Dairy cows come in all shapes and sizes and with more crossbreeding going on than ever, and the introduction of new European breeds, it’s not going to stop.

Tall and lean cows, short and fat cows – what is the best to have?

Breeding Worth (BW) ranks cows across breeds on their expected ability to produce milk. Liveweight is included in it because a heavy cow which needs lots of feed to maintain its body weight must be compared with a lighter one which can eat less.

The economic reality of all New Zealand dairy farms is the feed in/milk out ratio. If less feed can produce more milk, then that is the cow you want.

With the equation partly dependent on the weight of the cow, how accurate are the BW liveweight figures we’re dealing with?

The answer to that is not good. According to Dairy Statistics 2020-21, about 76% of cows milking in NZ were herd tested that year. Of those, only 2.5% had a weight added to the herd test figures.

So of the 4.9 million milking cows, only about 85,000 were weighed.

If you want the good news, cows which are part of the TOP system which helps select the next sires are weighed so there is some truth in that BW figure, but it could be a lot better.

It’s something you can do onfarm. More and more dairies have scales with EID tags making the recording easy as well as the uploading the data into MINDA.

And we weigh our young stock during the first two years of their lives to make sure they’re growing as they should. Why do we stop when they’re mature?

So when to weigh them?

Obviously not in the winter when they have a calf growing inside them, or in the spring when their udders are swollen with milk and condition is all over the place. Plus at that time you have enough things to do.

Instead, schedule it for summer when the foetus is still tiny and you and the cows are cruising through the sunny days.

Cows hit maturity around three years old so still weigh your two-year-olds but make sure you do it the following year as well. The system picks up their age and pregnancy status with the date of the weighing.

Weighing a mature cow once in its lifetime, the experts have said in the past, is enough as mature cows will stay the same weight (apart from their body condition score) throughout their lifetimes which could also explain possibly the low percentage of cows weighed in one season.

Don’t confuse liveweights with body condition scores. Body condition scores measure how much fat a cow has. It utilises this fat when it’s not getting enough energy from its feed and so loses it. It puts body condition on when it’s getting surplus energy to its needs.

Body condition is more about the health of the cow and how much it’s eating. Liveweight is all about genetics and what was fed and how much it was fed as a youngster.

Adding in weights to MINDA will affect a cow’s BW and the rankings will change in your herd. Until you add them, the weights are only an estimate of what they should be, given the information about the mums (which may also have never been weighed) and their half-sisters and cousins.

When the Southern Dairy Hubdid it this season, individual BWs moved as much as from positive $25 BW to negative $37 BW. At least they did it before culling decisions were made.

Don’t think this movement is a failure of the BW system. Instead, it’s more likely somewhere along the way, either with the cow or even its mum, tags got mixed up.

Which is another good reason to weigh your cows especially if you don’t DNA test your calves. It’s another check that everything is as it should be in your herd records.

Overall, your herd BW will probably stay about the same as some cows will go down and others will go up. What it does make you aware of is those girls who are not pulling their weight.

Because with greenhouse gas emissions, let alone the price of fertiliser and grain, the amount a cow eats to maintain body weight is key to current farming.

Instead of working out your kg milksolids (MS)/ha, work out your kg MS/liveweight. More milk with less hooves (calving costs, vet bills, cups going on) is better than good. It’s great.

So aiming for your 400kg cow to do its weight in milksolids annually is the target you want. Even better would be for it to do more than its weight.

So what has liveweight got to do with feed intake?

For an animal to live, forget producing milk or a calf, a cow has got to eat. There is an energy cost to everything it does from digesting its food, to keeping its heart beating and its lungs breathing and controlling its temperature.

And it’s not a straight-line equation. As liveweight increases, so does maintenance requirements but a lighter cow needs more energy per kg of body weight than a heavier cow. So a 400kg cow needs 50 megajoules of metabolisable energy (MJ ME)/day but a 450kg cow needs 54MJ ME/day and a 550kg cow only needs 63MJ ME/day.

But genetically, some cows are better at maintaining their bodyweight with less feed than others and these are the ones with the higher BW scores. It means of that 18kg DM you’re feeding them each day, they can put more of it into milk production.

With greenhouse gas emissions becoming important, if you can have fewer cows producing more milk then you can start ticking boxes.

And we’re doing this already. Our cow numbers are steadily dropping but at the same time we’re nationally maintaining milk production.

Whether that is by feeding more grain and higher quality pastures than before, or better genetics, who knows.

But it’s no surprise that cows with high BWs also have low nitrogen and methane/kg MS outputs. It means our breeding all these years has been successfully producing cows which convert feed better.

Nitrogen and methane (CH4) emitted by cows through urine and belching are feed losses. The cow has eaten the nitrogen, carbon and hydrogen but not converted it to milk or body condition or growing a calf or energy for walking.

By improving our breeding even more we will get there, and adding your cow weights into the growing data of our national herd will help keep our cows genetically on the right track.