CONICET | Buscador de Institutos y Recursos Humanos

The aim of this study was to evaluate the effect of dietary Acacia mearnsii tannin extract (TE) and managing the grazing schedule on N partitioning. The trial was conducted with 9 Holstein cows averaging 197 ± 12 d in milk, in a triplicate 3 × 3 Latin square design, through 3 22 d experimental periods. Cows were fed a ryegrass pasture (12.8% CP) and a total mixed ration (TMR, 16.4% CP). The 3 treatments consisted of morning grazing and afternoon TMR (AM), morning grazing and afternoon TMR added with 15 g/kg of TE (AMt), or morning TMR and afternoon grazing (PM). Individual dry matter intake was measured during 5 d of each period. Milk samples were collected during 2 d of each period for N analysis. Spot fecal and urine samples were collected twice daily during 3 d of each period. Fecal output was estimated using indigestible NDF as an internal marker. Urinary output was estimated assuming a daily creatinine excretion rate of 25.5 mg/kg of body weight. Statistical analysis was carried out using the PROC MIXED of SAS, where treatments were compared through contrast analysis. The treatments did not affect N intake or milk N excretion. Tannins did not affect fecal N excretion, but increased the fecal ADIN excretion (P < 0.01), and decreased total N and the urea-N excretion in urine (P < 0.05). Afternoon grazing increased fecal N excretion (P < 0.05), tended to decrease urinary total N excretion (P = 0.06) and decreased urinary N-urea excretion (P < 0.05). Although the treatments did not affect the total manure N excretion, the urinary N to fecal N ratio was reduced by 17 and 28% for AMt and PM respectively (P < 0.05). Results suggested that tannins inclusion in the TMR or afternoon grazing were effective to shift N excretion from urine to feces and to decrease the potential for ammonia emissions from manure of dairy cows. Table 1. Effect of tannin and grazing schedule on N utilization and manure excretion

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