Size matters. Mature cow weight—udder nonsense?

By Whitney Dollemore, Pastoral Research Officer, Katherine

Mature cow weight is pretty self-explanatory, but why is it so important to a pastoral business in the NT? We are at the mercy of the seasons in the NT, particularly how it affects pasture growth and quality. I know, you thought this was going to be an article about cows… well it is, but of course without grass there would be no cows. More importantly, you can produce more beef if you can narrow the gap between what food you can provide and the nutritional requirements of your breeders—but I digress.

The quality of pastures in the NT environment diminishes quickly after the end of the wet season and is particularly low by the end of the growing period. Average dry season (May–Sept) pastures in the VRD have low protein (5.5%) and low digestibility (52.5%). To meet maintenance requirements for protein and digestibility a late pregnant 400 kg breeder requires 7% and 52%, respectively. This is why we see body condition wain over the dry season even in non-lactating cows and why a breeder that is lactating in the dry season has no chance. A 400 kg lactating breeder maintaining weight producing ~5L milk/day requires 9% protein and 54% digestibility (Jackson et al., 2012). 

Just like humans, not all cows are built the same. Some are from the aristocratic portion of society, they look great, spend a lot of time eating and philosophising. Others are from the ‘peasant class’ and may not look so fancy but can produce so much on a diet of only potatoes! What has this got to do with my cows, you say?

Well, living in the NT we are providing potatoes and expecting lots of kilograms of weaners in return. If we expect to get weaning rates of 75% then we have to have the cows that can survive and reproduce under the environmental conditions i.e. the ‘peasant stock’. We desire a cow that will get pregnant at first mating (2yo) and that continues to bring in a weaner each year. Lactation and pregnancy increase the requirements for nutrients above that of maintenance.

So, a cow with a lower maintenance requirement, a smaller mature cow, will be better able to conserve body condition and reproduce more efficiently than a large mature cow. The Cash Cow Project found that cow hip height above 140 cm compared to less than 125 cm was a significant factor affecting the ability of a cow to reconceive within four months of calving (4.8%) and increased foetal calf loss by 3.7%, independent of breed (McGowan et al., 2014).

Ability to get in calf and raise a weaner each year is not solely about genetic selection, nutritional management walks hand in hand with genetic selection. Management strategies that can aim to match the nutritional requirements of the breeder with the dietary intakes include supplementation, early weaning, heifer segregation and control mating.

Talking exclusively about genetic selection if we are trying to select the ‘peasant’ stock—those animals that are more likely to produce a weaner each year—we are selecting for a moderate to low mature cow weight estimated breeding value (EBV). In the northern parts of the NT we are even selecting for a low mature cow weight (in the bottom 15% - Brahman BreedPLAN).

Jackson, D., Rolfe, J., English, B., Holmes, W., Matthews, R., Dixon, R. M., Smith, P. & MacDonald, N. (Eds) (2012). Phosphorus management of beef cattle in northern Australia. Sydney: Meat and Livestock Australia Limited.

McGowan, M., McCosker, K., Fordyce, G., Smith, D. R., Orourke, P. K., Perkins, N., Barnes, T., Marquart, L., Newsome, T., Menzies, D., Burns, B. & Jephcott, S. (2014).North Australian Beef Fertility Project - Cash Cow. Meat and Livestock Australia Ltd.