Identify ill-thrift early to get to real cause of problem
You may well ask 'why is he on about ill-thrift' seeing as lambing has not even finished yet. However, farmers generally only notice ill-thrift or poor growth rate during July and August and only then because the lambs are not becoming fit for sale.
The number of complaints by farmers about ill-thrift is directly related to the trends in lamb price. When prices are falling and lambs are not being sold, there are generally a large number of complaints. When prices are holding or even rising, like last year, there are few complaints, even though lamb thrive was no better than the previous year.
In the majority of situations, when farmers do seek a remedy from their adviser or vet at this time of year, they are recommended a worm dose or trace element supplement without any real discussion or problem analysis. The adviser has collected his fee, the vet his sale and the farmer feels happy that it was not his management, or the lack of it, that caused the problem.
Poor lamb drafting means that there is a problem, but gives no indication as to when the problem started. In my view, many cases of poor lamb thrive occur before the lambs are 10 weeks old and sometimes before the lambs are born.
Ewes underfed during pregnancy will have lighter lambs that will take longer to finish. A reduction of 0.4kg in lamb birth weight can readily occur due to poor ewe nutrition and this becomes a disadvantage of 1kg at weaning time, resulting in the lambs taking an extra week to finish.
Pasture quality and digestibility are at their highest during March, April and May. Twin lambs have the potential to gain at least 300g/day (singles 350g/day) during this period. As pasture quality drops during June, lambs' growth rate will also decline to around 220g/day. Growth rates after weaning are generally in the region of 150g/day.
As can be seen, March-born lambs on grass have the potential to grow twice as fast during the first 10 weeks of life than after weaning. A loss of 30g/day (which does not seem a lot) in lamb growth rate during this early period means a loss of 2kg in weaning weight and two weeks' delay in finishing. It comes down to providing sufficient grass (sward height 5-6cm), first for the ewes so that they have adequate milk supplies, and then for the lambs as milk yield declines and lambs become increasingly reliant on pasture.