Nitrogen balance and blood urea nitrogen by gestating sheep offered alfalfa silage wrapped with or without an enhanced oxygen barrier plastic after time delays up to three days

•Moist alfalfa round bales were wrapped with conventional plastic or plastic with an enhanced oxygen limiting barrier either the day of baling or 1, 2, or 3 days later.•Wrapping alfalfa silage with a plastic with an enhanced oxygen barrier improved nitrogen retention by sheep.•Delayed wrapping increased fiber-bound nitrogen concentrations and decreased apparent nitrogen absorption.•Nitrogen bound in the acid detergent fiber fraction was somewhat available to sheep. Exposing ensiling forage to oxygen can result in dry matter deterioration and reduce silage intake by animals. This study was conducted to investigate the effects of two different wrapping sources and time intervals between baling and wrapping on N utilization in gestating sheep offered alfalfa silage. Alfalfa silage was baled in large round bales (moisture concentration = 591 ± 43.0 g/kg), then wrapped with plastic using either an enhanced oxygen-limiting barrier (EOB) or no additional barrier (conventional wrap; CW) on the day of baling or 1, 2 or 3 d after baling. After 5 months of storage, silages were chopped, then offered randomly for ad libitum consumption to 16 gestating ewes [63.5 ± 1.71 kg body weight] to provide 2 observations per treatment for each of 3 experimental total collection periods. Each study period consisted of a 10-d dietary adaptation period followed by 7 d of total fecal and urine collection. Blood was collected immediately prior to feeding and four hours after feeding on the last day of each period. Data were analyzed using PROC MIXED of SAS for a 2 × 4 factorial treatment arrangement, while blood data were treated as repeated measures. When expressed as a portion of total N, neutral detergent insoluble N (NDIN) content of silage increased (P ≤ 0.02) linearly and quadratically, and acid detergent insoluble N (ADIN) concentrations increased (P < 0.01) linearly with longer wrapping delays. Nitrogen apparent absorption (g/d and g/kg N intake) and urinary N (g/d) decreased (P ≤ 0.04) linearly with extended time delays between baling and wrapping. Retained N (g/kg of N intake and of absorbed N) was greater (P = 0.04) from silage wrapped with EOB. Blood urea N concentrations (mg/dL) decreased (P = 0.02) quadratically with extended delay between baling and wrapping. Digestion of NDIN (g/kg NDIN intake) increased (P = 0.03) linearly with extending time delay when silage was wrapped with EOB and linearly and quadratically (P < 0.01) when wrapped with CW. Digestion of ADIN incre