[Published in CountryWide and The Dairy Exporter]

In recent years, grazing dairy heifers has become an industry in itself and dairy heifers are finding their way on to a diverse range of farms up and down the country. This means that almost no-one can escape the frenzy of enthusiasm that rural professionals are showing weighing, tracking and improving dairy heifer growth. This enthusiasm has led to a wealth of innovation and research, but also to some hefty myths and misconceptions. So, how heavy should a heifer be? How do we know if she’s on track? And can we adapt these methods for other farming stock?

When dealing with heifers there are two live-weight targets that farmers need to be concerned about. The mating target and the calving target. Research shows that body weight is a key driver the timing of puberty in cattle, heifers begin cycling when they weigh around 45% of their mature live-weight. The first few ovarian cycles can be hit and miss in terms of fertility so the mating target has been set at 60% of mature weight in order to ensure heifers are ready to conceive at the start of mating. I consider the mating target the more important target because it affects fertility in the first mating, but also in subsequent matings because later calving heifers have less time to recover after calving before the next mating season starts. Another reason to focus on the mating target is because it is the easier target of the two to targets to achieve. If the heifers are behind before mating, then it can be challenging to grow them well once they begin diverting their energy into pregnancy.

The second target, the calving target, is 90% of mature live-weight at the start of the calving season (around 2 years old). This target is important because it means the first calver is entering the herd in a fit state to calve and lactate (fatter cows produce more milk). I have noticed some confusion about what the calving target weight includes. The 90% includes everything; the cow, the calf she gestating and all the liquids and jiggly bits. If you are doing anything other than weighing a heavily pregnant cow, I urge you to find other ways to spend your time. An ideal time to monitor the calving target is when the first-calvers leave their winter grazing.

Both targets rely on estimating how big the heifers will be when they are fully mature. There are three methods for doing this. The best method for a group of animals depends on personal preference and the information available. The easiest method is to look up the average mature live-weight for the breed from a reference like the DairyNZ website (which currently lists 423kg for Jersey, 499kg for Friesians and 467 kg for crossbreds). If multiple breeds, crossbreeding and imported genetics are involved, then choosing the breed can be open to a fair bit of interpretation. However, on the whole this is a user-friendly approach and there is only one target to remember for each breed of cow.

If you have a unique mix of breeds or the live-weights from the book or internet look wrong for whatever reason. Then a great method is to weigh the mature cows from the same herd. Cows continue to mature for a few years after they start milking, so it is best to weigh those that are over 6 years old, leaving out any cows that have excessively high or low body condition scores. Use the average live-weight for each breed to set the targets.

The method that is most often recommended is using live-weight estimated breeding values (BVs). Using this method takes out some of the guesswork in choosing breeds and some applications (such as MINDA weights) will do the calculation for you. To estimate mature live-weight, add the heifer’s BV for live-weight to the live-weight of the genetic base cow (the ‘average’ cow that is used to formulate BVs). The genetic base used to be 503kg, but the base cow was updated in June and she now weighs, well actually I was not able to find anything official on the web, but the last I heard she

weighed 473kg (an increase in crossbreeding as well as efforts to increase the efficiency of cows has led to the average mature live-weight reducing). If a heifer has a live-weight BV of +10 kg, then we would estimate that her mature live-weight would be around 483kg (473 + 10 kg).

In the interest of dispelling myths, the BV method is not terribly more accurate than the other methods. The live-weight BV is based on what few mature live-weight records are available for the daughters of the heifer’s sire and maternal grandsire. Also, EBVs are calculated by taking the average of the parents BVs and are informative on what is happening on average (which is fantastic for their original purpose in breeding programmes), but an individual heifer is free to inherit any combination of her parent’s genes which can make her larger or smaller than we would expect. Overall, the confidence interval of an BV mature live-weight prediction is a 70kg range (the prediction plus or minus 35 kg). This is only marginally (about 4 kg either way) more accurate than choosing a mature live-weight from the NZ statistics or by weighing the herd.

You will note these confidence intervals in the estimated mature live-weight prediction (for any method) are pretty large. When setting a target, the ‘true’ mature live-weight is somewhere in a 70 kg or so range around our predicted mature live-weight. If a heifer is, say, 30kg below our calculated calving target, her weight alone can’t tell you if she is 30kg underweight or the target is set 30kg too high. Which brings us to a very important point. Live-weight targets are next to useless for assessing an individual animal. However, when used for a group of animals, most of these individual errors cancel each other out and we get a fairly good prediction of whether or not a herd of heifers is hitting the targets. The correct way to assess if heifers are growing ok, is to take the average live-weight and see if it meets the average target.

It seems that it doesn’t matter how the live-weight targets are achieved as long as they are achieved. There is no evidence to suggest that heifers growing at a consistent rate throughout the seasons are any better off than heifers that slow down for a period over winter or summer, as long as they hit the targets. It is possible to over-invest in heifers and it is not worth chasing every last kilogram. If an animal is already on target, adding an extra kilogram “for good luck” only makes sense in situations when pasture is abundant. It is true that fatter cows produce more milk, but this is a temporary benefit. Once that extra body fat has been used for milk production, it is gone for good. If turning high cost supplements into milk is appealing, then it is much more efficient to feed those supplements directly to a cow that is already in-milk.

Monitoring heifer live-weight is a great tool to maintain (or improve) the fertility and longevity of the milking herd. There is (understandably) some excitement in this space in the dairy industry, but can we use these lessons in other stock? Surely, astute beef farmers can follow these steps to set targets for beef maternal replacements. After all fertility and milking ability are important traits in the maternal beef cow. Perhaps, with a little ingenuity these methods may be adapted to other ruminants as well. If nothing else, I think all farmers can benefit from remembering there is a fair bit of natural variation in stock and always some errors involved when setting live-weight targets. Try not get too fixated on individual animal weights, in many cases the average live-weight is more informative for farm management decisions.