Performance of beef cattle grazing oats supplemented with energy, escape protein or high quality hay

The objective was to evaluate the effect of supplementing with energy and energy–escape protein concentrates or hay, on animal productivity, rumen and blood parameters of cattle grazing oats pasture (OP). Two experiments (Exp 1 and Exp 2) were conducted in two consecutive years, with Aberdeen Angus heifers grazing continuously OP. In Exp 1, 24 heifers (204 kg initial-weight) were grazing 55 days. The treatments were: unsupplemented control (CON1), or 2 kg per animal per day of ground corn (CORN1), or 75% ground corn + 25% corn gluten meal (C-GM). In Exp 1 herbage mass of OP decreased from 1777 to 1209 kg dry matter (DM)/ha from July to September. The DM concentration averaged 296 g/kg of fresh OP, and crude protein (CP) was below 100 g/kg DM; initially NPN was 47% of CP, decreasing later. Ca increased with time from 3.5 to 6.0 g/kg DM, the other minerals were not affected by date. The ratio K/(Ca+Mg) was high on July (3.80 meq/kg DM) and decreased in August to September (2.73 and 1.94, respectively). Average daily gain (ADG) averaged 921 g per day for C-GM versus 670, 759 for CON and CORN1, respectively. A non-significant decrease in rumen pH was detected for CORN1 (mean of 6.26). Rumen NH(3)-N was not affected by treatment, being the highest value 4.44 mg/dl in September (P<0.05). Plasma levels of Ca and Na were normal and not affected by treatment or date; Mg and K showed higher levels for CORN1 and C-GM versus CON1, K and P increased with date (P<0.05). In Exp 2, 36 heifers (192 kg-initial weight) were grazing OP during 140 days. Treatments in Exp 2 were: unsupplemented control (CON2), ground corn (CORN2) and alfalfa hay (AH). The animals did not reach the targeted intake level for supplements. Thus, daily intakes resulted 1.57 and 1.06 kg per animal for CORN2 and AH. In Exp 2, herbage mass was 1600-1700 kg/ha in May to September, decreasing in October. The lowest DM content was 20 g/kg of fresh OP in May to June, increasing in July. Initial CP was the highest (164 g/kg DM), it dropped in July increasing again in October; NPN content was 28% of CP in May, decreasing in October. Ca content raised from 3.6 g/kg DM in May to 6.0 g/kg DM in October. K decreased but Mg, P and Na have not changed their concentrations with time. The ratio K/(Ca+Mg) for OP was highest on May (6.23 meq/100 g DM), decreasing in October. ADG for CORN was 872 g, higher (P<0.05) than 718 g (CON2) or 781 g (AH). No differences were found for ruminal pH (mean=6.8); but for ruminal NH(3)-N the highest value was for AH (3.35 mg/dl), also varying with sampling date (P<0.05). Plasma levels of Ca and K decreased with time, being in July 8.76 and 19.22 mg/dl, respectively (P<0.05). Mg was not affected by either treatment or sampling date, P was higher for CORN2=7.91 mg/dl. Na levels were larger for CON2 and AH than for CON2, with 272, 282 and 256 mg/dl, respectively (P<0.05). Animal performance was improved by both energy and escape protein supplementation. However, treatments did not appear to modify to a great extent ruminal fermentation patterns or have any effect on blood mineral levels.