Tannins
The vegetative parts of guava have been reported to contain high levels of tannins: 10% DM in leaves and 11-30% DM in bark (Morton, 1987). A value of 15% DM of condensed tannins was reported in guava leaves (Suchitra et al., 2008). These tannins may have antinutritional effects when guava is used as fodder, and probably explain the low digestibility and degradability values observed in guava products. Conversely, high-tannin guava leaves have been considered a valuable additive for ruminant feeding as the tannins reduce methane emissions (Chatterjee et al., 2014; Hariadi et al., 2010).
Calcium oxalates
The vegetative parts of guava contain calcium oxalate crystals. High concentrations of calcium oxalates in guava bark may induce calculogenic minerals to accumulate, causing uroliths and penile obstruction in male goats (Adrian et al., 2011). In Hawaii, it has been observed that livestock grazing on pastures invaded with guava often suffer from bladder stones, particularly in goats that commonly strip the bark off the trunk (Adrian et al., 2012).