Condensed tannins fed to dairy goats: effects on digestibility, milk production, blood parameters, methane emission, and energy and nitrogen balances

Condensed tannins (CT) are plant polyphenols that can affect feed digestibility and are potentially able to reduce enteric methane emissions in ruminants. In this in vivo trial with 8 lactating goats, we investigated the effects of 4 levels of inclusion of a commercial CT extract from quebracho (0, 2, 4, 6% on DM basis; C, Q2, Q4, Q6, respectively). The experimental design was a repeated 4 × 4 Latin square with 28-d periods (24 d of diet adaptation and 4 d of sample collection) using metabolic cages and 4 open circuit respiration chambers. The inclusion of CT in the diets did not affect the dry matter intake (DMI) but caused a linear decrease in diet digestibility, with reductions up to -11% for dry matter (DM), -21% for crude protein (CP), -23% for neutral detergent fiber (aNDFom), and -13% for gross energy, when comparing the Q6 and C diets. However, ruminal total volatile fatty acids (VFA) concentration was not affected by CT, although there were changes in VFA proportions. Milk yield (g/d) was highest for Q4 (3371) and lowest for Q6 (3066). In terms of milk composition, CT induced a linear reduction of fat and CP concentrations. The reduction in CP digestibility resulted in a linear reduction in the milk urea level, up to -37% with Q6. Positively, CT linearly reduced the somatic cells count expressed as linear score. The feed efficiency was linearly decreased by CT inclusion. Furthermore, a shift from urinary to fecal nitrogen excretion was observed with CT. The retained nitrogen was always negative (on average -1.93 g/d). The methane yield (on average 19.2 g CH4/kg DMI) was linearly reduced by CT inclusion, up to -18% with Q6. Regarding the CH4 intensity, CT induced a linear reduction when expressed per kg of milk, but not per kg of fat and protein corrected milk. Moreover, the CH4 production per kg of digestible aNDFom was linearly increased by CT. The metabolizable energy intake (MEI) was not affected by the treatments, but the metabolizability (q = MEI/gross energy intake) was reduced as CT inclusion increased. From the results of the present study, it turned out that CT have a negative impact on feed digestibility and feed use efficiency. Condensed tannins can lower CH4 emissions from ruminants; however, the main mechanism of action is likely the decrease in feed digestibility. Furthermore, CT did not improve the N use efficiency. According to these findings, the positive environmental impacts of CT are only related to the shift from urinary to fecal N excretion.