You will still occasionally hear people say that grazing cows is no good for their fertility, but that is to misunderstand the drivers of fertility in dairy cattle. Many spring calving herds achieve excellent fertility at grass, and like so many things the key is to manage your expectations and your cows accordingly.
Grazed grass is the cheapest form of feed available on the farm but just like any other crop it needs to be utilized properly. Aim to maximise yield of grass on your farm by measuring what you grow, feeding it at the right covers and grazing down to the right residuals. This is what the NZ or grassland systems are all about, and done well, this system can maximise grass grown and utilised on the farm, and achieve excellent fertility.
Grass is an excellent feed with crude protein often above 20% and an M.E of above 12MJ. It also is rich in the right balance of oils to produce excellent quality eggs. However the challenge is that it is also a feed with variable dry matter and a low fibre content and occasionally can have extremely high protein levels producing very high levels of urea in the blood stream. Fertility problems often arise when farms try to pair very high yielding Holstein cows with a New Zealand style grazing system. The reason that things go awry is that if you turn out a high yielding Holstein cow, giving over 40 litres of milk, she is going to struggle to graze for long enough to achieve the dry matter intake to sustain that yield. Initially at least, genetics will win out over nutrition and the cow will still pump out the milk, buoyed by the protein in the grass. However she will be short of energy. This shortfall will be seen in falling milk protein percentages, and she will first of all try to make up this shortfall by taking fat off her back, resulting in the production of NEFA’s in the blood. These are directly toxic to developing eggs and this is probably the main reason fertility in very high yielding Holsteins can suffer at turnout.
On the other hand autumn-calving Holsteins or smaller, more aggressive grazing type animals giving 25 litres or so, will be able to graze enough at grass, on the right covers, to match their milk yield and energy requirements. It is often the case that fertility is excellent in these type of cows. When it isn’t, it is usually because of problems previously (e.g. overfat cows at calving especially after long dry periods in spring calvers) or over-estimating (or under-measuring!) the covers the cows are on. Not anything damaging in the grass per say. The one exception might be where the nitrogen level on the grass is off the scale, such as newly laid seeds. An excess of protein here can cause potentially dangerous levels of urea in the blood. Dangers include primarily depression of appetite, pushing cows further into a negative energy balance, and in extreme cases, urea poisoning, where cows become agitated, twitchy and demonstrate signs of abdominal pain, such as repeatedly lying down or kicking at their flanks. Where grass protein levels are high, it is sensible to trap this protein in the rumen, and support the cow’s metabolism by providing a source of starch, such as corn in the parlour, or buffer feeding with maize or wholecrop.
For the all year round calving herd the trick is to graze enough of the right cows to get your residuals right for the year, without compromising your highest yielding cows. Remember buffer feeding will alter grazing behaviour and many farms end up with the worst of both worlds, buffer feeding cows enough so that they won’t graze but not enough to supply their energy needs. In many herds you would be best off keeping a small number of fresh cows in and grazing a larger number of cows harder outside.
Aim to match the cows to the system you want to have, either by changing the type of cows you keep or by calving them at a time to maximise their ability as a grazing group.