Productivity and nitrogen metabolism of lactating cows fed pistachio hull with soybean meal partially replaced by slow-release urea

Pistachio hull (PH), a rich source of tannin, may interact with nitrogen (N) and affect N metabolism in dairy cows. This investigation aimed to assess the effects of feeding PH and two N sources (soybean meal: SBM or slow-release urea: SRU) on milk production, digestibility, and N metabolism in lactating dairy cows. The study utilized a 4 × 4 Latin square design with 12 Holstein dairy cows (body weight = 611 ± 39 kg; 84 ± 14 days in milk; 45.6 ± 5.3 kg/d milk), with treatments arranged in a 2 × 2 factorial configuration. The main factors were (1) the tannin source [without or with dried PH included at 76.5 g/kg of diet DM] and (2) two N sources [SBM vs. SRU]. Crude protein and total phenolic and tannin contents were greater, but NDF was lower in PH than in sugar beet pulp. Feeding diets containing 7.65% PH resulted in reductions in milk yield, milk urea N, and milk efficiency but milk fat and protein concentration increased. Milk yield was similar between SBM and SRU. Feeding PH in replacement to sugar beet pulp did not interact with N sources, except for feed intake, as the PH + SRU diet tended to decrease feed intake (P = 0.09). Feeding PH was also associated with decreased dry matter, crude protein, and NDF digestibility. Dry matter digestibility decreased when a portion of SBM was replaced by SRU. Feeding PH in replacement to sugar beet pulp lowered ruminal ammonia-N concentration and increased acetate-to-propionate proportion. Feeding PH vs. sugar beet pulp had no effect on N intake and N excretion in urine and milk, but it increased the amount of N excretion in feces and decreased apparent N efficiency (milk N/N intake). Alanine aminotransferase was lower in cows fed SRU only in PH- but not sugar beet pulp-containing diets. Blood creatinine was lower when SRU was included in the PH-containing diet, but the opposite was observed in diets containing sugar beet pulp. Feeding PH instead of sugar beet pulp did not affect blood urea-N but resulted in lower glucose and aspartate aminotransferase concentrations. Overall, cows fed PH in replacement to sugar beet pulp were less efficient in converting feed nutrients into milk production, likely because of its negative effect on nutrient digestion. Partial replacement of SBM with SRU had no influence on lactation productivity and urea-N concentration in milk and blood. This may imply the suitability of SRU in providing a sustained supply of N in the rumen, likely qualifying it as a viable source of N to replace a portion of SBM in dairy diets.