What studies have measured in pasture eggs

Hens out on grass eat plants and insects as well as the feed in their troughs. Farm scientists have set up trials to see what that changes in the eggs they lay. The trials vary in size, they come from several countries, and each one measured its own set of things.

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What studies have measured

Turkey, 2021: four kinds of outdoor area

A team from Turkish universities, writing in the journal Animals in June 2021, kept 400 Lohmann Brown hens for 16 weeks. Each group had free access to a different outdoor area: bare soil, chicory, white clover, or a mix of chicory and clover.

The hens on clover and on the mix ate more of the plants than the hens on chicory. The hens on bare soil ate more of their concentrate feed than the clover and mix groups did, and their eggs had thicker shells than every other group's. The team found no effect on laying.

The make-up of the fats in the yolk also differed between groups. The team measured more polyunsaturated fats in yolks from the mixed-plant group than in yolks from the bare-soil and clover groups.

Poland, 2024: cages against an outdoor run

Researchers in Italy and Poland, writing in Animals in April 2024, compared 120 eggs from older hens on one company's farm in southern Poland. Half came from enriched cages and half from free-range hens with a fenced outdoor area.

They found no difference in the total fat or the cholesterol in the yolk. The free-range yolks had a lower share of monounsaturated fats and a higher share of polyunsaturated fats.

United States, 2023: five ways of housing hens

A team at North Carolina State University, writing in Animals in February 2023, studied brown laying hens at a state research station. The hens lived in five systems, from battery cages to free range.

Eggs from the free-range hens were the heaviest, and their shells and yolks were the darkest. Housing made no difference to the strength or stretch of the membrane round the yolk, or to the egg solids.

Chile, 2026: two breeds on the same grass

Researchers at the Universidad Austral de Chile, writing in Poultry Science in October 2026, moved 30 hens of two breeds around grass plots for 91 days. They looked at 551 eggs.

The local Chilean Creole hens ate more of the grass than the Lohmann Brown hens, and their yolks were more deeply coloured. The two breeds laid at the same rate, and their eggs weighed the same. The team analysed the fats and vitamins in pooled yolk samples only, and in their words this gave exploratory evidence.

United States, 2025: one farm through the seasons

A Michigan State University team, writing in PLOS ONE in September 2025, followed eggs from one pasture-based farm in southern Ohio from May to December. The study had no indoor flock to compare with.

They measured the fats, vitamins, minerals and yolk colour each month. Several of them changed over the season, and the yolk colour score was highest in December.

Yolk colour

A Purdue University team explained in Poultry Science in 2021 that hens cannot make carotenoids, the yellow and red pigments that colour a yolk. In their words, the color of the yolk is derived from the carotenoid pigments present in their feed. They added that some producers use feed additives with yellow and red carotenoids, from natural and artificial sources.

The same team fed caged hens one of three kinds of maize: white, yellow or orange. On a standard colour fan, yolks from the white maize scored 1 to 2, the yellow scored around 6 and the orange around 9. The team also bought eggs in Indiana shops in 2018. Yolks from the orange maize were comparable in colour to the darkest of those, which were free-range or pasture eggs.

Grass is one source of these pigments. Maize and feed additives are others.

Taste tests

One study asked people to taste the eggs. Researchers in Chile, writing in Poultry Science in November 2020, gave 30 untrained tasters four kinds of egg without showing them the shells. Two kinds came from family farms, where the authors say hens are mainly kept in backyards. One was industrial free range, and one came from caged hens.

The tasters preferred the family farm eggs. The authors wrote that yolk color was the most influential parameter in making this difference.

The family farm hens differed from the others in breed, feed and flock size as well as in outdoor access. The authors describe these hens as local breeds, often fed on pasture, insects, kitchen scraps and some grain. Chilean rules define a family farm as one with fewer than 500 birds.

What is not known

A review of the research, published in Poultry Science in September 2026 by researchers in Chile and New Zealand, found that egg results vary with botanical composition, pasture availability, season, genotype, and basal diet. Put simply, the plants in the field, how much grass there is, the time of year, the breed and the feed all play a part.

The same review says that long-term comparative studies are still needed under commercial free-range conditions.

The studies above add their own limits. Some followed only a few dozen hens or a few hundred eggs. They come from Turkey, Poland, Chile and the United States, and none of them was carried out in the UK. Each trial measured its own list of things, so their results cannot simply be added together.

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