The influence of a diet of either pasture or hay on the development of lactic acidosis in sheep was investigated using a grain challenge approach. Twenty-four Merino wethers with a mean live weight of 36·7 (s.e.3·6) kg were used; 12 were adapted to grass pasture and 12 to hay (lucerne and oaten hay, 60: 40) for 4 weeks before being given 1 kg of crushed barley via stomach tube. Six sheep in each group were also given virginiamycin (VM; 50 mg/kg barley) with the grain to test the efficacy of this antibiotic in controlling the bacteria responsible for the development of acidosis. Changes in volatile fatty acid (VFA), pH, lactate and bacterial count in the rumen and faecal pH and dry matter (DM) were measured for a 24-h period following administration of the barley. Daily intakes of hay were measured for a 10-day period following grain engorgement. Total ruminal VFA increased (P < 0·01) over time and tended (P = 0·08) to be higher in sheep adapted to hay than in those adapted to pasture (67·5 v. 59·8 mmol/l). The molar proportions of VFA changed (P < 0·01) over time in favour of propionate in both groups. Ruminal pH was higher (P < 0·001) in pasture-adapted sheep, but declined (P < 0·001) in both groups over time following the introduction of barley. This decline in pH was associated with increases in ruminal concentration of VFA in pasture-adapted sheep and VFA and lactate in hay-adapted sheep. The addition of VM resulted in a higher (P < 0·001) proportion of propionate and a trend towards higher (P = 0·24) faecal pH and DM content. Faecal pH and DM content declined (P < 0·001) over time and was lower for the pasture-adapted sheep. The introduction of either barley alone or barley with VM from both hay and pasture diets increased (P < 0·05) the viable counts of total bacteria, Streptococcus bovis and lactic acid bacteria. Bacterial isolates were purified and identified by complete sequencing of the 16S rRNA gene to determine the predominant bacteria during the overfeeding of grain. Isolates from medium selective for S. bovis were all identified as this species when VM was not given. VM had no effect on counts of viable bacteria, but inhibited the growth of S. bovis.

This study has shown that sheep given hay are more susceptible to lactic acidosis, the signs of which can be reduced by VM.

This trial investigated the effects of virginiamycin on sow and litter performance. Virginiamycin was added to the sow diet during the period of gestation and lactation at the levels of0, 40 and 60 mg/kg, throughout two breeding cycles. Virginiamycin did not significantly affect sow weight change during gestation or lactation, or litter size and weight; there were no significant effects on food consumption (P > 0·05). However, when added at a level of 40 mgl kg virginiamycin tended to produce a better condition of sows which resulted in a significant reduction of replacement rate (P < 0·05), even though the proportion of gilts culled in the treated and untreated groups was high. The most evident result was a significantly shorter weaning-to-conception interval for the virginiamycin group at the end of the second reproductive cycle (first period, 5·9 days less than the control group, 2·5 days less than the 60 mg/kg group; second period, -9·5 days v. control group, P < 0·05, and -7·1 days v. 60 mg/kg group, P < 0·05), allowing a numerical improvement in productivity. Piglet performance was not significantly improved by the use of virginiamycin even though there was an evident trend in favour of the 40 mg/kg group which presented a higher proportion of heavier piglets (P < 0·05) and a reduction of pre-weaning mortality. A significant quadratic regression was found for weaning-to-conception interval and numeric productivity, indicating that the dosage more efficient to improve these parameters was 32 mg/kg. The results obtained in the virginiamycin 60 group were generally inferior to those of the 40 mg/kg group and also to the control group and seem to demonstrate that the 60 mg/kg dosage was too high.

The present paper discusses the results of a trial study, which was carried out to demonstrate the potential beneficial effects of virginiamycin (VM) on sow and litter performance. VM was added to the sow food at the levels of 0, 20 and 40 mg/kg, for a period of two breeding cycles, covering pregnancy and lactation. VM supplementation of the sow food improved sow performance by: (i) decreasing sow weight loss from farrowing to weaning (first period: 8·78 v. 3·54 v. 2·88 kg, P < 0·05; second period: 8·98 v. 3·93 v. 2·32 kg, P < 0·05), (ii) decreasing the duration of the reproductive cycle (first period: 154·5 v. 152·2 v. 151·2 days, P < 0·05; second period: 153·8 v. 151·5 v. 250·6 days, P < 0·05) and (in) increasing milk fat content (second period: 63·7 v. 81·3 v. 83·3 g/kg, P<0·05). Litter performance was also improved in terms of: (i) litter size at weaning (first period: 8·16 v. 8·88 v. 9·18, P < 0·05; second period: 8·98 v. 9·30 v. 9·76, P < 0·05), (ii) body weight at weaning (first period: 5·78 v. 6·29 v. 6·56 kg, P < 0·05; second period: 5·88 v. 6·38 v. 6·60 kg, P < 0·05), (Hi) average daily gain (first period: 172 v. 189 v. 197 g, P < 0·05; second period: 178 v. 292 v. 198 g, P < 0·05) and (iv) food conversion ratio (first period: 0·356 v. 0·331 v. 0·324, P < 0·05; second period: 0·363 v. 0·334 v. 0·325, P < 0·05). These beneficial effects of VM were more pronounced at the higher of the two inclusion levels.