A supplemental feed adds data to your existing Google Shopping feed without replacing it — useful for adding custom labels, overriding specific titles, testing description variants, or supplying data your ecommerce platform can’t export natively.
Primary feed contains your full product catalogue from Shopify or WooCommerce
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Supplemental feed contains just item IDs + the specific attributes to add or override
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Merchant Center matches by item ID and merges, with supplemental values taking precedence
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Result: primary feed attributes + supplemental overrides = the final product data Google uses
When to use a supplemental feed
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Adding custom labels from external data
Your cost-of-goods data lives in a spreadsheet or ERP system, not in Shopify. A supplemental feed maps item IDs to margin tiers (high/medium/low) without requiring any Shopify changes. Trajekt can generate this supplemental file from a CSV upload or external API connection.
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Adding lifestyle images alongside product shots
Your primary feed uses white-background product images (correct for standard Shopping). For Performance Max, you want to include lifestyle images as additional_image_link values. A supplemental feed adds these image URLs without changing your primary feed structure.
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Testing alternative titles or descriptions
A/B testing feed titles without Merchant Center’s native testing tool: create a supplemental feed with alternative titles for a subset of products. Run them via a separate Shopping campaign product group to compare performance.
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Promotional and sale flagging
Your summer sale isn’t tracked in Shopify as a collection or tag, but you want custom_label_3 = “summer-sale” on the relevant products. A supplemental feed maintained alongside your promotional calendar applies these flags for the duration of the event.
How to set up a supplemental feed in Merchant Center
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In Merchant Center Next: Products → Data sources → Add data source
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Select “Supplemental data source” (not Primary)
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Choose “Scheduled fetch” and enter your Trajekt supplemental feed URL
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Set fetch frequency and link to your primary data source
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Merchant Center merges supplemental data with primary feed during next processing cycle
Frequently asked questions
A supplemental feed is a secondary product data source in Google Merchant Center that adds or overrides specific attributes for products already in your primary feed, without replacing the entire feed. Supplemental feeds are matched to primary feed products by their item ID. You can use them to add custom labels, override titles or descriptions for specific campaigns, add additional images, or correct data that your primary platform can't supply.
Use a supplemental feed when: your primary ecommerce platform can't supply certain attributes (like custom labels or supplemental images) in its native export; you want to override specific attributes for a subset of products without changing your main feed; you're running a campaign-specific test with different titles or descriptions; or you want to add seasonal pricing or promotional flags without modifying your product data in Shopify or WooCommerce.
In Google Merchant Center, go to Products > Data sources > Add supplemental data source. You provide a CSV or XML file containing only the fields you want to add or override, plus the item ID of each product they apply to. Merchant Center matches rows by item ID and applies the supplemental data on top of your primary feed data. Supplemental feed values take precedence over primary feed values for the same attribute.
Yes. Trajekt can generate a supplemental feed CSV targeting specific products — for example, outputting a file containing just item_id and custom_label_0 values for products that meet a certain rule condition. This supplemental file is hosted at a Trajekt URL that Merchant Center fetches on schedule, keeping the supplemental data current alongside your primary feed.
The most common use case is adding custom labels that your ecommerce platform can't supply natively. For example, adding a 'margin-tier' custom label based on your cost-of-goods data held in a separate system, or a 'clearance' flag for products you're liquidating that isn't tracked in Shopify. The supplemental feed adds the label without any Shopify changes.
No. Merchant Center processes supplemental feeds separately and merges them with your primary feed during the nightly processing cycle. There's no performance impact on your primary feed. Processing time for a small supplemental feed (a few thousand rows, just item IDs and a couple of attributes) is typically a few minutes.
Merchant Center allows multiple supplemental data sources per account. There's no strict limit on supplemental feeds, though Google recommends using them purposefully rather than proliferating them. For most merchants, one to three supplemental feeds serve the common use cases: custom labels, additional images, and performance testing overrides.
Primary and supplemental feeds from one platform
Trajekt generates and hosts both your primary Google Shopping feed and supplemental override files — all updated on your schedule.