Debunking 3 Google Merchant Center Supplemental Feed Myths

19 June

Most brands I see treat Google Merchant centre supplemental feeds like a spare tyre. They only think about it when something is broken. They use it to fix a disapproval, patch a missing GTIN, and then forget about it.

This is a huge mistake.

Relying only on your primary feed from Shopify is like bringing a knife to a gunfight. It’s the bare minimum. The real performance gains in Google Shopping and Performance Max come from the data you add after the primary feed is set up.

Supplemental feeds are not just for fixes. They are your primary tool for strategic optimisation. They let you test, segment, and enrich your product data in ways your primary feed can’t. If you’re not using them, you’re leaving money on the table.

Google Merchant centre supplemental feed: Beyond basic fixes

The biggest myth is that supplemental feeds are just for error correction. I’ve audited dozens of eCom accounts where the only supplemental feed was created three years ago to solve a specific disapproval. It hasn’t been touched since.

This misses the entire point. A supplemental feed is a layer of control. It allows you to overwrite or add to your primary product data without messing with your website’s backend.

Think of it this way. Your primary feed from Shopify provides the foundational data: title, price, image link, description. A supplemental feed adds the marketing layer on top.

We use them for strategic tasks, not just reactive fixes.

A/B testing product titles. Does “Organic Cotton Tee” perform better than “Soft Breathable T-Shirt”? You can’t easily test this in Shopify. With a supplemental feed, we can send 50% of impressions to one title and 50% to another to find the winner.
Adding custom labels for bidding. Your primary feed doesn’t know which products have the highest margin or which are bestsellers. We use a supplemental feed to add custom_label_0 with values like “High Margin” or “Bestseller” to build smarter bidding strategies in PMax.
Applying promotional text. You can add “On Sale” or “Free Shipping” to product titles during a promotion without changing the product page itself. This is much faster and cleaner.

This moves the feed from a simple data repository into an active campaign asset. It’s a core part of any serious Google Shopping Feed Optimisation strategy. It’s how you tell Google exactly which products matter most to your business.

Supplemental feed management for small businesses

The second myth is that this is too complex for small teams. Founders think they need a developer or an expensive third-party tool to manage feeds.

That’s not true.

The simplest and most effective supplemental feed is a Google Sheet. If you can manage a spreadsheet, you can manage a supplemental feed. Shopify users can get this running in under 15 minutes.

Here’s the basic process:

Create a new Google Sheet.
The first column must be id. This is the product ID that matches the ID in your primary feed.
Add columns for the attributes you want to add or change. For example, custom_label_0 or title.
In Google Merchant centre, go to Products > Feeds.
Click ‘Add supplemental feed’. Name it, select ‘Google Sheets’, and link the sheet you just created.
GMC will ask you to connect it to a primary feed. Select your main Shopify feed.

That’s it. Now, any ID you put in that Google Sheet will have its data merged with the primary feed. You can find the exact steps in Google’s official documentation on creating a supplemental feed.

You don’t need to add all your products. You can start with just your top 10 bestsellers to test a new title format. The key is to start small and make incremental improvements.

For brands we work with, this is often one of the first things we implement as part of our Google Ads management. It gives us immediate control without needing to wait for developer resources. Don’t let the perception of complexity stop you from taking control of your data. If you’re unsure how to take control of your data, a free Google audit can help identify opportunities in your current setup.

Why primary feeds aren’t always sufficient for optimisation

“My Shopify app handles the feed. Isn’t that enough?”

I hear this all the time. The answer is no. Primary feeds, especially those generated by standard apps like Google & YouTube on Shopify, are a starting point. They are not an optimisation tool.

They have several limitations.

First, they are slow. Most primary feeds only update once every 24 hours. If you sell out of a key product, that product could keep showing in ads for almost a full day, wasting spend. A Google Sheet supplemental feed can be set to fetch every hour.

Second, they are rigid. The feed app pulls data directly from your Shopify product fields. If you want to test a marketing-focused title like “Perfect Summer Beach Towel”, you have to change the actual product title on your website. This affects SEO and on-site user experience. A supplemental feed lets you create different titles just for your ads.

Third, they lack marketing context. Your primary feed contains product attributes, not business attributes. It doesn’t know which products are high-margin, which are seasonal, or which you’re trying to clear out. This is the data you need for smart bidding.

This is where a supplemental feed becomes essential for granular control. It lets you layer business intelligence onto your product data. This is crucial for advanced campaign types that rely heavily on data inputs, and it’s a key part of our PMax Optimisation Framework. Without this extra data layer, you’re just giving Google the same generic information as all your competitors.

Advanced strategies with Google Merchant centre supplemental feeds

Once you’re comfortable with the basics, you can use supplemental feeds for more advanced tactics. This is where we see significant performance lifts for the brands we manage.

Sophisticated Bidding with Custom Labels Custom labels are the most powerful tool you have. You get five of them (custom_label_0 through custom_label_4). We use a supplemental feed to populate these based on business goals.

custom_label_0: Margin (e.g., ‘high-margin’, ‘low-margin’)
custom_label_1: Product Status (e.g., ‘bestseller’, ‘new-arrival’, ‘clearance’)
custom_label_2: Seasonality (e.g., ‘summer’, ‘winter’, ‘evergreen’)

With these labels, you can create asset groups in PMax or product groups in standard Shopping that bid more aggressively on your most profitable products.

Title A/B Testing This is a simple but effective strategy. In your primary feed, you have your standard, SEO-friendly title. In a supplemental feed, you create a new title column with a more direct-response, marketing-focused version. Then, you create a feed rule in Merchant centre to test them. For example, a rule could be: “If id contains ‘t-shirt’, set title to supplemental feed title”. You can run this for two weeks, measure CTR and conversion rate, and then roll out the winner. We’ve seen title optimisations alone lift CTR by 20-30%.

Enriching Product Data with USPs Your product description is often long and detailed. For ads, you might want to highlight one key selling point. Use a supplemental feed to add a short_title attribute that includes a key benefit. For a skincare product, the main title might be “Vitamin C Radiance Serum 30ml”. The supplemental short_title could be “Brightens Skin in 7 Days”.

These strategies are impossible if you only rely on a primary feed. They require a separate, flexible data layer that you control completely. If you want an expert to review your current setup, you can always request a free Google audit from our team.

Elevating your product data with supplemental feeds for competitive advantage

ultimately, Google Ads is an auction. The winner isn’t just the one who bids the most. The winner is the one with the most relevant ad. In Google Shopping and PMax, relevance is almost entirely determined by the quality of your product data.

Using supplemental feeds is how you create a higher quality data set than your competitors.

By adding custom labels, testing titles, and enriching descriptions, you give Google’s algorithm better signals. Better signals lead to your ads being shown to more qualified buyers. This improves your click-through rate.

A higher CTR tells Google your ads are relevant, which can improve your quality score and lower your cost-per-click. Better targeting and more persuasive ad copy also lead to higher conversion rates. The combined effect is a significant improvement in your return on ad spend (ROAS).

This isn’t a one-off trick. It’s a long-term strategic advantage. While your competitors are stuck with the generic data from their Shopify app, you are feeding Google a stream of highly optimised, business-aware information. You can see from our results how this focus on fundamentals pays off.

Not sure if your Google Ads structure is costing you?

We audit Google Ads accounts weekly — PMax, Shopping, Search. The free Google Audit shows you where budget leaks and what to fix first.

Get the audit →

If your Google Merchant centre account only has one primary feed in it, you have a massive opportunity.

Setting this up correctly is one of the highest-use actions you can take to improve your Google Ads performance. If you want an expert pair of eyes on your feed strategy, my team can help.

Dan Nikas