The effect of different levels of poplar (Populus) supplementation on the reproductive performance of ewes grazing low quality drought pasture during mating

A grazing experiment, conducted for 71 days from 31 January to 12 April in the late summer/autumn of 2001 at Massey University’s Riverside Farm, in Masterton (New Zealand), compared the effect of poplar supplementation (Populus deltoides × nigra, clone Veronese), during mating, on ewe production and reproduction when grazing low quality drought pasture. A rotational grazing system with 300 mixed age Romney ewes () was used, with 100 ewes per treatment. All ewes were offered 0.70 kg dry matter (DM)/day of low quality pasture, containing 84% dead matter, with pre- and post-grazing pasture masses of 1040 and 531 kg DM/ha. Ewes were randomly allocated to three treatment groups, being: high supplementation, low supplementation and control. The high and low treatment groups were offered 1.50 kg/ewe/day (fresh) and 0.75 kg/ewe/day (fresh) poplar cuttings, respectively. The effect of poplar supplementation on liveweight (LW) and body condition score (BCS) change; reproductive rate at pregnancy scanning, lambing, docking and weaning; and wool production and staple length was measured. The poplar diet selected contained 22 g/kg DM of phenolic glycosides and higher levels of nitrogen (N; 28.4 g/kg DM versus 17.8 g/kg DM) and condensed tannin (CT; 7.0 g/kg DM versus 1.5 g/kg DM) and was of higher organic matter digestibility (0.66 versus 0.52) than the pasture diet selected. Voluntary DM intake of poplar progressively increased with time of supplementary feeding for both treatments (P<0.01), due to increases in both DM content and the diameter of stem consumed. Reproductive rate was low in the control ewes (121 lambs born/100 ewes mated), and poplar supplementation increased ewe reproductive rate by approximately 20 and 30% units for the low and high treatment groups, respectively, compared to the control group, at scanning, lambing, docking and weaning. The increase in reproductive rate in supplemented ewes was due to increases in both conception rate and fecundity, with a higher proportion of pregnant ewes, and a higher proportion of multiple pregnancies, in the supplemented groups. Ewes in the high and low treatments lost slightly less LW (−67 and −71 g/day versus −82 g/day; P<0.05) and BCS (−0.78 and −1.27 units versus −1.31 units; P<0.05) as a result of poplar supplementation, but these differences did not occur in the post-treatment period. There were no treatment effects on wool production or staple length, and only small treatment effects on the LW of single- and twin-born lambs at birth and weaning. Poplar cuttings are a beneficial supplement for increasing the reproductive rate of ewes grazing drought pasture during the pre-mating and mating periods. Poplar supplementation increased intakes of DM, metabolisable energy (ME) and crude protein (CP), and increased the estimated g CP/MJ ME eaten during the mating period. Increased concentrations of total N, CT and water-soluble carbohydrate (WSC) in the diet of supplemented ewes would also be likely to increase outputs of undegradable dietary protein and microbial protein from the rumen, per unit of CP consumed. A combination of these mechanisms, especially the likely increased absorption of protein, probably explains the increased ewe reproductive rate from poplar supplementation.