to ewes. It’s good to understand things that change coumestrol, so you can decide just how risky flushing ewes on lucerne might be.

Rachel Fields recently completed her PhD in 2017 at Lincoln University researching the causes of elevated coumestrol in lucerne and ways to mitigate the risk to ewe reproductive performance. A full copy of her thesis can be found here: https://researcharchive.lincoln.ac.nz/handle/10182/8614?fbclid=IwAR0UNFn6vPGCl-Mk4kbjfSz-BAQGwrbaXvTvp6EgCjcvRTyH3_kRPI7lpcg

As can be the case for many plants it seems that things that stress the lucerne plant encourage it to produce more coumestrol as a survival mechanism. This seems a bit odd because coumestrol doesn’t help the plant fight against pests or disease, so we are currently unsure why coumestrol is produced. All we can do is work on strategies agronomically to minimise stress on the lucerne plant best we can.

From Rachels work, the factors that influence coumestrol content of lucerne include one or more of the following:

1. Fungal pathogens: Warm, humid conditions that favour fungal growth kick off risk of coumestrol problems. Fungi implicated as those causing increased levels of coumestrol include common leaf spot, Stemphylium leafspot, Leptosphaerulina leaf spot, Antracnose, spring black stem, yellow leaf blotch, downy mildew and Verticillium wilt. The good news is that fresh new growth shouldn’t necessarily contain high coumestrol following removal of older fungal damaged plant material.
2. Aphids: More aphids = higher risk
3. Stage of development of the lucerne stand: Short leafy material contains less coumestrol than flowering stands.
4. Plant part: There appears to be a relationship between leaf and stem coumestrol content that varies depending on whether plants are infected with fungal disease or not.
5. Water stress: Plants that are getting stressed through water deficit can also have issues of increasing coumestrol levels.

Things that are less likely to lift coumestrol include viral damage, mechanical damage to leaves, and rust.

With fungal disease being the main risk for lucerne to accumulate coumestrol, it makes sense that things that increase risk of fungal disease are worth monitoring. Rainfall and high humidity (above 95%) will influence if coumestrol will likely be an issue for ewes. Lots of consecutive dull overcast days are also a risk factor.

Overall risk will clearly be quite different between different regions around NZ and certainly different risk profile within and between different seasons. Rachels research also notes the importance of walking lucerne stands and closely inspecting plants, including looking at the lower sections of the foliage that can be carrying increases disease.

Managing lucerne stands for reduced risk of coumestrol challenges

Below are some considerations and strategies to minimise coumestrol accumulation in lucerne:

1. Regional variation in coumestrol accumulation: Regions with lower humidity (e.g. Marlborough) have fewer issues with coumestrol than higher risk areas such as Hawkes Bay prone to more issues of higher humidity.
2. Inspecting stands for fungal disease: Walk stands to look for obvious presence of fungal disease and/or aphids. If the stand has lots of fungal foliar damage, consider taking this as lucerne silage or baleage and allow the stand to freshen up again pre-tupping. Alternatively, graze with drystock such as ewe hoggets that won’t be mated in the current season. Fresh lucerne regrowth post-harvest will likely be lower in coumestrol than the previous forage, grazing early post-silage cut. Rachels research found coumestrol levels were lower 4 weeks post-cut than 6 weeks post cut. Note that the lower DM yields across 4 weeks regrowth vs. 6 weeks means you’ll need to readjust your stocking rate plans to account for less lucerne on offer.
3. New vs existing stands: New season lucerne stands accumulate less coumestrol than older stands – likely with a cleaner base to the stand / less dead plant material etc.
4. Fungicides: Research concluded that carbendazim fungicide wasn’t useful for reducing risk of fungal damage or coumestrol accumulation.
5. Lucerne cultivar: At present there doesn’t appear to be cultivar differences in coumestrol accumulation but selection of cultivars with increased aphid and disease resistance may assist with minimising plant stress.

Managing ewes on lucerne stands that “may” contain elevated levels of coumestrol

Any possible risks of coumestrol issues may be worth accepting if there’s nothing much else for ewes to eat. If feed is limited and lucerne is the only decent green feed on offer, you may need to flush on lucerne if not much else is available. Given the exceptionally high quality of lucerne, flushing but suffering the potential effects of a reduced ovulation rate is better than feeding very little at all, or only poor-quality pasture containing large amounts of ‘tag’. This is true particularly if post-early rains pasture is rotting down and creating other challenges of e.g. facial eczema and even ironically, zearalenone-induced mating issues in some regions of NZ, east coast north island particularly.

Another option is allowing ewes to start their flushing process on lucerne before switching to something else before they go to the ram. As long as the lucerne is removed from the diet at least two weeks before the ewes go to the ram, ovulation rapidly resumes in a normal manner. So, it might be that you start ewes flushing on lucerne post-drought rain, then as pastures are recovering, you move the ewes back onto pasture two weeks before the ram. The bonus of this is that you give the grass-based pastures an extra time to recover and build pasture mass while the ewes are on lucerne. This strategy means ewes are less likely to overgraze permanent pastures with detrimental impacts on pasture persistence. Two points to consider if adopting the strategy are:

1. Don’t switch ewes off the lucerne if you’re sending them back to fresh air and a nice view – that is, underfeeding. Stock are better they’re on lucerne with possible coumestrol issues than underfeeding.
2. Once ewes have mated and conceived (and here’s hoping most do from the first cycle) then ewes can return to lucerne.