Balancing Grazing Quality & Quantity

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As spring progresses, ryegrass begins to attain phenomenal rates of daily growth and proceeds from a vegetative to reproductive phase. Usually at this stage we notice per cow production beginning to decline. 

This is nothing out of the ordinary, as many cows will now have been mated and we anticipate post peak decline, however, pasture quality may also be contributing to the degree of reduction. This is where management practices can achieve significant gains.

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Ryegrass in the reproductive phase runs up a ‘flag’ leaf in order to support seeding. These mature tillers contain higher volumes of stem relative to leaf in supporting reproductive functions. The higher stem to leaf ratio means that our cows will be consuming increased levels of indigestible Neutral Detergent Fibre (iNDF), resulting in reduced Digestibility of Organic Matter in Dry Matter (DOMD). 

Lignin is the compound in mature ryegrass tillers that contributes to increasing iNDF.  Lignin is what we have observed when we say, “the grass in hardening up”. It is indigestible when it is consumed by cows; the only contribution it makes is to increased fecal volume. 

Managing ryegrass tillers to contain lower levels of lignin as spring progresses will have immediate production benefits, as this effects DOMD and metabolisable energy. Metabolisable energy from forages is calculated using the following equation; ME= 0.16 x DOMD% (AFRC and Lincoln University standard formulae). Every percentage point increase in forage DOMD adds 0.16 MJ of metabolisable energy to a forage feed source. Even small gains in pasture digestibility can have significant production benefits, as per the example below:

 

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The difference between consuming pasture at 12 MJ versus 11.5 MJ in late spring will only be a matter of days, with the production benefit being as much as 140g MS per cow daily (assuming DMI is the same). 

With the advent of reduced DOMD comes slower rate of passage. Early spring, when pasture dry matter is low and DOMD is high, rate of passage through the rumen may be as high as 11% per hour (Utsumi, unpublished). As pastures 
become higher in iNDF, which is directly correlated with reduced DOMD, rate of passage through the rumen slows due to rumen ‘bugs’ requiring more time to break down the additional fibre. We often observe reductions in total dry matter intake (DMI) with this seasonal change. The key point is to graze ryegrass tillers at the time when they have the highest energy content and DMI will be optimal.

Metabolisable protein content of pasture typically trends lower as DOMD reduces. Herds observing rates of post peak decline greater than 7% month on month and rapid increases in cow body condition score could benefit from a review of metabolisable protein supply.

For more information on Balancing Grazing Quality & Quantity, talk to your local Ingham Dairy Nutrition Specialist or visit your Fonterra Farm Source or RD1 store.


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