Causes of elevated coumestrol in lucerne and mitigation of the subsequent risk to ewe reproductive performance

Abstract

Coumestrol in lucerne can reduce the ovulation rate of ewes, which lowers lambing rates. This thesis isolated the factors with potential to increase the coumestrol levels of lucerne. The management of ewes during the mating season to avoid suppressed ovulation was also investigated. This enabled management strategies to be developed to mitigate the risk of depressed ovulation rate in ewes grazing lucerne during the mating season.

Agronomic factors investigated were lucerne cultivar, cutting frequency, development stage, aphid herbivory, fungal infection, water stress, and fungicide application. Fungal pathogens explained most of the differences in coumestrol with a moderate relationship between fungal damage and coumestrol content in field and glasshouse experiments. Lucerne inoculated with Stemphylium sp. had high coumestrol levels, with 169 ± 25.1 mg/kg DM measured in entire shoots, compared with 3.4 ± 0.84 mg/kg in control plants. Pea aphids were a minor cause of a coumestrol response, with an increase from 2.4 ± 0.39 mg/kg DM to 5.3 ± 0.65 mg/kg DM measured in a glasshouse experiment. Flowering did not produce a coumestrol response, with simultaneous increases observed in vegetative and flowering plants. Wilted lucerne had a coumestrol content of 1.3 ± 0.43 mg/kg DM which was not different to well-watered lucerne (2.2 ± 0.59 mg/kg DM) and only increased to 3.0 ± 0.57 mg/kg DM with further stress.

Despite fungal pathogens causing the main coumestrol response, carbendazim fungicidal treatments predicted to decrease coumestrol accumulation were ineffective. In leaves inoculated with stemphylium a newer cultivar ‘Stamina 5’ had higher coumestrol (396 ± 82.4 mg/kg DM) than ‘Wairau’ (143 ± 35.6 mg/kg DM), a 40 year old industry standard. In the field, five cultivars currently on the market did not differ in coumestrol content throughout a growing season and all reached at-risk levels (> 25 mg/kg DM) with an average content of 56.2 ± 3.24 mg/kg DM by late-May 2015. Removal of herbage was an effective method to reduce coumestrol as regrowth material typically had lower levels than the prior herbage.

Based on the field data, a model which used relative humidity and rainfall terms was created to predict when lucerne was likely to have heightened coumestrol. From this model a risk assessment analysis was created for a range of New Zealand locations. The assessment showed that four week old lucerne regrowth was at lower risk of elevated coumestrol than the standard six week regrowth which ewes are typically grazed on. Blenheim was least likely of the four locations to have at-risk levels of coumestrol and in all locations except Napier, the risk of high coumestrol increased as the autumn progressed. This would have greatest effect in Otago/Southland, where mating is concentrated late in the season. Indirect prediction of coumestrol content was also identified through ewe lambs that showed increased growth of teats and udder protrusion as an indicator of the crop oestrogenicity.

Ideally, high quality forage provided by lucerne is beneficial for live weight and thus reproductive performance, but grazing must avoid coumestrol-induced impairment of ovulation rates. A grazing experiment removed ewes at different intervals prior to a CIDR-induced ovulation. This experiment demonstrated that the risk of impaired lambing performance due to moderately oestrogenic lucerne consumption decreased with time on grass prior to ovulation. Removal of ewes two weeks prior to ovulation sufficed to mitigate the risk of decreased lambing performance in this situation.

The main results of this thesis indicate coumestrol levels are elevated by fungal infection of lucerne, but grazing management can be utilised to mitigate the effects on ewe reproductive performance.