Our aim was to evaluate whether increasing soluble carbohydrates in the herbage by changing the time of the grazing session or including Acacia mearnsii tannin in the diet would affect intake, digestion, N partitioning, and productive performance of dairy cows fed a diet combining ryegrass herbage with partial total mixed ration (PMR). We hypothesized that both strategies could reduce the concentration of NH3-N in the rumen, reducing urinary N excretion. Nine Holstein cows were used in a triplicate 3 x 3 Latin square experiment with 3 experimental periods of 22 d. The cows were fed a fixed amount of PMR [60% of the predicted individual dry matter intake (DMI)], and an unrestricted amount of herbage in 1 grazing session of 5 h/d. The treatments were (1) morning grazing session and afternoon PMR meal (AM); (2) morning PMR meal and afternoon grazing session (PM); and (3) morning grazing session and afternoon PMR meal supplemented with 15.0 g of tannins/kg of PMR dry matter (TAN). Milk production was not affected by treatments. Although the protein concentration was lower for TAN than for PM, no differences were detected for the yield of any component between treatments. The concentration of individual or grouped fatty acids in milk fat was not affected by treatments, except for 16:1 cis-9 and.9-desaturase ratios 14:1/14:0 and 16:1/16:0, which were lower for TAN. Treatments did not affect total DMI, but PM tended to increase herbage DMI and reduce dry matter and crude protein digestibilities. Treatments did not affect cow eating and ruminating behavior except for the proportion of time spent eating PMR, which was higher for PM and TAN. Although no relevant effects of treatments on ruminal fermentation, purine derivatives excretion in urine, or N excretion in milk were detected, both PM and TAN decreased the total N excreted in urine by an average of 8% compared with AM. In conclusion, changing the grazing session from the morning to the afternoon and including tannins in the diet were effective in decreasing the excretion of urinary N but did not change the productive performance of dairy cows fed PMR and ryegrass herbage.

The aim of this study was to evaluate the effect of the dietary inclusion of Acacia mearnsii tannin extract (TA) on nutrients intake and digestibility, and nitrogen (N) retention by sheep given a tropical grass-based diet. The trial was conducted with six castrated male sheep in a cross-over design in two 21-days experimental periods. The sheep were housed in metabolic cages and offered Tifton 85 hay (Cynodon dactylon) ad libitum plus concentrate at a rate of 12 g of dry matter (DM)/kg body weight (BW). The treatments were concentrate without (Control) or with 10 g TA/kg DM (Tannin). Concentration of TA in the diet was 3.8 g/kg DM and did not affect the feed intake or apparent digestibility. The TA decreased the true digestibility of n compounds (P<0.05) whereas did not impact the N retention, microbial N flow to the small intestine or the efficiency of rumen microbial protein synthesis. In conclusion, a low dietary concentration of TA did not impact the nutrients supply and N use by sheep fed with a tropical grass-based diet.

This study was carried out to evaluate the impact of including Acacia mearnsii tannin extract (TA) as a feed additive on nutrition and productive performance of dairy cows grazing a high‐quality temperate pasture and receiving supplementation with a concentrate feedstuff. Fourteen multiparous Holstein cows were assigned to either of the following treatments: concentrate without or with 20 g TA/kg dry matter (DM). Concentrate intake accounted for 32% of the total DM intake. Tannin addition increased the herbage DM intake by 22% (p < .05). There was no effect of TA inclusion on milk yield, milk composition, milk nitrogen (N) excretion, milk and plasma urea‐N concentration, urinary excretion of total N, urea‐N, and purine derivatives. However, TA inclusion increased the N intake and retention, total N excretion in manure, fecal N to urine N ratio, and decreased the dietary N efficiency for milk production and the percentage of ingested N excreted in urine (p < .05). In conclusion, supplementing dairy cows grazing a high‐quality temperate pasture with a concentrate containing 20 g TA/kg DM showed the potential of decreasing the proportion of ingested N excreted in urine without affecting the productive performance.

In addition to these differences in degradability that will result in different ruminant performance, other factors such as availability and price determine which cereal to use. The high cost of some feeds encourages the use of alternative local sources of energetic feed, notably the starchy tubers in temperate and tropical countries (^{WHEATLEY et al., 1995}). Among the starch-rich tuber feeds studied in substitution of cereals for ruminants are cassava (Manihot esculenta Crantz) (^{WANAPAT & KANG, 2015}), beet and by-products (^{EVANS & MESSERSCHMIDT, 2017}), potato (Solanum Tuberosum) (^{BABAEINASAB et al., 2015}) and sweet potato (Ipomoea batatas) (^{MATHER et al., 1948}).

Context: Tannin extracts have been investigated as natural feed additives with the potential to decrease the enteric emission of methane and urinary N and to improve the productive performance of ruminants. However, the impact of this additive in dairy cattle grazing tropical grass pastures has not been extensively evaluated. Aims: To evaluate the impact of the Acacia mearnsii bark extract (TA) on productive and nutritional variables in dairy cows grazing a Cynodon dactylon pasture and receiving supplementary corn silage and concentrate. Methods: Fourteen multiparous Holstein dairy cows were assigned in a randomised block design to either of the following two treatments: concentrate without TA (control) or with 10 g of TA/kg dry matter (DM). The concentrate and corn silage accounted for ~0.30 and 0.30 of total DM intake respectively. Key results: The dietary concentration of TA was 2.9 g/kg DM and it did not affect nutrients intake, milk production and composition, rumen microbial protein synthesis, plasma urea concentration or faecal N excretion. Total N and urea N excreted in urine were higher, whereas total N excreted in manure tended to be higher in TA treatment. Conclusions: No productive advantage was obtained by including TA in the diet of dairy cows grazing Cynodon dactylon pasture. Instead, it increased the excretion of labile urinary N. Implications: The effect of the use of TA as a feed additive for cows fed tropical grass-based diets is negative from the environmental point of view.