Put diesel in a Formula One car and things do not go so well. For cows, carbohydrates are the major fuel source, accounting for more than half of the animal’s energy needs. When carbohydrates – molecules containing the elements carbon, oxygen and hydrogen – are broken down in the rumen they form simple sugars, like glucose and fructose, which literally fuel the beast.
The building blocks
The main carbohydrates in dairy cows’ diet are fibre (hemicellulose and cellulose), starch, and soluble sugars like sucrose.
Starch has become regarded as a high-octane super-fuel, while the stalwart pasture – a mix of soluble sugars and fibre – is losing its lustre. Not so, DairyNZ senior scientist Dr Jane Kay said.
They are all made up of the same sugar molecules, Kay said. Starch and fibre are just glucose molecules joined together, while sucrose is a glucose and a fructose molecule joined together. It is the nature of the weld, or bond, holding the sugar molecules together that is the difference.
The stronger the bond between the sugar molecules, the more slowly the carbohydrate molecule is broken down by the micro-organisms in the rumen.
Get the right recipe for mating
DairyNZ animal scientist Dr Jane Kay says getting the feeding right between calving and mating is more about ensuring the cows get enough energy rather than the type of feed being offered.
“In most cases, we’d say feed your cows to achieve your target grazing residuals, and milk production and reproduction will be okay.”
Feeding starch (eg maize grain) or sugar-based (eg molasses) supplements in the weeks or months before mating had been suggested to improve reproductive performance. The potential biophysical mechanism responsible for this is not well understood, but has been linked with greater production of hormones like insulin and insulin growth factor-1 (IGF-1).
Insulin and IGF-1 are involved in mediating the growth of follicles. Follicles are found on the cow’s ovaries and are the home of eggs or the cells developing into eggs, called oocytes. Insulin and IGF-1 increase the sensitivity of the ooctyes to other hormones that make them develop more quickly. If the oocyte grows more rapidly, then the cow should cycle earlier.
While New Zealand research indicated that cows would cycle one day earlier, on average, if blood IGF-1 levels were increased by 1 nanogram/ml in early lactation, blood IGF-1 levels only explained three percent of the variation in the time taken for cows to cycle – 97% of the variation was due to other factors.
Pasture-based research showed that cows supplemented with a starch-based feed at 3-6.5kg DM/day did experience a small increase in blood IGF-1. However, it did not start to show until 3-4 weeks post-calving and did not show an effect on ovulation or pregnancy rates.
‘But it is still the same Lego blocks. Once it’s pulled apart, away you go, it just takes a little longer.’
High starch diets could also cause early embryonic death, Kay said.
“There is a recommendation out of Nottingham University to increase the starch in the ration before mating and then decrease it after mating. However, the benefits of this dietary strategy on reproductive performance are inconsistent.”
One study with a small number of cows showed improvements in cycling and 3- and 6-week in-calf rates when starch was increased in the diet of pasture-based cows. However, this study was not designed to investigate reproductive performance and was unusual in that there was no increase in milksolids production with the added starch. There was also no increase in final pregnancy rates.
Kay said onfarm experiments were starting this spring investigating if there was any effect on reproduction of feeding high starch (cereal grain) and high fibre (palm kernel and soyahulls) supplements, in conjunction with Waikato-based Anexa Animal Health.
