Understanding lamb mortality

Introduction

The Ontario lamb industry consists of diverse production systems, breeds and markets. This diversity creates many opportunities but makes it more difficult for producers to determine what production results are "normal" or "good."

Every flock has lamb mortality. Because there are so many reasons lambs do not survive, it is difficult to determine what changes in a flock could improve lamb survival. Even when post-mortems are done on all dead lambs, the results are often inconclusive. An extensive 1999-2000 health study done in Quebec^{footnote 1[1]} found that 68% of the post-mortems conducted on lambs that died in the first 2 days of life were inconclusive.

In a global literature review of lamb mortality research, Dwyer^{footnote 2[2]} reported that lamb mortality can be categorized into the following areas:

• trauma in birth process
• failure to adapt to post-natal life
• inability to maintain body temperature
• low lamb vigour
• poor maternal bond
• infectious disease
• functional disorders
• predation

Many factors influence and potentially cause problems in these areas - nutrition of the ewe during gestation, sanitation, ventilation, size of litter and birth weight. With lambs dying from a variety of different causes, it is even more difficult to determine what a "good" lamb survival rate would be for your particular sheep operation.

Actual mortality rates

Dwyer^{footnote 2[2]} found that estimates of pre-weaning mortality ranged from 10%-30% and that most of the mortality was in the first 3 days of life across different breeds and production systems. Several sources of Canadian mortality data over the past 30 years find average mortality rates of 12%-16%.

In a 1982-83 survey of sheep diseases in Canada, Dohoo^{footnote 3[3]} found that 116 farms estimated that total lamb mortality was a conservative 12.1%. The causes of mortality were grouped into 12 areas, with 38% of lambs dying of unknown causes.

In a 1999 productivity and health management survey of 64 Ontario sheep producers, Fischer and Menzies^{footnote 4[4]} found an average total mortality of 11.9%, with 5.5% stillborn, 5% mortality pre-weaning and 1.9% mortality post-weaning.

Table 1 shows the average mortality recorded by Ontario producers in the Sheep Flock Improvement Program^{footnote 5[5]} for the major breeds and across all breeds recorded in the program.

Adapted from Belanger, et al. (2001).

A portion of the 1999-2000 Quebec study focused on lamb mortality and showed an average mortality of 15.8% (Table 2), similar to the average results recorded on the Sheep Flock Improvement Program. The Quebec study also reported a range of 6.8%-31.7% mortality among the participating flocks, suggesting that a range of 10%-30% mortality could be a reasonable expectation in Ontario flocks.

Achievable mortality rates

After extensive research in reproductive performance, Shelton and Willingham^{footnote 6[6]} stated that 10% mortality after birth is a practical minimum under production conditions. Veterinarians often state that total mortality of 10% or less is an achievable, realistic number.

In the Quebec study^{footnote 1[1]}, one flock had a pre-weaning mortality rate of 6.8%, illustrating that a 10% total mortality rate is possible.

The advantages and risks of lambing on grass or lambing in the barn are different, but, as Dwyer^{footnote 2[2]} found in a literature review, a similar range of average pre-weaning mortality of 10%-30% is reported, regardless of the system. It is also expected that more prolific flocks will have more mortality.

In a 9-year summary of mortality at Spooner research station, Berger^{footnote 7[7]} reported that, on average, 9.9% of the lambs were born dead or died before weaning. The range of mortality between years was 5.6%-15%. The average litter size per year ranged from 1.96-2.36. This illustrates that even with prolific flocks, a pre-weaning mortality of 10% is achievable, with a total lamb mortality of 10% still possible.

On farm

It is important to keep mortality records to help identify possible causes and solutions. If your lamb mortality is high, it is also important to conduct post-mortems and record the results. Because mortality is caused by many factors, the solution is often not found very easily. In the Quebec study, Arsenault^{footnote 8[8]} reports post-mortem results that illustrate this problem (Table 3a and 3b).

Because so many post-mortems result in an inconclusive diagnosis, it is also important to keep other flock records that may help determine management changes that could improve lamb survival. Statistical analysis of the Quebec data determined that the risk of mortality in the first 10 days was greater for a lamb if it had a low birth weight (<4.0 kg), was born to a ewe over 4 years of age or was a male from a litter of three or more lambs (Table 4).

This means that changes in factors such as the average age of your flock, average birth weight and condition score of your ewes at lambing might influence the mortality rate of your flock. Lamb mortality can have a large influence on the profitability of an operation. It is important to remain dedicated to reducing mortality and conducting post-mortems even though, due to the large number of possible causes of mortality and inconclusive diagnoses, conclusions can be evasive.

Adapted from Belanger, et al. (2001).

Conclusion

Lamb mortality is influenced by a large number of environmental factors that are often difficult to control under farm conditions.

Keep production records that measure mortality rates and continually monitor and reduce the risk areas in your operation that contribute the most to mortality.

Every farm should have an optimum goal of 10% total lamb mortality; some operations will be able to achieve an even lower rate. If your farm operation has a pre-weaning mortality rate of more than 10%, look for opportunities to improve lamb survival.

References

Arsenault, J. 2002. Qu'en est-il de la sante de nos troupeaux ovins? Proceedings of Symposium ovin 2002. pp. 59-73.

Belanger, D., Arsenault, J., Dubreuil, P., Girard, C. 2001. Rapport du projet sur l'evaluation du statut sanitaire des troupeaux ovins du Bas-St-Laurent et de l'Estrie. Pgs Faculté de Medecine Veterinaire Universite de Montreal. Accessed October 21, 2010.

Berger, Y.M. 1997. Lamb mortality and causes - A nine-year summary at the Spooner Agricultural Research Station. In: Proceedings of the 45th Annual Spooner Sheep Day. Dept. of Animal Sciences, UW-Madison. pp. 33-40.

Dohoo, I.R., Curtis, R.A., and Finley, G.G. 1985. A Survey of Sheep Diseases in Canada. Can J. Comp Med 49:239-247.

Dwyer, C.M. 2008. Genetic and physiological determinants of maternal behaviour and lamb survival: Implications for low-input sheep management. J. Anim. Sci 86:E246-E258.

Fisher, J.W., and Menzies, P.I. 2000. An analysis of costs and returns of managing sheep flock health. Final report. Accessed October 18, 2010.

Kennedy, D. 2010. Sheep Flock Improvement Program Annual Report 2009. Ontario Ministry of Agriculture, Food and Rural Affairs. pp. 20-25.

Shelton, M., and Willingham, T. 2002. Lamb Mortality. Sheep and Goat Research Journal, vol. 17, no 3, 2002, p. 15-16