Not enough dairy farmers are paying close enough attention to the dry matter (DM) content of forage. They should assess DM more regularly and modify diets accordingly.

Adam Clay, ruminant manager with Trouw Nutrition GB, says the dry matter content of silages can vary considerably during the winter.

“A wide range of factors will affect silage dry matter, yet all too often we see diets that are formulated based on a silage analysis taken at the start of the winter which then remain unchanged.”

While most farmers appreciate different cuts will have different DM contents, many overlook the fact there will be variations within a single cut depending on sward and different weather conditions at cutting.

Furthermore, weather at feed-out will affect the DM of silage going into the feeder wagon.

Dry matter will even differ depending on where it is in the clamp.

“In a farm study carried out last winter, we found silage at the bottom of the clamp was typically 1.5% drier than the crop at the top. During the winter the average dry matter reduced from 41% to 36.6% as the proportion of different fields changed.”

Every 2.5% change in dry matter will increase or decrease dry matter intake by 1kg/cow/day, which is worth +/- 2 litres/cow (see table).

“If we assume the diet was developed based on 40kg fresh weight and 27.5% dry matter, it would be expected that the silage would support M+9 litres. But what would happen if the dry matter dropped to 25%?

“The immediate consequence would be that cows would be short of energy, which would show first as a sudden drop in yield. If the lower dry matter persisted, the likelihood is that cow condition would drop and fertility would also suffer. Cows would be clearing the trough more quickly because daily dry matter intakes would have fallen.”

A study on eight trial farms last winter showed if dry matter was reduced and no action was taken to compensate for wetter silage, farms would’ve lost on average £169 worth of milk a cow.

Mr Clay says declining dry matter is not the only issue – an increase in forage dry matter will also have an effect on performance. “If dry matter increases, there is a real possibility that cows will fail to clear troughs, as the total dry matter offered will exceed intakes, leading to a waste of feed and higher costs. There is a risk late-lactation cows will put on excess condition, but the biggest opportunity is that if forage dry matter increases it may be possible to fine-tune the diet to reduce feed costs.

“If a dry matter increase allowed an extra two litres from forage, this would allow a reduction of about 1kg in the total concentrate fed to each cow daily, providing such intakes can be achieved.”

Instead farmers should have silage analysed more regularly. Mr Clay says it is also vital the sample is representative of the whole clamp. He recommends using a sample taken from 15cm behind the clamp face comprising at least nine sub-samples taken in a W-shape across the face.

“Investing time to check and react to forage dry matter will be time well spent and help ensure more productive, healthy cows,” he concludes.

 Effect of changing forage dry matter content on dairy performance   
Dry matter (%)  Dry matter supplied from 40kg fresh weight  Energy supplied at 10.5MJ/kg DM (MJ)  Production supported 
25  10  105  M +7 
28  11.2  117.6  M +9.5 
30  12  126  M +11.2 

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