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Friday 9 December 2016

Calculating dairy cow intakes can be tricky

Dairy

Mary Kinston

Published 16/08/2011 | 05:00

Milk production has been sliding steadily over the last month and figures of 1.3-1.6kg of milk solids per cow per day have been common for groups over recent weeks. I find it interesting how comparable members of a discussion group can be, with often only 0.2kgMS/cow between farmers. Low figures of 1.0-1.2 have been presented on occasion, especially where once-a-day milking (OAD) is in play or herd health issues, eg IBR, high SCC, have been a problem.

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However, benchmarking at one particular discussion group in the first week of August revealed a large divergence in milk solids per cow varying from 1.9kg to 1.4kgMS/cow/day, with meal being fed at 0-2kg/ cow/day.

Within this group there were a range of cow breeds and type, varying from large Holstein Friesian cows to small Jersey cross cows with liveweights varying from around 600kg to 450kg respectively.

Calving date was comparable, though calving rate did vary between herds. Interesting as this may be, its relevance actually lies with the impact this was having on cow appetite, feed requirement, and the average pasture cover.

As the group walked around the farm in question it became apparent, on scoring the paddocks in terms of pasture mass, that the lads that had cows producing the high levels of 1.8-1.9kgMS/cow were underscoring pasture mass compared to the others.

Obviously the conversation moved towards the method of assessment, and most actually suggested that they used the cows to re-assess their measurements through back calculation, and then the penny dropped!

As target pasture cover is calculated using the formula below, the assumed intake per cow was obviously having an impact on the perception of available grass cover in the paddock, as shown in the example.

Target pre-grazing cover = (stocking rate x intake of pasture x rotation length) + residual cover eg 3.5cows/ha x 15kg DM grass/cow x 22days + 50kgDM/ha = 1205kgDM/ha versus 3.5cows/ha x 18kg DM grass/cow x 22days + 50kgDM/ha = 1436kgDM/ha.

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As a result, the entire group was asked about intake and all the lads, irrespective of cow type, weight or production, were using figures of 17-18kgDM/cow.

Whilst it may have seemed reasonable on an individual basis, when considering that production varied by 0.5kgMS/ cow and liveweight by 150kg between herds across the group, this assumption appears rather surprising.

Determining the intake of a cow can be a tricky task over and above milk production and maintenance, because factors such as condition score/ liveweight change, walking, and pregnancy have an impact on the partition of nutrients from the energy consumed.

For example, condition score loss will supplement milk production as a cow heads from calving to peak lactation. Cows walking up steep gradients will have a higher demand for energy than cows walking on the flat and the distance travelled per day also has an impact.

Pregnancy will also have an increasing demand for the energy consumed for the last 12 weeks of gestation.

However, there are methods to estimate the intake requirement and if we assume that in mid-lactation milk production is being met by intake, with no weight gain or loss, and that cows are walking no more than 2km per day on the flat, with feed quality around 11MJME/kgDM eaten, the table above illustrates how intake per cow may vary given different production levels and herd average liveweights (Lwt).

If we consider the variation within the group, a herd producing 1.4kgMS/cow/day with an average liveweight of 450kg Lwt would require 15.2kgDM/cow/day, whereas a herd producing 1.8kgMS/ cow/day at 600kg Lwt would still require 19.8kgDM/cow/day.

This is a difference of 4.6kgDM per cow per day, and certainly food for thought.

On a practical level, intake may not change through lactation as the table suggests, as cows may also use energy for weight gain and obviously an improvement in cow condition score until dry off is desirable, so these figures should be inflated to reflect that.

On the grassland management front, as we head towards the end of August it's time to extend the grazing rotation and allow pasture cover to build.

Aim for 30 days by August 31 and have a go at doing an autumn feed budget.

This is especially important if you hope to feed little to no meal until dry off because of milk quota.

Dr Mary Kinston is a dairy consultant and discussion group facilitator and can be contacted on mary.kinston@gmail.com

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