New Zealand’s pasture-based dairy system is characterised by a seasonal calving pattern that matches pasture growth, starting in the spring and, of course, finishing in the autumn. The aim of this type of system is to have a calving interval no longer than 365 days. To be able to achieve this target, it is necessary to begin mating all cows between 80 to 85 days after calving (as a dairy cow’s pregnancy is 282 days), with most farmers mating around the 23rd October in preparation for calving on 1st August. Furthermore, a tighter calving pattern is preferable for production and reproduction performance of the herd; through increasing the interval from calving to mating, there is more opportunity for more cycles as well as more days in milk. From a reproductive point of view, a tighter calving pattern provides the opportunity to cycle all cows (or the majority of them) before the start of mating, and therefore helps to reduce costs of treating the non-cycling cows. This is also important because typical conception rates for the
first cycle are only around 30-40%; natural conception rate improves with each cycle and is optimal at the 3rd cycle at 55-65%. To make the calving pattern tighter for a herd, it is necessary to get cows in calf as soon as possible, hence it is important to focus on heat detection, especially in the first 3 weeks of the planned start of mating (PSM). Premating heat detection is also an important tool in helping both to identify cows that are not cycling (so decisions can be made for potential intervention) and to record cows that are cycling in the herd, and thus provide an indication on the success of mating in the first cycle post PSM.
There are several factors that influence the percentage of non-cycling cows (extended anoestrus period) within any given herd, but, from the animal nutrition point of view, the most common cause is a low BCS at calving (mature cows BCS < 5.0) with extended negative energy balance (NEB) after calving. This impacts on the BCS of the herd due to
the excessive mobilisation of energy reserves in response to milk production. Loss of some BCS (less than 0.5 BCS units) post calving is a normal function of the modern NZ dairy cow – we tend to run into problems where cows lose BCS over 1 BCS unit between calving and mating. Cows which calve at BCS below the recommended target are at a higher risk of delaying their cycling. For example, there have been reports of a 6% decrease in pre-mating heat detection when cows calved at BCS 4.0 instead of BCS 5.0. On the other hand, a feed shortage supply after calving, which prolongs the NEB of the cows, will also impact on BCS due to the mobilisation of excessive fat reserves of the cow. It is common to find that thinner cows delay their reproductive cycle as a physiological response to the shortage or poor quality of feed before mating. Consequently, using the pre-mating heat detection records in conjunction with scoring the herd 3-4 weeks before PSM, will bring the opportunity to plan ahead with the aim of putting some strategies in place (if required) that may help to improve the nutritional level of thinner and younger cows. At PSM at least 85% of cows should have BCS 4.5 or above. Anything below this target may well have a significant impact on the reproductive performance, through a lower submission rate and/or a reduced conception rate as described previously, and therefore result in a poor `6-weeks in-calf` rate (4-5% reduction of `6-weeks in-calf` rate when cows are mated at BCS 3.0 instead BCS 4.0).
Checking whether your cows are in a state of NEB can be done by looking through your fat-to-protein ratios – typically these should be in the normal range of 0.73-0.77 and optimally 0.75-0.80 depending on the breed of cow. Figures which are less than this indicate higher fat mobilisation which, in turn, indicates that cows are not being fed sufficiently. This can be seen when spring storms blow through – even cows offered
sufficient feed stand in the corner with their backs to the wind. Subsequently we often see a spike in milk fat % which shows short term mobilisation of fat stores.
One common approach to minimise loss of BCS of thinner/younger cows is to split the herd into two mobs – this is usually done when the majority of calving has been completed and can allow 4-6 weeks pre-mating. These light and/or young cows are then given an opportunity to improve their energy balance to a positive state and actually put on BCS/milk better pre-mating (which improves the mating success).
The light mob can have feeding preferences such as use of supplementation or grazing in front of the cows (getting the first part of every paddock) with the aim to get the best of the pasture allowance without putting pressure on them. In the meantime, the main herd follows the light mob and grazes to the desired post-grazing residual targets. In
this way pasture quality is not compromised. Another strategy commonly used is the oncea-day milking (OAD) after calving; this helps to reduce the nutritional demand for milk production and therefore reduce the length of the NEB period. This approach will likewise help to minimise loss of the BCS of thinner and younger cows.
There are feeding options which can improve the result of mating; all the science and evidence we have to date shows that reaching optimal BCS pre-PSC and minimising loss of BCS between calving and mating play the biggest roles in achieving good success rates. Through discussing these options with trusted advisors sufficiently early, decisions can be made to ensure success or at least minimise disasters on farm this mating.
Centre for Dairy Excellence Mark Youngs