About 13 million lambs die each year in Australia from cold stress although this may be in combination with mis-mothering and starvation. New-born lambs are highly sensitive to chilling during the first few days of life. The lower the temperature of the air and the higher the wind speed, the more likely a lamb is to be dangerously chilled, especially while its coat is wet from birth fluids. The risk of death is greatly increased if rain keeps the lamb wet for a longer period.
Lambs that are born as twins, or in larger litters, face even greater risks to their survival. Twins are generally lighter than single lambs and lighter lambs have a relatively greater surface area from which to lose heat. Moreover, survival following birth depends on the lamb being able to suckle successfully within a short time and the amount of milk available will depend on the number of siblings surviving birth. Twins they must compete for milk from a ewe which sometimes may be greatly weakened by nutritional stress in late pregnancy. Mis-mothering is an additional risk for twin-born lambs.
The Mortality Model
LambAlive is based on a comprehensive analysis of lamb mortality in Merino and crossbred flocks at CSIRO's Ginninderra Experiment Station near Canberra. The program estimates risk of lamb mortality by calculating a "chill index" from historical daily weather records. A calibration based on data collected from the field experiments is then applied to the chill index to predict the level of lamb mortality.
The statistical model underlying this analysis provides a prediction of average lamb mortality that would be expected over a number of years. Obviously it is impossible to predict with any precision at joining the weather that will prevail at lambing 5 months later. Thus the program only indicates the likelihood that on average over a number of years losses will be higher or lower for lambings commencing on alternative dates
The familiar "grazier alerts" issued by the Australian Bureau of Meteorology warning of possible lamb losses are based on an empirical model (Nixon-Smith 1972) derived from basic research into lamb mortality (e.g. Alexander 1962) and field observations made in western Victoria. The Nixon-Smith model assumes that the rate of heat loss by new-born lambs is a function of air temperature and wind speed, with an additional cooling effect due to evaporation from the birth coat of moisture resulting from wetting by rain.
In LambAlive a simple adaptation of the same empirical model is used to calculate a chill
index appropriate for the new-born lamb born at any time during a 24 hour period:
The results from a large grazing experiment near Canberra with Merino and Border Leicester x Merino ewes grazed over several years at a wide range of stocking rates were used to establish a relationship between the proportion of lambs dying each day during lambing and the chill index recorded for the same day (Donnelly et al. 1982; Donnelly 1984). The statistical model describing this relationship estimates the odds for a lamb surviving the first three days of life, taking into account the chill index, the relative body condition of the ewes at lambing and whether the lamb was born as a single or in a litter.
The levels of mortality predicted by LambAlive only include lambs that were born alive but died within the first three days of life. Other causes of death, including lambs that are born dead, are not considered. Note, however, that the cause of death may initially be due to mis-mothering and starvation rather than excessive cold leading directly to hypothermia. Thus a twin-born lamb may be abandoned by its mother and ultimately die from starvation and hypothermia; such deaths are included in the losses calculated by LambAlive.
The spreadsheet also assumes that a lambing period is of 17 days duration. This is the length of an oestrus cycle at mating and, unless stated otherwise, all losses are calculated over 17 day periods starting on the specified date. If SireSine raddles are fitted to the rams at joining and the ewes are joined for 5 weeks, the flock can easily be split into early and late lambing groups, each with an expected lambing period lasting 17 days.
However, such a detailed approach to management is not essential to obtain useful information from the spreadsheet.
In deriving the calibration several assumptions were based on inspection of the experimental data. Firstly, the numbers of deaths on the second and third days of life were approximately 2/3 and 1/3 respectively of those lambs recorded dying on the first day. These proportions were used to weight the chill index over the first 3 days of life of the lambs. Secondly, the maternal weight at lambing (body weight less the weight of the uterus and its contents) is calculated from the mean weight of a group of ewes of the same age and grazed in a common flock, and lambing on days with a similar level being recorded for the chill index.
Another important assumption is that lambings occur uniformly throughout the day. There is no reliable data to show that more ewes lamb at night than during the day. Therefore average daily values for wind and temperature and total daily rainfall are used to calculate the chill index.
Finally, it takes five to seven days on average for a weather system to cross the southern part of the Australian continent from west to east. Each system will have embedded in it one, or sometimes several, cold fronts that may be rain-bearing. The passage of an individual front usually lasts about one day. Thus if a flock of ewes lamb over a 17 day period, then on average 6% of ewes will lamb each day so that only a small percentage of the lamb drop will be at risk during the passage of an individual weather front. Depending on the severity of the weather during the passage of the front, all, none or a proportion lambs born at that time may perish. However, during the period between fronts the risk of losses from hypothermia is much reduced.