Scanning will not pay if it is not accurate and if it is not accurate it is better not to scan at all. There are very many reasons why this is so but I will mention only three.

• Twin lambs born among the singles and any lambs born amongst the “dry” ewes, because of scanning errors, will be very poor lambs indeed, and in fact worse lambs than they would have been had scanning not been conducted at all.
• Where single-bearing ewes are managed as twins in late pregnancy, because of scanning errors, the result is difficult births, desertion and death of the lamb and possible death the ewe as well.
• Twin-bearing ewes managed as singles can develop pregnancy toxaemia and die.

Few things in agriculture are perfect and scanning is no exception, however, with a competent operator, the potential accuracy is more than satisfactory. The stage of pregnancy on scan day is critical and where scanning is done at the right time the following accuracy can be expected:

• Of every 1000 ewes said to be pregnant, 995 should have a lamb (99.5%);
• Of every 1000 ewes said to have a twin, 999 should have a twin (99.9%);
• Of every 1000 ewes said to be singles, 990 should have singles (99.0%) and 10 will be twinners.

Sheep brought straight in from the paddock or fed on hay overnight before scanning can be harder to scan and error levels may rise.  Freshly shorn sheep can be more difficult to scan than those in the wool because they are more active when handled and this will contribute to more scanning errors.

When some producers fail to mark 200% of lambs from the twin-bearing ewes, they conclude that scanning is inaccurate. The real reason for the failure to mark all of the lambs that scanning identified, is the massive losses which occur among twin-born lambs in the first three days of life.  This issue is covered in greater detail in other pages of this website. The flip side of this situation is the massive potential for improved reproductive performance and general productivity when high survival rates are realized from twin-lambing ewes.