Feed your Ayrshires

Adequately meeting metabolizable protein requirements throughout lactation.

Overview

Understanding Stepdown: When Production Suddenly Drops

This article revisits the causes behind sudden drops in milk production observed in some Ayrshire cows, as first discussed in the 2023 edition of the Canadian Ayrshire Review. It underscores the importance of meeting metabolizable protein requirements and offers practical tools to identify, prevent, and correct this issue.

A Word to Remember (Or to Add to Your Vocabulary): Stepdown

Ayrshire Canada has highlighted a recurring concern: sudden and often unexplained drops in milk production in certain cows. While not new nor unique to the Ayrshire breed, this issue affects overall herd performance. To describe this phenomenon, the term “stepdown” was introduced.

A stepdown is defined as a sharp decline in lactation persistence—specifically, a drop of more than 10 percentage points between two milk recording tests. It typically presents as a persistence index falling to 80% rather than the expected 90%, without any valid explanation. If the next test shows a sharp rebound to 110%, the previous drop was likely just a recording error, not an actual drop in performance.
However, if this type of sudden fluctuation is repeated (a decline not followed by a rebound in production), it is no longer a coincidence: your herd is likely experiencing a stepdown issue. It is crucial to address it quickly to protect your herd’s health and the profitability of your operation.

Chart 1 & Cart 2: Cows AY80 (2^nd and 3^rd lactation) and 1646 (4^th lactation) show a true stepdown, whereas cow 6654 (Chart 3) demonstrates only a recording error.

Stepdown: A Widespread but Manageable Phenomenon

A thorough analysis conducted by Catherine Chaput* from Agrinova, based on data from 186 Ayrshire herds over a three-year period, reveals striking contrasts: some herds are only minimally affected by stepdown, while others are heavily impacted.

In Chart 4, herd 1006 has a stepdown rate of only 18% (an excellent performance), while in herd 1014, 80% of cows were affected.

The study also identified a link between high genetic potential for protein production and increased vulnerability to stepdown. In other words, cows with stronger genetic potential require higher nutritional support—especially in terms of metabolizable protein. If these needs aren’t adequately met, the risk of stepdown rises significantly.

Metabolizable Protein: The Cornerstone of Milk Production

Unlike fat, the dairy cow has no internal reserve of protein. Her diet must meet this need entirely. For Ayrshires, known for their strong protein yields, this fact becomes even more critical. Even a short-term nutritional deficiency can quickly reduce performance.

At the start of lactation, when the cow reaches her peak production, milk protein levels are typically lower, while lactose output—which drives milk volume—is at its highest. As lactation progresses, lactose decreases but protein concentration increases. At this point, the cow’s genetic potential becomes the main factor influencing milk protein percentage. Meeting the cow’s metabolizable protein needs throughout the entire lactation cycle is therefore essential.

Remember that metabolizable protein is the sum of digestible microbial protein (produced in the rumen) and digestible undegraded protein.

It is this metabolizable protein that enables the cow to synthesize milk proteins, including alpha-lactalbumin, which is essential for lactose production, and therefore, for milk volume. A deficiency of 55 g/day in metabolizable protein can cause a 1 kg/day drop in milk production, and lead to a chain reaction of issues: over-conditioning, calving difficulties, ketosis, reduced fertility, lower revenues, higher veterinary costs, etc.

Remember:

A deficit of 55 g/day results in a loss of 1 kg of milk/day.
A cow producing 27 kg of milk at 3.4% protein will require 1,311 g of metabolizable protein per day solely for milk production.
A cow producing 20 kg at 4.5% has the same requirements as one producing 30 kg at 3% — that is, 1,286 g/day.

So… is stepdown inevitable? No. Or at least, it can be significantly diminished. Is it only of genetic origin? Certainly not. Otherwise, there wouldn’t be as much variability between herds. Is the Ayrshire cow different? Yes, but mainly because Ayrshire cows produce a lot of protein! If we provide our cows with what they need, they will, in turn, provide us.

Stepdown is a warning sign. A call to react. It is often linked to unmet nutritional needs. Let’s be data-savvy and act quickly.

*Catherine Chaput, agr., M. Sc., Project Manager in Research and Innovation, Agrinova