29 DECEMBER 2015
A cows lactation may begin with the act of calving, but the performance of that lactation can be traced right the way back through her transition period, her dry period and even back to the end of her previous lactation. The dry and transition periods of a cow have a huge impact on the incidence of metabolic disorders such as milk fever, retained foetal membrane and ketosis. Those periods are also directly related to egg quality and therefore to fertility into lactation.
The big question is: What are the options for managing dry cows to maximise lactation performance?
LOW CALCIUM OR 0% CALCIUM
The ideal dry cow diet is one that provides little or no calcium (30g per day). unfortunately, this is almost completely unachievable, especially on our predominantly grass silage based diets where the forage alone could be providing in excess of double that. where alternative forages are available that can help, but the use of a calcium binder can simulate the same effect on any.
Just like a mycotoxin binder carries mycotoxins safely through the cow, a calcium binder prevents the cow from absorbing calcium, whilst ensuring there is still an adequate supply in the bloodstream, by stimulating the cows’ own calcium mobilising systems. This creates a perfect balance of calcium in the cow around the time of calving, which greatly reduces the risk of milk fever, held cleansings, ketosis and twisted stomachs.
DCAB & Partial DCAB Systems DCAB stands for Dietary Cation Anion Balance and measures the balance of positive and negative ions within the cows’ diet. The ions that are measured come from Potassium (K) and Sodium (Na) on the positive side, and from Chloride (Cl) and Sulphur (S) on the negative side.
A highly positive DCAB score, promotes intakes and milk yield and is easily sustained by the generally high levels of positive ions (cations) that we find in most of our feedstuffs and particularly in grass silages. Unfortunately, a high DCAB score adversely affects dry cows. In particular, these very high positive scores are closely linked to milk fever and, as a result, can also have an impact on retained foetal membranes, displaced abomasums and other performance related problems right through lactation.
To minimise these effects, we can feed specific ingredients to bring the overall DCAB score down. Products such as Magnesium Chloride or Ammonium Chloride are required to pull down the DCAB value of a diet and can be supplied as a straight ingredient, through the water supply, or as part of a dry cow concentrate.
In a ‘full’ DCAB system, the DCAB score is made significantly negative. At these low levels, cows become very sensitive to changes in the DCAB value and as a result it is necessary to regularly check the pH of their urine. Typically urine pH would be around 8.0, but on the full DCAB system could drop as low as 6.0. Reducing DCAB by this much drives the cow into a metabolic acidosis. Just like with rumen acidosis, this additional acid loading has to be buffered. The cows own response to this is to mobilise calcium to buffer the acid. It is this mobilisation of calcium that significantly reduces her risk of milk fever.
A ‘partial’ DCAB system, aims to pull the DCAB value of the diet down to around 0 meq/kg. Typically, a partial DCAB diet will have a DCAB range of -50 to +50 meq/kg. At this level, there is no need to check urine pH making it significantly easier to manage and administer than the full system.
In principle the two systems work exactly the same, however with the partial system being less extreme, it is also slightly less effective. When used correctly though, it will still do a great job of reducing the incidence of milk fever. The downside of both of these, is that the DCAB score will be constantly moving as a result of dietary changes. Even simply moving back through a silage clamp, the DCAB scores from different fields will have a range of values. As a result, forages need to be tested regularly for their mineral content to make sure that the correct levels of negative anions can be provided.
With a typical forage mineral analysis taking up to 10 days, it can be difficult to keep up with changes to the diet, as well as proving potentially expensive. If DCAB values move up out of the optimum range, milk fevers will start to occur; whilst too negative a DCAB score can actually cause milk fevers if there isn’t enough calcium in the diet.
Keeping dry cows in one group is not without its problems, but is a very practical system, particularly in smaller herds where grouping cattle in low numbers really isn’t feasible. The principle of this is that cows are dried off as normal at around 8 weeks pre-calving. They can then be fed on just one diet, right the way through until they reach the point of calving. This means that there are fewer changes of diet, giving the rumen a much better chance of remaining stable.
The issues surrounding a one group system are those of intakes and energy. There will be a big variation in the requirements of cows that are spread throughout the dry period. In the early part of the dry period, cows will eat quite a lot, but actually have a fairly low energy requirement. In the second half