Coumestrol produced by alfalfa (Medicago sativa L.) can reduce the ovulation rate of ewes. This study isolated agronomic factors that affect coumestrol levels in alfalfa. Fungal diseases explained most differences in coumestrol. Alfalfa inoculated with Stemphylium vesicarium contained 169 ± 25.1 mg kg -1 dry matter (DM) compared with 3.4 ± 0.84 mg kg -1 DM in controls. However, in field-grown alfalfa there was a low relationship (R 2 = 0.388) between coumestrol content and visual fungal damage score which indicates a need for alternative methods of prediction. In the field, a range of cultivars all reached coumestrol levels reported to pose a risk to ewe reproductive performance (>25 mg kg -1 DM). In S. vesicarium-inoculated leaves, the modern cultivar 'Stamina 5' had 396 ± 82.4 mg kg -1 DM compared with the 40-yr-old industry standard 'Wairau' at 143 ± 35.6 mg kg -1 DM. Flowering in isolation did not produce a coumestrol response, with increases detected or not detected simultaneously in vegetative and flowering plants. Pea aphids had a minor effect on coumestrol content. When alfalfa was subjected to ~5 aphids per stem for 4 wk coumestrol increased from 2.4 ± 0.39 to 5.3 ± 0.65 mg kg -1 DM. This study highlights fungal pathogens traditionally not considered problematic in alfalfa stands due to limited effects on stand persistence and yield, as important causal agents of elevated coumestrol levels. Plant breeding to reduce their impact is recommended.

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Grazing ewes on lucerne crops that contain elevated coumestrol concentrations before and during their mating period can decrease ovulation rate, which results in fewer multiple birth lambs. In this experiment, the ovulation rate of ewes grazing a moderately oestrogenic (29.1 ± 2.88 mg coumestrol/kg DM) lucerne crop for 6 weeks was compared with those transferred to grass pasture for 1 or 3 weeks, and a Control group on grass throughout. Ordinal regression predicted an exponential relationship with a sharp initial increase in ovulation rate from 0 to 14 days on grass that levelled off when ewes were on grass for durations greater than 3 weeks. Of importance, moderate coumestrol levels in lucerne remained throughout the experiment, and liveweight was unaffected by grazing treatment. This experiment quantified the decrease in risk of impaired lambing performance due to moderately oestrogenic lucerne consumption in relation to time on grass before ovulation. Specifically, removal of ewes from lucerne 2 weeks before ovulation mitigated the risk of decreased lambing performance.

A report of mammary development in 6-month-old ewe lambs grazing lucerne on a commercial farm in North Otago provided an opportunity to investigate the possibility to use this as a tool to detect oestrogenic lucerne pastures. Of the 36 ewe lambs grazing lucerne at this farm, protruding mammary glands were visible in 19. In contrast, none of the 22 ewe lambs grazing grass had protruding mammary glands. The lucerne-fed ewe lambs also had larger mean teat width than the grassfed ewe lambs (17.9 ± 0.53 mm versus 13.4 ± 0.78 mm respectively. The lucerne-fed animals were returned to grass pasture 4 weeks before the introduction of rams. There was no effect of the earlier 16 weeks of lucerne grazing on the number of fetuses present in ewes near mid-gestation compared with ewes on grass throughout this period. These findings from a commercial farm suggest that oestrogenic lucerne may be detected by monitoring ewe lambs for mammary development. This would enable farmers to manage pre-mating grazing of ewes with minimal risk of lowered ovulation rates. Keywords: sheep, coumestrol, oestrogen, fecundity, Medicago sativa, alfalfa

Abstract The objective was to determine whether grazing lucerne for 7 days before breeding and for the first 7 days of breeding affected the reproductive performance of ewes with different body condition scores (average of 3.2 versus 3.7). This was compared with grazing a grass-based sward with a high proportion of dead herbage that was available at the time of breeding in a summer dry region (Wairarapa) in autumn 2016. The lucerne contained large amounts of fungal infected stem and dead herbage, which induced high levels of coumestrol (107 ± 15 mg/kg DM). By the end of the feeding period the grass and lucerne on offer were both of low quality (7.8 MJ ME/kg DM). Ewes that grazed lucerne for 7 days before breeding and for the first 7 days of the breeding period produced 18 fewer (P