Effect of dietary Quebracho tannin extract on feed intake, digestibility, excretion of urinary purine derivatives and milk production in dairy cows

The aim of this study was to evaluate the effects of dietary Quebracho tannin extract (QTE) on feed intake, apparent total tract digestibility (ATTD), excretion of urinary purine derivatives (PD) and milk composition and yield in dairy cows. Fifty Holstein cows were divided into two groups. To reach a similar performance of both groups, cows were divided according to their milk yield, body weight, days in milk and number of lactations at the start of the experiment averaging 33.2 +/- 8.2kg/d, 637 +/- 58kg, 114 +/- 73d and 2.3 +/- 1.6 lactations, respectively. The cows were fed a basal diet as total mixed ration containing on dry matter (DM) basis 34% grass silage, 32% maize silage and 34% concentrate feeds. Three dietary treatments were tested, the control (CON, basal diet without QTE), QTE(15) (basal diet with QTE at 15g/kg DM) and QTE(30) (basal diet with QTE at 30g/kg DM). Two treatments were arranged along six periods each 21d (13d adaptation phase and 8d sampling phase). The ATTD of DM and organic matter were reduced only in Diet QTE(30), whereas both QTE treatments reduced ATTD of fibre and nitrogen (N), indicating that QTE impaired rumen fermentation. Nevertheless, feed intake was unaffected by QTE. In Diet CON, urinary N excretion accounted for 29.8% of N intake and decreased in treatments QTE(15) and QTE(30) to 27.5% and 17.9%, respectively. Daily faecal N excretion increased in treatments CON, QTE(15) and QTE(30) from 211 to 237 and 273g/d, respectively, which amounted to 39.0%, 42.4% and 51.7% of the N intake, respectively. Hence, QTE shifted N excretion from urine to faeces, whereas the proportion of ingested N appearing in milk was not affected by QTE (average 30.7% of N intake). Daily PD excretion as indicator for microbial crude protein (CP) flow at the duodenum decreased in treatment QTE(30) compared with Diet CON from 413 to 280mmol/d. The ratios of total PD to creatinine suggest that urinary PD excretion was already lower when feeding Diet QTE(15). While there was no effect of Diet QTE(15), treatment QTE(30) reduced milk yield, milk fat and protein. Both QTE treatments reduced milk urea concentration, which suggest that ruminal degradation of dietary CP was reduced. In summary, adding QTE at dosages of 15 and 30g/kg DM to diets of lactating dairy cows to improve feed and protein use efficiency is not recommended.