An appetite for change

By Dairy News

FARMERS OFTEN report that rapid changes to a dairy cow’s diet can lead to dramatic reductions in milk yield, presumably caused by variable feed intake, disrupted rumen function and possible health problems.

This often happens in late spring or early summer when concentrates are introduced into a mostly pasture diet, or in autumn when pasture availability increases and concentrates are withdrawn.

A research project by Agriculture Victoria researchers at Ellinbank is focusing on reducing these negative effects by redefining the recommendations around diet changeovers in dairy systems.

An initial experiment conducted by PhD student and Research Scientist, Victoria Russo, investigated the use of different strategies for introducing increased amounts of wheat grain (8 kg DM, equivalent to 40 per cent of total DM intake) into the diet of late lactation cows previously being fed only lucerne cubes.

The wheat was introduced either rapidly over six days, or gradually over 12 days, and either in large increments of 2.7 kg or small increments of 1.3 kg.

The results were unexpected in that they showed that no matter which strategy was used, there were no detrimental effects of grain introduction on intake, milk yield or ruminal pH, and nor were there any signs of acidosis.

“We think the lucerne cubes helped to buffer the pH of the rumen, preventing the drop in ruminal pH that is typically seen when a highly fermentable starch source is fed with fresh pasture,” Ms Russo said.

“This suggests that the effects of diet changeovers on rumen function are driven not only by the characteristics of the grain being introduced but also by those of the forage.”

In light of the results of that first experiment, Ms Russo then refined a method for assessing the buffering capacity of different forages by titrating them against hydrochloric acid and used it to screen 150 samples of forage, both fresh and conserved.

“We found a stark variation in the buffering capacity of different forages,” she said.

“There was variation in the initial pH, and also in the amount of acid needed to change the pH of the forage. The more acid that is required, the greater the buffering capacity.”

The next step was to pick some forages that showed both good and poor buffering capacity in the laboratory and test them in cows.

“We picked two perennial rye-grass cultivars that we fed fresh, as well as rye-grass hay and lucerne hay,” Ms Russo said.

“We fed these forages to cows as 100 per cent of their diet for three weeks before suddenly introducing them to 8 kg DM of wheat per day.

“We found that some of the forages were much better at protecting against steep drops in ruminal pH than others.

“Following the grain challenge the two hays, in particular, resulted in ruminal pH that didn’t drop as quickly or as low as the fresh forages. Generally these results were in line with what we had predicted in the lab.”

Overall the results so far suggest that the effects of diet changeovers on rumen function are driven not only by the characteristics of the grain being introduced but also by those of the forage.

If farmers need to introduce grain quickly it pays to also consider the base forage as a way of preventing dramatic declines in ruminal pH.