The effect of stage of harvest on the process of digestion in cattle fed winter oats indoors. II. Nitrogen digestion and microbial protein synthesis

The effects of dates of harvest of winter oats (Avena sativa L.) on the sites of nitrogen (N) digestion and ruminal microbial CP synthesis were studied in cattle fed indoors. The forage was cut from randomly selected areas of a single field at five dates from autumn to spring: A (25 May, autumn); EW (25 June, early winter); W (9 August, winter); ES (22 September, early spring); LS (20 October, late spring). The trial was arranged as a completely randomized design considering dates of harvest as treatments. Two heifers weighing 567 ± 17.8 kg fitted with cannulas in the rumen, proximal duodenum and distal ileum were given ad libitum access to feed. Animals were dosed with chromic oxide and Co-EDTA as external markers. The N content of the forage (g kg−1 dry matter) varied between 37.2 in A and 16.5 in LS (P < 0.0001) and N intakes (NI, g kg−1 live weight (LW)) ranged from 0.81 to 0.23 with the lowest value in LS (P < 0.02). Mean ruminal ammonia (NH3-N) concentration ranged (P < 0.0005) from 32.5 mg dl−1 in A to 5.1 in LS. Non-ammonia N (NAN) flow to the small intestine ranged from 0.46 to 0.25 g kg−1 LW (P < 0.001). For A, EW, and W forages the NAN flows in relation to NI (NANNI, g g−1) were lower (P < 0.05; mean: 0.69) than in the ES and LS forages (mean: 1.12). The NANNI was negatively related to NI kg−1 digestible OM intake (DOMI), NANNI = 1.63 − 0.015 g NI kg−1 DOMI; (r = −0.94; P < 0.001). Microbial CP synthesis (g N kg−1 OM apparently digested in the rumen, OMADR) ranged from 31.5 to 46.0, tended to be lower in A (P < 0.15) than in other forages and averaged 0.69 of the duodenal NAN. Ruminal CP degradability (g g−1 NI) tended to be higher (P < 0.13) in A (0.81) than in ES (0.65). The date of harvest of winter oats can influence ruminal N digestion and intestinal supply of N. Duodenal NAN flow is dependent on NI and NANNI depends not only on NI, but also on OM digestibility of winter oats. A high ruminal ammonia concentration in winter oats with high N content may be due to inefficient microbial capture of ruminally degraded CP, even when the efficiency of microbial synthesis (g N kg−1 OMADR) may be high.