My initial thought and principal rule-out was nitrates in forages or water.
In my experience, Neospora-induced abortions are typically mid-term, long before the fetus is viable.
In 40 years of veterinary practice and diagnostic pathology, I saw exactly zero cases of fungal abortion... I've seen photos, and would recognize it if it passed in front of me.
Ergonovine/ergovaline from fescue endophyte causes
peripheral vasoconstriction, shunting more blood to internal organs, not less. That's part of why we see increased heat stress/summer slump in cattle on high-endophyte fescue... they are less able to dissipate heat, because there is less circulation to extremities.
Simlarly, cold weather also causes peripheral vasoconstriction, in this case to preserve body heat... resulting in shunting of more blood to internal organs, including the uterus, resulting in enhanced fetal nutrition and larger birthweights (though not necessarily an increase in dystocia).
Conversely, if you were in a long stretch of really high temperatures, one could postulate that peripheral vasodilation - to help dissipate body heat - might have the effect of decreasing birth weight... though I don't know that 23 pounds is in that realm!
The Mare Reproductive Loss Syndrome deal in 2001 was associated with Eastern Tent Caterpillars, and it appears that the offending party was the 'setae', the little hairs on the caterpillars.
https://pubmed.ncbi.nlm.nih.gov/20081071/