Can mating ewes on Lotus corniculatus be used to reduce lamb mortality between birth and weaning? : a thesis presented in fulfillment of the requirements for the degree of Master in Animal Science at Massey University
A grazing experiment was conducted over 74 days from late summer to early autumn in 2004 at Massey University's Riverside dryland farm, in the Wairarapa Region, New Zealand. The experiment compared the effects of grazing ewes on Lotus corniculatus (birdsfoot trefoil; cv. Grassland Goldie) versus perennial ryegrass (Lolium perenne)\white clover (Trifolium repens) pasture for 10 weeks during pre-mating and mating time, on reproductive performance of the animals and mortality of lambs from birth to weaning. Shorn mixed age Romncy ewes with an initial mean live weight of 56.7 Kg and mean body condition score of 2.9 were rotationally grazed on L. corniculatus (n = 300) or pasture (n = 300) at a herbage allowance of 2 Kg DM/ewe/day. At the end of the L. corniculaus feeding, animals from both treatment groups were joined and grazed on pasture until weaning. Total condensed tannin (CT) concentration in the diet selected was 22g/Kg DM for L. corniculatus, with only trace amounts in pasture. Digestible organic matter in dry matter (DOMD; O.66 vs 0.64) and metabolizable energy (ME) concentration (10.75 vs 10.42 MJ/ Kg DM) were slightly, but significantly, higher in L. corniculatus than in pasture. There was no difference in the nitrogen (N) content (g/Kg DM) neither in the in vitro organic matter digestibility (OMD) between both diets, whilst the concentration of neutral detergent fiber (NDF) was lower in L. corniculatus than in pasture. Liveweight gain (g/day) during the experimental feeding period was lower in the L. corniculatus group (94 g/day) than in the control pasture group (130 g/day), whilst increments in body condition score (BCS) during the same period were similar in both groups, with a mean total increment of 0.11 units. There were no significant differences in the reproductive rate of ewes (lambs born/100 ewes mated); however, there was a tendency in the lotus group to have a higher reproductive rate at scanning and lambing when data was corrected to similar liveweight gain during mating. There was no difference in fecundity rate (proportion of singles, twins and triplets) at lambing between treatment groups; however, there was a tendency for the lotus group to have a higher proportion of triplet lambs when data was corrected to similar liveweight gain during mating. The mean conception rate was 96.5% with no difference between treatment groups. Ewes in the lotus group lambed earlier than their counterparts in the control pasture group, with the difference being significant in twin and triplet-bearing ewes (P<0.05); differences in mean lambing dates were 0, 2 and 6 days for single, twin and triplet-bearing ewes respectively. Birth weight of female triplet lambs born to ewes mated on L. corniculatus was higher than for their counterparts born to ewes mated on control pasture (P<0.05); there was no other nutritional effect on either birth weight or weaning weight. Lamb mortality was directly proportional to birth rank regardless of nutritional treatment group, with mortality being of 14.7%, 26.7%, and 45.6% for singles, twins and triplet lambs respectively. Ewes' wool production was similar for both treatment groups. Control pasture had an unusual high nutritional quality for a dryland area during autumn, which was due to an unusual wet autumn. The good nutritional status of ewes in the control pasture group was reflected in the high reproductive parameters measured in this experiment, leaving little room for differences when compared to the parameters observed in the lotus group. The co-variance adjustment to equal liveweight gain during mating suggests that in a normal dry autumn, mating on L. corniculatus would have increased the number of triplet pregnancies. It was concluded that the high lamb mortality rate in the lotus group was influenced mainly by the combination of three factors: a heavy storm during the third lambing week, a trend to more of triplet lambs in this group, and a higher proportion of triplet bearing ewes lambing just before or during the storm in this group than in the control pasture group. It is suggested that the effect of L. corniculatus feeding during mating and early pregnancy on triplet pregnancies may be due to the increased EAA absorption from the action of the condensed tannins. Factors which may have influenced the results of the present experiment are discussed, and suggestions arc made concerning the design of future field experiment of this type.