An annual pattern of milk composition has been well recognized on dairy farms across the world for years, with the highest milk fat and protein concentration in milk observed during the winter and lowest occurring in the summer. This trend is manipulated solely by season, and impacts housed and grazing cows similarly. So, when we get to spring, and then turnout for some, and milk butterfats start to decline- how do we know if this is real milk fat depression or not?
Is it simply a seasonal response which we have little manipulation over, or are we seeing changes which can be regulated by the feed, avoiding loss of milk and associated impacts throughout lactation? We can have a better idea by analysing multiple bulk milk parameters, individual cow milk recordings, along with the diet and identifying what is really going on.
Butterfat and Protein
Firstly, look out for sudden or ‘out of season’ changes to butterfat or protein % on bulk tank recordings. As we head into spring we would expect butterfat to drop slightly, but mostly as a result of ‘dilution’ as yields go up (see Figure 1), and total milk solids should be consistent. We want to minimise any further drops that could be caused by SARA (Sub Acute Ruminal Acidosis) which is fairly common on lush early grass, reducing the rumens ability to synthesis milk fat pre-cursors. Reductions of >0.3% butterfat could indicate SARA, which could lose you -2.5L/cow, on top of the reduced value per litre. It is important to provide enough palatable buffer feed, containing high digestible fi bre feeds such as sugar beet which contains ‘acid-regulating’ pectins, and ensure minerals are balanced.
On the contrary, if butterfat is abnormally >1.35 times the protein, the cows are in early lactation, and losing condition, they are likely mobilising body fat for energy and may be in ketosis. The energy level of the ration should be reviewed, is there enough fermentable carbohydrates for the rumen and importantly is there enough rapidly available glucogenic energy for the higher yielding cows. Sometimes a top up energy source such as propylene glycol may be needed for particularly high yielding fresh cows. Protected fats, ensuring they are the right fatty acid profile, can also supply much needed energy boosts. Protected fats like Dynalac is formulated for earlier lactation and will provide the correct fatty acid profile, on top of a balanced diet, needed for milk production and spare body condition; which in turn will help fertility. Looking after the early lactation or transition cow is critical for the remaining lactation performance, so make sure milk quality parameters are monitored and acted on if needed.
MUN
Another important indicator on the daily milk results which is often overlooked, is MUN (Milk Urea Nitrogen). This tells us how much Nitrogen from protein is excreted in the milk and positively correlated to blood urea levels. MUN can tell us a lot about the diets balance of rumen available energy to protein, indicate if there may be sorting of the ration, whether the protein supplied in the diet is optimum for this production level, and if we are maximising rumen efficiency. MUN is often a precursor to drops in milk yield and protein percent and can indicate metabolic problems. If milk urea’s are consistently >300 mg/L or 0.030% (NML reports) then the protein in the diet is not being utilised effectively. Excess protein in the rumen is turned into Ammonia through Proteolysis, this is absorbed into the blood but will be ‘detoxified’ by the liver to Urea and this uses up valuable energy, which could otherwise be used for milk production. This is then excreted in urine and milk which is a waste of nutrients from the cow and lost to the environment. Consistently high circulating Urea can also be detrimental to fertility, reducing heat expression, pregnancy rates and ability to hold service (Butler et al, 1996).
However, during grazing, milk ureas are often above 0.03% and hard to manage, as the high rumen available protein is not being fully captured due to lack of digestible fibre in grass. We can help prevent them getting excessively high and therefore utilise more of the protein available by feeding a buffer feed or concentrate low in oil and RDP (rumen degradable protein), high in quality digestible fibre (Sugar beet) and slowly fermentable starch (Maize). Another benefit of supplementing additional digestible fibre at grass is that it will encourage cudding, where the cow will produce saliva containing urea which recirculates it back to the rumen for another chance to incorporate it into microbial protein.
On the other hand, low levels of MUN may indicate a shortage of RDP however this must be taken with caution, as there are many other factors influencing the rumens function and ability to utilise protein and generate microbial protein. However, both scenarios are economically and metabolically adverse to any commercial dairy herd.
Figure 1: Shows the seasonal effect on milk constituents throughout the year, no matter what system, but when either fall outside these ratios we need to take appropriate action with the feeding
