Achieving 4% Intake

The Importance of Pasture Quality on Dairy Cow Productivity

Milk production from cows eating grass is influenced by two main factors:

1. How much the cow can consume (controlled especially by cow size and stage of lactation). 
2. What the cow consumes in terms of composition and quality.

Broadly speaking pasture quality can be measured as metabolisable energy expressed as megajoules of metabolisable energy per kg of dry matter (MJME/kgDM), which for most forages is derived from digestibility.

It is important to realise the significance of reduced or increased pasture quality. There is a “double gain” or “double loss” when pasture quality increases or decreases because intake will increase with quality and energy density (MJME/kgDM) as well and vice versa:

In reality quantity of intake and quality of pasture are interrelated and cannot be treated separately.

Example 1.
If a 400kg Jersey cow consuming 3.8% of body weight at peak milk consumes 15.2kgDM/day of 11mJ ME/kgDM pasture this is consumption of 167 MJME/day. Maintenance plus grazing energy would amount to approx 58 MJME.  If we assume no weight loss or gain occurring then 109 MJME is available for milk production or approx 1.60kg MS/day.  If this same animal is presented with 12mJ of ME/kgDM grass and consumes 16kgDM/day then this is intake of 192 MJME/kgDM.  Using the same calculation production would be 2.0 kgMS/day.  The extra 0.4 kg MS/cow/day has been achieved with management of quality (probably with more skill than cost).  For an average sized herd of cows of say 200, this would amount to extra daily income of $200 at current payouts, and a difference of $300 per ha in economic farm surplus if this difference in quality can be maintained for 100 days of the year.

Example 2.
If a 500kg Friesian cow consuming 3.8% of body weight at peak milk is presented with ryegrass entering it’s reproductive stage in late October/November and an ME value of 10 MJME instead of the 11 MJME grass it received last rotation.  It was producing 2.0 kgMS/day on higher quality grass (11ME), but intake reduces to 3.5% of body weight or 17.5kgDM/day.  ME intake on the lower quality pasture then is 175 MJME.  This cow might require 65 MJME for maintenance and grazing leaving 110 MJME for milk production or 1.54 kgMS/day.

The second example encompasses the period of “post-peak decline” in productivity of spring calving dairy cows on pasture.  In northern latitudes of NZ this period of post peak decline often amounts to 12% decline in productivity per month for 2 months as ryegrass quality declines just at a time when cows have peaked in production and mating is occurring. 

Supplementation of pasture to address quality deficits

Whilst supplements can be added to a grass based diet to address protein excess, deficiency, carbohydrate deficiency, fibre deficit, etc it is important to include economic calculations in decisions on supplementation.  When many supplements are added to a pasture based system (dairy farm) they tend to substitute for grass (less grass will then be eaten by the cow).  If the system was self sufficient for pasture grown before addition of the supplement then loss of grass quality can easily result, meaning less pasture of a lower ME value will be harvested per ha, cow productivity can fall (or can only be maintained by constant addition of high quality supplement).  The result can be an economic mess unless good principles are followed.

Highly stocked farms have the opportunity to import larger amounts of supplement onto the farm to minimise the negative effects of the pasture diet without compromising pasture harvest.

Increased stocking rate often maintains quality of pasture in the above situation and ensures continued harvest of high quality pasture.  This approach is “less risky” for the farmer than attempting to increase per cow productivity to higher levels than previously achieved by reducing stocking rate as extra skills are required for the latter, and more work may be required with a lower stocking rate in order to maintain grass quality (especially topping and silage making), and factors outside the farmers control may limit per cow productivity (eg. disease, endophytes, weather factors).  Whilst increasing per cow production should remain a target and will increase the % of food consumed being converted to milk, the biggest drivers for farm profitability in pasture based systems in NZ appear to be increased or maintained pasture harvest, achieving higher peak milk by calving cows in better condition (“the higher they fly the longer they glide”), increased lactation length and aiming for moderate annual increases in per cow production (but not necessarily the maximum possible).

Table of “styles” of dairying on grass based systems in Waikato

Higher per cow production with lower stocking rate (eg.450-600MS, 2.5-3 cows/ha)	Moderate per cow production with higher stocking rate (380-480MS, 2.8-5 cows/ha) and limited supplement (500-1200kgDM/cow) import	Low per cow production (250-350) on self sufficient “all grass system”
-        More difficult on mainly grass diet. -        Tends to involve reduced pasture harvest by mechanical control of quality, increased senescence and decay, higher residuals, more supplementation to enhance intake. -        Improved efficiency of conversion of feed eaten into milk? -        Greater risk of waste and financial loss but can be quite profitable -        Need to control costs of off farm purchases	-        Cows more likely to harvest extra pasture/ha of higher quality. -        Likely to require supplement to maximise lactation length, achieve high peak milk, maintain peak. -        Intermediate efficiency of conversion of feed to milk. -        Easier to achieve for most dairy farmers due to environmental constraints, and less financial risk. -        Need to control costs of off farm purchases -        Potentially the highest income earners if done well	-       Average performance level with high % pasture harvest (“utilisation” but can depress growth with overgrazing) -       Minimal supplementation of minerals and off farm feeds -       Cows used more as buffer in feed supply -       increased herd fertility and health problems. -       More dependant on climate weather conditions. -       Lower cost structure likely (farm working expenses/gross income)

All NZ dairy farms are pasture based.  Even the most intensively supplemented farms are still deriving half their food from grazed pasture and the vast majority of the 14,000 dairy farms are over 90% pasture based.  The costs of supplemented systems need to be carefully monitored.  Confusion still exists about the true cost of grass when comparing to other feeds.  If capital cost of land is included along with variable costs on a dairy farm then pasture is more likely to be costed at 12-20c/kgDM, which is cheaper than most imported onto farm feeds but not much cheaper.  Published supplement trial results show average responses near 0.07kgMS/kgDM of supplement but higher responses can occur with skilful use and careful measurement.  Extended lactation length using supplements to pasture can provide larger responses nearer 0.1-0.12kgMS/kgDM of supplement.

Whether a dairy system relies solely on pasture harvested by cows or supplements imported plus pasture harvested (a “supported system”), the most important principle is to maintain or increase pasture harvest, as overheads are relatively fixed for pasture on a farm.  An understanding of what imported feeds can do to a dairy system is essential before advising indiscriminate use of supplements.  Financial risk is increased but gains can also be greater if done well.

In summary then, increased per cow production improves the % of food converted to milk that is eaten, but close monitoring of pasture harvest/ha and economics are required to ensure a financially beneficial result. 

Dr. Soren Moller
Dairy Consultant/Director
Intelact Nutrition Limited