What commonly causes enlarged uteruses in swine? Mouldy grain containing the fungal by product called zearalenone.

[The chemical metabolite product of Fusarium fungus, named ] zearalenone mimics the effects of the female hormone estrogen and induces feminization at dietary concentrations of less than 1 mg/kg, while higher concentrations interfere with conception, ovulation, implantation, fetal development, and the viability of newborn animals.

Mycotoxins:  Risks in Plant, Animal, and Human Systems Council for Agricultural Science and Technology, Ames, Iowa, USA. Iowa State University, Ames ISBN 1-887383-22-0. Task Force Report No. 139  January 2003, Council for Agricultural Science and Technology, Ames, Iowa, USA, P 58

The lead up to this post is an earlier GMO Pundit item about a new feeding study that reveals differences in uterus size between groups of swine fed different diets.

A good web article that helps by providing context for this new report is the following:

Zearalenone Toxicity in Swine
Zearalenone Toxicity, a Significant Factor in Reduced Swine
Published on: 03/31/2011 Author/s : Fang Chi, Jonathan Broomhead, Chia Chung Chen (Amlan International)

They have this to say:

The presence of zearalenone in feed is unavoidable and zearalenone toxicosis is hard to treat. The most practical way to treat zearalenone toxicosis is to use an enterosorbent to prevent the initial dietary absorption by the gut and subsequent conjugated zearalenone compounds from being reabsorbed via enterohepatic circulation. Due to its rapid absorption in the small intestine, the inactivation of zearalenone after ingestion becomes extremely critical in stopping toxicity. By understanding the absorption and metabolism of zearalenone, producers are able to select the right methods to control and treat zearalenone toxicosis more effectively and economically.
The appearance of a red and swollen vulva in gilts and increased abortions and stillbirths during gestation may indicate zearalenone contamination of the feed. The mycotoxin zearalenone is the greatest contributor to economic loss in swine reproduction.1 It is produced by Fusarium fungi and commonly found in grains worldwide. Production, storage, and climate conditions all contribute to growth of these fungi.2 Due to its similar chemical structure to estrogen, zearalenone causes estrogenic effects in swine,3 and affects all age groups including gilts and sows that are the most sensitive. Its clinical and subclinical symptoms are illustrated in Figure 1.

FIGURE 1. Clinical Effects of Zearalenone Toxicosis

Understanding zearalenone's biological effects and its metabolism can help swine producers manage mycotoxicosis properly and prevent economic loss. The chemical structure of zearalenone is 6-(10-hydroxy-6-oxo-trans-1-undecenyl)-b-resorcyclic acid lactone (Fig ure 2). Its optically active isomer l-form is isolated from the mycelia of Fusarium graminearum. It is a colorless crystalline solid with a melting point of 164-165° C, indicating that zearalenone is hard to destroy under normal feed processing. In fact, when zearalenone was present in g round corn no decomposition was seen after 44 h at 150° C.4 Zearalenone is slightly soluble in water and is more soluble in acetone, ether, benzene, alcohols and aqueous alkali. Due to its similarity to estrogen, zearalenone and its metabolites can bind to estrogen receptors, causing estrogenic effects in pigs....continues

@ Zearalenone Toxicity in Swine - Engormix

Figure 6.6, showing swollen reproductive organs in mice, from Mycotoxins:  Risks in Plant, Animal, and Human Systems Council for Agricultural Science and Technology,  Ames, Iowa, USA. Iowa State University,  Ames ISBN 1-887383-22-0. Task Force Report No. 139  January 2003,  Council for Agricultural Science and Technology,  Ames, Iowa, USA