Reduction in lamb mortality by means of grass windbreaks : results of a five year study.

Abstract

REDUCTION IN LAMB MORTALITY BY MEANS OF GRASS WIND-BREAKS: RESULTS OF A FIVE-YEAR STUDY G. ALEXANDER*, J.J. LYNCH**, B.E. MOTTERSHEAD** and J.B. DONNELLY*** SUMMARY The importance to lamb survival of reducing wind velocity over a lambing paddock has been demonstrated in a 5-year study involving 1153 fine-wool Merino lambs at Armidale, N.S.W., using wind-breaks of a sterile Phalaris; survival of singles was increased by 10% and survival of multiples by 32%. Shearing the ewes just prior to lambing had no consistent effect on lamb survival. Wind (> 15 km/h) combined with rain and/or with low air temperatures (< 5OC) increased mortality by increasing the proportion of lambs that died of hypothermia immediately after birth, and resulted in some l-2 day-old lambs dying with milk in the Vascular damage to the central nervous system (CNS) increased the abomasum. susceptibility to hypothermia. The presence of peripheral oedema (cold injury) was an unreliable autopsy criterion that death was due to cold exposure. INTRODUCTION Cold exposure is a well-recognized cause of lamb mortality (Obst and Day 1968) that is reduced by lambing indoors (Watson et al. 1968) or lambing in a stand of mature Phakris aquatica giving protection from wind (Egan et al. 1972). However, the necessary inputs of capital, labour and managerial skills have disAs a step towards the couraged practical use of these methods in Australia. development of inexpensive shelter systems for field lambing, wind-breaks in the form of hedges of a sterile but vigorous Phak~i~ hybrid, reputed to be less palatable to sheep than P. aquatica, were established in small lambing paddocks The study terminated after 5 years (1974-78) and (Alexander and Lynch 1976). the effects of the shelter on survival rates of the lambs are reported, together with an assessment of the causes of death. METHODS P. aquatica and Trifo%m repens pasture, during July-August when l-2 day periods The study was conducted at Armidale, N.S.W., on a flat, treeless area of of rain, sleet or light snow, interspersed with 3-4 day periods of fine, cool days and frosty nights, are expected (Fig. 2 of Lynch and Alexander 1976). The layout of the paddocks and hedges, the fine-wool Merino sheep and their management and the observations made on weather and sheep have been described in detail elseIn brief, l-l.5 m high hedges of the P?zaZaris where (Lynch and Alexander 1976). hybrid (P. aquatica x P. amndinaeeae) 20 m apart and at 90� to the prevailing westerly wind were established in four 0.4 ha paddocks. Four similar adjacent Sheep shorn 3 to 6 days before the due paddocks were left without shelter. lambing date were lambed in two sheltered and two unsheltered paddocks, and Groups of ewes were added to unshorn sheep were lambed in the other paddocks. the paddocks every 3-4 days, according to mating date, and recently-lambed ewes Paddocks were floodwere removed to a common area, within l-3 days of lambing. Lamb weights and lit at night and animals were observed at least once hourly. rectal temperatures, and any untoward behaviour were recorded about 2 h after birth and thence at various intervals. In all, 830 single lambs, 160 pairs of Lambs that died during the 3-4 twins, and one set of triplets were observed. week lambing period were autopsied within 48 h, mostly within 24 h of death. ** *** * CSIRO Division of Animal Production, Prospect, N.S.W. 2148. CSIRO Division of Animal Production, Armidale, N.S.W. 2350. CSIRO Division of Mathematics and Statistics, Prospect, N.S.W. 2148. 329 Animal hduction in Australia Mortality data for singles and multiples were analysed separately using a generalized linear model (Nelder and Wedderburn 1972) assuming a binomial error distribution and applying a complementary log log linking function in the form Y = ln[-ln(l-p)] where p is the mortality as a proportion of the total number of lambs. Tests of significance were done using an analysis of deviance, the change in deviance after sequential fitting of treatment effects being distributed approximately as x2 . The 'treatment', 'bad' weather, was arbitrarily defined as a combination of wind (> 15 km/h at 2 m above ground) with rain and/or with a screen temperature < 5OC at any time during the first 6 h of life. RESULTS Mortality Total lamb mortality over the 5 years is partitioned in Table 1 according to treatment. Overall 9% (37/413) of single lambs born in shelter died, compared with 17% (73/417) unsheltered, representing an improvement in surviva2 rate (91.0% with shelter vs 82.5% withoutshelter) of lO%;and 36% (62/173) of sheltered multiples died compared with 51% (77/150) of unsheltered multiples, an improvement in survivalrate of 32% (64.2% vs 48.7%). TABLE1 Total mortality of lambs over 5 years In the analysis of deviance of the pooled 5-year data, mortality was decreased by shelter (P < 0.001 for singles and P = 0.005 for multiples), and inThe mortalities creased by bad weather (PC 0.001 and P = 0.004 respectively). of multiples with and without shelter in bad weather were similar, and the weather x shelter effect was significant for singles (P = 0.006) but not for multiples (P = 0.28). There was no main shearing effect with singles (P = 0.16) but a Weather x shearing interactions marginal effect with multiples (P = 0.064). (P = 0.078 for singles and P = 0.018 for multiples) indicated that in the absence of shelter, mortality of singles from shorn ewes was increased by bad weather to However, mortala greater extent than mortality of singles from unshorn ewes. ity of multiples from unshorn ewes was increased by bad weather more than mortalThere was considerable variability from year ity of multiples from shorn ewes. to year in the significance of the various treatment effects and interactions. In a separate analysis of the pooled data, shelter, when winds were < 15 km/h, also significantly reduced mortality of singles in wet weather [3% (l/33) with shelter vs 21% (6/29) with no shelter] (P < 0.05) but not fine weather, and of multiples in fine weather [31% (38/124) with shelter vs 48% (56/116) with no shelter] (P < O-01), but not wet weather. 330 Animal production in Australia Causes of death Declining body temperature within two hours of birth appeared to be the direct cause of death of 18% (59/323) of multiples, but only 5% (40/830) of singles (P < 0.001) (Table 2); most died without standing or attempting to suck. The incidence of hypothermia in sheltered singles was not related to weather during the first 6 h of life, but the incidence increased in bad weather with unsheltered singles (P < 0.001) and with multiples whether sheltered or not (P < 0.001) (Table 2). In this death-class, 35% of the singles and 24% of the multiples, showed evidence of haemorrhage of the CNS. TABLE 2 Proportion of lambs in three death-classes Twenty-one lambs aged from 18 to 53 h were found to have milk in the abomasum at autopsy. Their deaths coincided with periods of inclement weather, but with only 7/21 was the weather bad during the first 6 h of life. Nineteen of the 21 had experienced periods ranging from 14 to 35 h of bad or merely wet weather that began sometime between birth and 36 h of age and continued to within 6 h of death or to 13 h in one instance. The remaining two, both singles, experienced temperatures of<5OC for most of their 33-44 h life, but the winds were < 15 km/h and there was no rain. The 212 incidence of CNS haemorrhage in these lambs was significantly higher than the 2/19 in the others (P < 0.05); 25% (3/13) of the singles and 12.5% (l/8) of mult' 1es in this class showed CNS damage. ip Among the dead lambs (57 singles and 72 twins) that did not fall into either of the above classes ('remainder', Table 2), mortality appeared to be little influenced by bad weather, except in unsheltered multiples; but their low mortality (4.6%) corresponded with a very high incidence (54.6%) of similar multiples In this remainder class, evidence dying with low post-natal temperatures. 331