THE importance of pasture quality and of milk producers understanding what constitutes the feed their herd grazes was emphasised by researchers at the recent Northern Dairy Conference.
Veterinary science researchers have linked the trend towards increasing sugar content in forages in order to lift milk yield, to a higher risk of the costly digestive disorder acidosis.
Considering the numerous advantages of cutting-edge pastures bred for higher sugar content, the key for farmers appears to be in accounting for the sugar and starch content in pastures when formulating rations.
So if you’re not already analysing pastures, it’s time to start.
Helen Golder, from the Faculty of Veterinary Science at The University of Sydney, said there had been a lot of emphasis on breeding perennial ryegrasses and clovers with high sugar content during the past few decades.
These pastures have been commercially available in Australia for the past four years, with about 5000 hectares planted so far, but in countries like New Zea-land there are 20,000ha planted.
“They were bred to cost effectively increase milk yield – they provide energy for the rumen microbes so they can better use the protein content of pastures and increase nitrogen efficiency,” Ms Golder said.
Developers of higher sugar content pastures claim they can increase a cow’s milk production across a grazing season by six per cent, along with offering digestibility increases of up to three per cent.
“They also have persistency due to strong extensive root systems and farmers can often get extra silage cuts, reducing the need for grain in the bails,” Ms Golder said.
With the advent of such pastures, it was more important than ever to take notice of sugar and starch content figures on feed analysis sheets, she said.
“Sugar content of pastures is often referred to as the water soluble carbohydrate content, and you’ll often see a WSC or NSC (non structural carbohydrate, which is the WSC and starch content) figure on feed analysis sheets,” she said.
“They are highly variable, ranging between 100 and 300 grams a kilogram dry matter in ryegrass.”
Variability comes between species and seasons, among other things.
Subtropical pastures have a higher sugar content in winter than in summer; sugar content is usually higher in annuals verses perennials; they are lowest overnight and are higher on sunny, cold days rather than cloudy days and generally lower nitrogen paddocks produce higher sugar contents.
So, depending on the time of day it is cut, sugar content in hay and silage can be greatly different.
Ms Golder presented data showing in hay, the average WSC as a percentage of dry matter is about 17 for oaten and wheaten hay, 11 for grass and nine for lucerne, compared to as little as three for corn silage and six for legume silage.
“Traditionally, we think of hay as providing fibre to the diet but it is important to know it is also contributing sugar,” she said.
“We need to be aware of how easy it is to exceed the daily recommended intake of NSC in the diet, which is 32pc.”
An example she presented showed a 600kg lactating cow eating 16kg of dry matter a day of ryegrass could be taking in 1.6 to 4.8kg of sugar a day.
If it was also supplemented with 6kg of grain with a starch content of 55pc it was getting 3.3kg of starch a day.
As a result, the cow was eating up to 8.1kg/day of rapidly fermentable carbohydrate, equal to 37pc of its diet.
Ms Golder has just led an experiment involving 30 Holstein heifers divided into groups where grain, fructose and/ or histidine were fed, with rumen samples taken afterwards.
The groups fed sugars had significantly lower PH levels, suggesting they had the potential to become acidotic.
There was more than a 50-fold increase in lactic acid levels in the sugar groups as well.
However, the research indicated lactic acid may not be as heavily involved in acidosis as what has previously been thought, Ms Golder said.
She pointed out the study involved feeding sugar for just one day.
“In reality dairy herds will be subjected to high sugar content pastures for multiple days so there is likely to be a greater magnitude of the effect,” she said.
A Dairy Australia event, the conference was held on the Gold Coast in October and covered a range of topics including staying motivated, cow rep-roduction and pasture research.
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