Rickets is a disease of young growing animals characterised by defective calcification of growing bones. Bones are subject to pressure distortion and fractures. Rickets can be caused by a deficiency in vitamin D, calcium and/or phosphorus or an imbalance of the calcium: phosphorus ratio. I had seen this condition sporadically in the Forbes district since 2005. However in August 2010 we had an increased number with 7 cases investigated and/or reported between 5 August and 24 August. All of these cases involved second or first cross lambs grazing oat crops.
On 5 August I was called to a property as 14 of 400 4 month old second cross lambs were recumbent. They had been mustered the day previously and were to be weaned. They had been vaccinated with 6 in1 and drenched the previous day. The lambs had been on grazing oats since birth and up until a week ago were supplemented with a loose lick containing equal parts lime/salt/causmag. They had also been supplemented with oats in a creep feeder and oaten hay. On examination all 14 were in sternal recumbency. They were bright however reluctant to rise. One had a fractured hind leg. It was euthanased and a post-mortem confirmed a proximal fractured tibia. The frontal bones were easily compressed.
The producer was advised to treat these lambs with calcium borogluconate and lucerne hay. He put the remainder of the mob on a lucerne paddock. There were no more cases and the majority of the recumbent lambs recovered.
On 6 August 2010, I was called to examine 10 of 400 4-month-old crossbred lambs who had gone down after the mob was mustered so the ewes could be shorn. These lambs had been on grazing oats since birth. They were given a range of supplements supplied as loose licks including commercial loose licks. The majority were in sternal recumbency and reluctant or unable to rise.
Once again, the owner was advised to treat the clinical cases with calcium borogluconate and put the mob on a lucerne paddock. The majority recovered.
On 23rd August DV Nik Cronin was called to examine 17 of 400 6 month first cross weaner lambs that went down or became lame after mustering. They had been grazing oats for prolonged periods. A loose lick was provided but not always utilized. Two lambs had broken limbs.
There appears to be some confusion as to the underlying cause of rickets in sheep. Osteodystrophies generally are due to a dietary deficiency or disturbance in the metabolism of calcium, phosphorus and Vitamin D. Calcium deficiency can be caused by an absolute deficiency in calcium (primary) or an imbalance of the calcium: phosphorus ratio (secondary). Phosphorus deficiency can be caused by an absolute deficiency of phosphorus (primary) or excess calcium in the diet (secondary). Vitamin D deficiency is caused by an absolute deficiency in Vitamin D (primary) or deficiency due to rachitogenic factors (secondary). Carotene is the best known of these. Other causes listed include internal parasitism, copper deficiency and malnutrition.
Rickets in lambs grazing cereals appears to be multi-factorial. There is a complex interaction between Vitamin D, calcium and phosphorus. Vitamin D facilitates the absorption and incorporation of calcium and phosphorus into bone. If the ratio of these two minerals is correct absorption of the minerals take place readily. However if there is an imbalance in this ratio excess Vitamin D may be required for the animal to adjust itself. Conversely if Vitamin D is lacking more minerals and a more correct ratio of the minerals will be required.
In 2010 all the cases seen and reported were on grazing oats and in many cases despite supplementations containing calcium. A green oat crop has a calcium: phosphorus ratio of 1.5:1. Lambs should receive a ratio of 2:1 - 1.5:1. This would indicate an adequate calcium:phosphorus ratio. A well growing 30 kg lamb requires 6.6g of calcium and 3.2g of phosphorus daily. Oats contains 3g/kg of calcium and 2g/kg of phosphorus. These lambs consume approximately 2kg of forage dry matter per day. These figures indicate close to adequate calcium and phosphorus levels. Some green feeds, particularly oats, contain a rachitogenic factor in the form of Vitamin A. The ewe supplies enough Vitamin D in her milk for the first 4-6 weeks of the lamb's life. After this the primary means of maintaining Vitamin D in sheep is via photobiosynthesis from the sun. These cases occurred after the shortest day of the year and after a period of cloudy overcast weather. In 2010 we had an early break in the season allowing for early sowing of the oats. The season remained good allowing for prolonged grazing of oats. Good nutrition (energy/protein) combined with good genetics for growth resulted in high growth rates and a greater requirement for calcium and phosphorus for skeletal growth.
The biochemistry results indicate Vitamin D deficiency in the two cases tested. Serum calcium levels were moderately low in all cases. There was a varying result for serum phosphate levels with case 1 and 2 having marginally low phosphorus levels and case 3 having adequate phosphorus levels.
The majority of these lambs respond to treatment with calcium however the condition has already had a negative impact on their growth. Prevention is the best option. As we cannot control sunlight hours and this problem seems to sometimes occur despite supplementation of calcium, prophylactic treatment of autumn drop crossbred lambs grazing winter cereals particularly oats with Vitamin D appears to be a good option and is relatively inexpensive. The most common form of Vitamin D supplementation is as an injectible product in combination with Vitamin A and E. However different products had varying ratios of Vitamins AD and E. This needs to be taken into consideration when recommending a product.
Recommend dose rate is 1/2ml (at marking) - 1ml (at weaning).
Thanks to DV Tony Morton for his advice on Vitamin AD and E products.