Working out how much pasture and crop you’re harvesting off the milking platform is the first place to start.
At its simplest the calculation is:
Total energy consumed (MJME or kg DM)
– imported supplement eaten
– supplement made on effective area and exported
= Total pasture and crop eaten (MJME or kg DM)
To work out the total kilograms consumed DairyNZ also has a table but you need to know your herd’s average liveweight and per-cow production based on total milk production (milk to the factory and milk other than colostrum that didn’t go in the vat-peak cows milked).
That number will include the period cows are dry so you need to subtract the amount of feed they consume during the dry period if they are wintered off-farm. Use kg DM/cow offered times the utilisation factor – for example 14kg DM/cow/day x 85% utilisation.
Table four.
You could either buy-in all the deficit as supplement or use a combination of supplements with other tactics such as de-stocking through culling or drying off cows.
It’s a similar story if you match feed supply over a six-month period, except the amount of supplement will be less.
If you set cow numbers so demand is met by pasture for 10-12 months (where cows are wintered off) you’ll have surplus pasture through the height of the growing season that can be conserved for use in the shoulder growing months.
To work out how much additional feed is required you need to know what the average growth rate is over each period and the average intake per cow.
If average growth over three months is 73kg DM/ha/day and cows are consuming 18kg DM/cow/day the stocking rate can be calculated.
73kg DM/ha/day x 90 days = 6570kg DM total supply
Adjusted for utilisation at 85% = 5584kg DM
18kg DM x 90 days = 1620kg DM demand
5584kg DM/ 1620kg DM = 3.4 cows/ha
If the average over 10 months is 46kg DM/ha/day and demand over that longer period averages 15kg DM/cow/day, a similar calculation shows the stocking rate should be 2.6 cows/ha.
DairyNZ estimates at the average 55g MS/kg DM response to supplements the breakeven price is 3.5% of milk price.
At $4/kg MS that’s 14c/kg DM – an important fact to remember when going through this exercise. The lower the payout the lower the price a farmer can afford to pay for bought-in feed.
DairyNZ’s pasture and crop eaten ready reckoner is at www.dairynz.co.nz/media/253642/1-16_Pasture_eaten_calculator.pdf.
