Postgrazing Sward Height and Milk Production: Getting the Residual Right in Supplemented Herds

Every grazing dairy farmer eventually has the same argument with a paddock. Graze it harder and you use more grass and keep the sward in better shape for the next round. Leave more behind and the cows eat better but you waste some grass. Supplements complicate the picture because you can buy back some of the intake the paddock denies. The question is how much milk that actually protects.

The number that frames this decision is the postgrazing sward height, the height of the grass left after cows leave the paddock. An Irish trial gives some of the cleanest numbers available, and this article uses it to build a practical way of thinking about the residual in a supplemented herd.

What the residual tells you

Postgrazing height is a window into what the cow could eat. A low residual means cows were forced to graze deep into the sward, where the leaf is thin and bites are small. A higher residual means cows could be selective and fill up faster. Because it is measured after the cows have left, it also shows you whether your allocation matched your herd's appetite. If you want the wider picture of grass and hay quality, our guide on forage quality covers the stored-feed side.

The Irish trial

A study published in the journal animal followed 72 Holstein-Friesian cows that calved around mid-February. In the first period, from early February to mid-April, cows grazed to either a severe postgrazing height of 2.7 cm or a moderate height of 3.5 cm. In the second period, running to the end of October, cows stayed in their first-period groups but were assigned to either 3.5 cm (severe) or 4.5 cm (moderate).

Early lactation: tight grazing cost milk

Grazing to 2.7 cm instead of 3.5 cm in early lactation reduced, per cow per day:

• milk yield by 2.3 kg
• milk protein yield by 95 g and fat yield by 143 g
• lactose yield by 109 g
• grass dry matter intake by 1.7 kg

Protein concentration fell by 1.2 g/kg and fat concentration by 2.2 g/kg. Cows on the severe treatment also ended the period lighter.

Later lactation: the carryover was mixed

After the ten-week restriction, the early treatment did not carry over to milk or milk solids yields in the second period, though it did affect milk fat and lactose concentrations. Cows that had been restricted early in the season were heavier and carried slightly more condition later in lactation. Moving the residual from 3.5 to 4.5 cm in period two raised milk and milk solids yields, driven by higher grass intake, and raised body weight.

The authors' conclusion is the most useful sentence in the paper: cumulative milk production after the restriction was unaffected, but the large loss in milk solids during the restriction was not recovered. In other words, cows partly rebuilt themselves later, but the solids you did not make in spring stayed lost.

Where supplements fit

If a tight residual costs intake, can concentrates cover it? Partly. Penn State work summarized on Farms.com gives a sense of the arithmetic:

• Each pound of concentrate cuts pasture intake by roughly half a pound (a substitution rate of about 0.4 to 0.6), though total intake still rises.
• The milk response per additional pound of concentrate falls from about 1.2 to 1.4 lb over the first 4 lb to about 0.4 to 0.6 lb at 16 to 20 lb.
• At a milk-to-feed price ratio near 2.0, feeding 16 to 20 lb to high-genetic-merit cows can still make sense; at ratios near 1.0 it generally does not.

The Penn State figures describe a specific set of cows and prices, so rerun them with your own. The point is that concentrate is a diminishing-returns tool. It is best used to fine-tune a well-managed grazing plan, not to rescue one where cows are asked to graze too hard.

A practical way to set the residual

1. Treat early lactation as the protected period. The Irish trial found the loss from tight residuals in spring was not recovered. If you will be generous at any time of year, do it for fresh cows.
2. Measure your residuals, not just your covers. Walk paddocks after the cows leave, record heights and compare them across the herd's groups.
3. Match allocation to the herd. Persistently low residuals in a paddock mean the herd was underfed; persistently high residuals mean you gave away grass you could have used.
4. Think about the next rotation. A lower residual can change regrowth and quality, and later-season growth, although the trial summary I reviewed focused on the cows rather than the sward.
5. Price the trade. Compare the value of the solids lost with the cost of concentrate or silage you would feed to hold the residual up.

A caution about reading the numbers

The trial used cows on an Irish spring-calving system with grass-based diets. A herd using a heavily supplemented TMR plus grazing for a few hours, or a herd in a very different climate, may see a different response. Heights are also measured differently across regions and tools, so compare your readings only to benchmarks measured the same way. Your local extension office, or a grazing specialist, can tell you what residual is realistic for your forage species.

Bottom line

In the Irish trial, forcing cows from a 3.5 cm to a 2.7 cm residual in early lactation cut daily milk by 2.3 kg and permanently lost a chunk of milk solids, while moving to a more generous 4.5 cm later in the season raised yield and intake. Supplements can help but they come with diminishing returns and substitution. For more on building a grass-based system, see the agriculture directory, and explore farm.fm's page on rotational grazing.