The science behind a great heifer: monitoring growth

By Nicola Dennis

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 confusion. 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 when heifers are about 15 months old and the pre-calving target when they are about 22 months old. Research shows that body weight is a key driver 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 to achieve. If the heifers are behind before mating, then it can be challenging to put extra weight on them through winter, and once they begin diverting their energy into pregnancy.

The second target, the pre-calving target, is 90% of mature live-weight. This target is important because it means the first calver is entering the herd in a well-grown state. I have noticed some confusion about what the calving target weight includes. The 90% includes everything; the cow, the calf she is gestating and all the liquids and jiggly bits. An ideal time to monitor the pre-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 and resources (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 if the live-weights from the book or internet look wrong for whatever reason, then another method is to weigh the mature cows from the same herd (I realise this is a tall order for most heifer graziers). 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. Then you can use the average live-weight for each breed to set the targets.

The method that is most often recommended uses estimated breeding values (BVs) for live-weight. 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 cow has slimmed down in recent years (owing to an increase in cross-breeding and breeding for increased efficiency), she now tilts the scales somewhere around 470 kg (not finalised at the time of writing). Programmes such as InCalf are likely to continue to use the old base weight of 503 kg until the official base weight has been decided on. Using this method, if a heifer has a live-weight BV of +10 kg, then we would estimate that her mature live-weight would be somewhere around 480kg (e.g. 470 + 10 kg).

If you found the live-weight BV method too confusing, then do not despair! As it turns out, the BV method is not terribly more accurate than the other methods for estimating mature live-weight. 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, BVs 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 can inherit any combination of her parent’s genes which can make her larger or smaller than we would expect. Overall, the confidence interval of a mature live-weight BV prediction on any individual animal 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 predictions (for any method) are large. When setting a target, the ‘true’ mature live-weight for any individual 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 has been overestimated by 30kg. Which brings us to the very important conclusion that individual live-weight targets have substantial limitations. However, when used for a group of animals, most of these individual errors cancel each other out and the range for error is much smaller. The larger the group of heifers the more confidence we can have in our target. For example, a group of 15 heifers have an error margin which is around 9kg either side of the target (which is pretty acceptable, in my books) and in a group of 50 heifers this reduces to around 5 kg (which is even better!) Therefore, the correct way to assess if heifers are growing correctly, is to take the average live-weight and see if it is adequate when compared with the average target.

It seems that it doesn’t matter how the herd 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 the herd meets its targets. It is possible to over-invest in heifers and it is not worth chasing every last kilogram. If the herd is already on target, adding an extra kilogram or so “for good luck” only makes sense in situations when pasture is abundant. It is true that fatter 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. That way, the supplement goes directly to milk production rather than sitting on a heifer’s back (as fat) waiting to be metabolised into milk at a later date.

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 that 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 most cases the average live-weight is much more accurate and a lot more informative for farm management decisions.