Plant analysis backs up earlier advice on feeding swedes to cows

Results from analysis of Southland swede plants collected last season is supporting DairyNZ advice to farmers that feeding maturing swede crops increases the risk of ill-health in cows.

The industry body accordingly is recommending that farmers do not feed Herbicide Tolerant (HT) swedes to cows in spring when the animals are in late pregnancy or early lactation. DairyNZ is also advising caution if farmers are considering other leafy varieties.

DairyNZ arranged for swede sample analysis from 11 Southland farms in September last year following issues with cows becoming ill and some dying after feeding on swedes.

Staff collected swede samples, dissected plants as quickly as possible and froze the samples in liquid nitrogen to stop any spoiling of the plant material and breakdown of the glucosinolates (GSLs), the naturally occurring compounds in brassicas that have been linked to cow health problems.

Swedes were dissected into up to six plant parts so that each section could be analysed separately. Up to 150 plant parts were analysed from three swede varieties across the 11 different farms.

Key findings from the plant analysis are:

* Total GSL concentrations are higher in the HT swede variety than in the non-HT swede varieties. While there is not much difference in GSL concentrations in the bulb and crown between HT and non-HT swedes, GSL concentrations in the other plant parts are generally higher for HT swedes, with a pronounced difference in the upper leaf and upper stem.

* The risk of ill-health and death in cows increases when total GSL concentrations increase as swedes enter the reproductive stage (elongated stem, new leaf, flowers and seed heads).

* Different parts of the plants included different individual GSLs

* The concentrations of individual GSLs varied between plant parts.

* HT swedes have higher concentrations of GSLs in “reproductive” plant parts, increasing the risk of ill-health for cows grazing swedes with elongated stems and the appearance of flowers.

* No single GSL stands out as significantly different between plant variety and plant part.

DairyNZ Southland-South Otago regional leader, Richard Kyte, says the new plant data supports the current DairyNZ advice that farmers should be very cautious when feeding swedes.

“This analysis confirms that feeding maturing swede crops increases the risk of ill-health,” he says. “We’re continuing to advise farmers to focus on managing a number of factors involved in feeding swedes this season, including the proportion of swede that makes up the diet of their cows.

“And these new plant results back up our earlier farmer survey findings that feeding swedes on the milking platform (farm) in spring when cows approach calving and early lactation increases the risk of ill-health.

“The farmer survey we released earlier indicated that in spring 2014, there was a higher risk of ill-health for Herbicide Tolerant (HT) swedes compared with other varieties of swedes in spring. Given those findings and now the plant analysis, we’re recommending that farmers do not feed HT swedes to cows in spring when the animals are in late pregnancy or early lactation and when the risk of all the factors that can lead to ill-heath and potential cow deaths can rapidly combine. In spring, air temperatures are expected to increase rapidly, leading to “bolted swedes” and bolted HT swedes have much higher levels of total GSLs.

“At the time of planting their swedes, farmers have no idea what the following winter/spring is going to be like. The spring is a key risk time as swedes will be closer to going into the reproductive stage,” he says.

But Kyte says climatic and growing conditions for swedes this season are very different from last year.

“We identified through our farmer survey a departure from the ten year climate average for the region in 2014. The warmer air temperatures and fewer frost days may have enhanced both leaf growth and maturity last year. We’re seeing quite a different growth pattern this season. Plants don’t appear to be bolting. Last season was a bit abnormal,” he says.

The general advice being given to farmers is that special care is needed with HT swedes and other leafy varieties like Aparima Gold and Triumph, when warm air temperatures from northerly weather conditions cause swedes to regrow and change quickly.

DairyNZ is still monitoring the health of a small number of cows this season. It expects to release a final report to farmers in October.

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