Managing Forage Quality for Milk Production
There have been a number of factors that have made achieving historical peak milk solids production per cow quite the challenge this season. With the ever improving milk solids payout, most producers are going to want to hold their cows as high as is physically possible in order to optimise milk solids sales. In order to realise this, there are a number of practical management issues that will need to be approached with best practice.
Milk production tends to rise and fall on forage quality. Now that we are in late spring/early summer, pasture naturally becomes reproductive and with that comes a decline in nutrient density. This decline is due mainly to reductions in fibre digestibility as plants become more mature and harden up. Forage neutral detergent fibre (NDF) increases with plant maturity, but the bigger challenge is that the digestibility of the NDF falls.
Researchers Oba and Allen were able to show that every 1% decrease in forage NDF digestibility resulted in a net loss of 0.17kg dry matter intake (DMI). So we can quickly calculate that a 5% decrease in NDF digestibility results in cows dropping around 1kg DMI daily, this will be reflected in declining milk solids production. In practical terms, this is like grazing ryegrass at the 3.5 leaf stage versus 3 leaf stage.
We suggest that you do utilise every available resource at your disposal to maintain the best possible forge quality. If you can’t apply adequate grazing pressure, pre-graze topping (or post if you prefer this) followed by a suitable application of fertiliser offers invaluable assistance. We’re often asked, “But what if it gets dry after I have topped?” If it doesn’t rain, whatever cover you have will be compromised. But if there is some moisture about, your topped paddocks will be way ahead. Alternately, if forage intake is becoming limiting, silage, summer crops and pellets will need to be increased on a per cow basis.
Ideal rotation length
Use leaf emergence rate to help calculate your ideal rotation length, remembering the 3-leaf principle (ideally grazing at 2.5 – 3 leaf stage). Use a rising plate meter or learn how to do a quadrant cut to assess pre and post grazing residuals (see our Herbage Mass Assessment video). https://dairynutritionspecialists.co.nz/resources/pip-gale-talks/ Then use this data to crunch the numbers on daily forage intake per cow.
Look to maintain forage intakes above 3% of your herd’s mature liveweight. For example, 500kg cows should be consuming at least 15kg (500kg x 3%) as forage daily. You can then balance the rest of the ration with pellets to achieve production targets. Pre-graze topping has the added benefit of reducing trash on your paddocks, which aids in reducing the risk of high spore counts when facial eczema is a potential threat.
Why digestible forages are important
There has been a lot of discussion over recent years on the topic of “Fully fed cows…” We all know from experience that as forage quality declines, gut fill becomes a constraint as rate of passage through the rumen slows and intake falls… but cows may well have consumed all they can physically eat. We also know that if we could offer them more digestible forages, they could consume additional kilograms of dry matter and milk solids production would be maintained at higher levels.
During mid-lactation, Dr. Dave Mertens NDF equation for optimal DMI tends to hold true when evaluating optimal daily DMI per cow. His theory is that cows will consume 1.2 – 1.3% of their liveweight as NDF daily.
The equation to calculate this (at 1.3%) is 130/total ration NDF% expressed as a percentage of liveweight.
For example, a 500kg cow being offered a ration with an average NDF of 43%:
130/43 = 3.02
500kg x 3.02% = 15.1kg DMI daily.
If the same cow were offered higher quality forage and the total ration NDF dropped to 38%:
130/38 = 3.42
500kg x 3.42 = 17.1kg DMI daily for the same cow.
Now, this equation obviously has some limitations and should not be applied at every stage of lactation, but it does give a good indicator as to why DMI falls as we head into summer.
Slowing post-peak decline
Maintaining total DMI at the highest possible level is the starting point in arresting post-peak decline. Then you need to evaluate the balance of nutrients in the total ration. Rumen degradable protein (RDP) can become a production limiting factor, look to move to a pellet that offers at least 16% crude protein. Look for cows in your herd that are rapidly gaining body condition score (BCS) well before 200 days in milk (DIM). They are telling you that they require additional metabolisable protein in order to partition energy away from BCS gain and towards the udder.
If you are unsure how to assess the nutrient delivery of your current ration for optimal feed conversion efficiency and milk solids yield, contact your local Dairy Nutrition Specialist.