Negative associative effects

The advanced version of the Pasture Consumption Calculator enables users is able to select negative associative effects of 0, 2.5 or 5%. When selected, these values represent the reduction in estimated ME of pasture and conserved forage supplements - for example:

  • If '0%' is selected, it is assumed there are no negative associative effects between feeds.
  • If '2.5%' or '5%' is selected, the estimated ME value of pasture and conserved forage supplements is reduced by 2.5 or 5% respectively.

What are negative associative effects?

When two or more feeds are mixed in the ration, the digestibility and therefore metabolisable energy (ME, MJ/kg DM) of the total diet may be less than the sum of the individual components and their relative proportions in the mix. This interaction is commonly termed ‘negative associative effects’, and is most common when highly fermentable starch-containing supplements are fed with fresh herbage or conserved fodder.

When a mixture of feeds are fed to dairy cows they have effects on how each is digested. As an example, when grain supplements are fed to cows, particularly if significant amounts are fed at milking, the pH of rumen fluid is reduced. This in turn impairs the digestion of structural carbohydrate (cell wall) in pasture and conserved forages. This is an example of negative associative effects, as a reduction in digestion of the cell wall in pasture and forages reduces the ME available to the cow. In practice, this means the milk production response to supplements is lower than what might be expected or predicted.

The degree to which associative effects occur depends on many factors.

  • The type of pasture, conserved forage and concentrate being fed
  • The amounts of each being consumed, their nutritive characteristics (composition, physical characteristics and rates of fermentation)
  • Management practices that affect the pattern of feed intake, such as pasture allocation and the method/frequency of feeding supplements, all impact the scale of response.

The method and frequency of feeding is particularly important as this dictates the consistency of nutrient supply to the microorganisms in the cow's rumen.

In pasture-based dairy systems, grazing cows consume most of their pasture in the 3 to 4 hours following each milking. When high quality pastures are eaten they are rapidly fermented producing acids which lower rumen fluid pH. A stable rumen fluid pH of 6.0 or above leads to more efficient digestion. When grain/concentrate supplements are fed at milking, they are rapidly fermented and reduce rumen fluid (see the figure below). This reduction in pH leads to a change in the rumen microorganisms and reduced digestion of plant cell wall material.

Figure 1: Changes in rumen pH over a 24-hour period - Perennial-pasture only (pasture only), Pasture with grain fed twice daily in the dairy (pasture + grain), Total mixed ration (TMR) and Partial mixed ration fed once daily with once a day grazing (PMR).

The magnitude of associative effects in grazing cows is determined by the amounts and nature of the pasture consumed and the amounts and characteristics of the concentrate fed in the dairy. When cows are fed well-formulated total mixed rations (TMR), nutrients are provided in a manner that leads to a more consistent and stable fermentation. This means, associative effects between feeds are much lower as rumen fluid pH is more uniform and does not fall to low levels. It would be expected that grazing cows supplemented with feeds in a well formulated mixed ration on a feed pad would have less fluctuation in rumen fluid pH than those grazing pasture and supplemented with grain in the dairy, but greater fluctuations than cows fed a TMR.