How different factors influence egg size

The amino acids lysine and methionine are two significant drivers of egg size in hens’ diets

A new trend for supermarkets to prefer large eggs over medium is creating pressure in the poultry supply chain. Jake Davies looks at ways that producers are meeting the market needs without compromising animal welfare.

Just over half of the eggs a hen lays over its lifetime will be classified as large or very large (weighing more than 63g) and retailers and shoppers have developed a taste for them in the past 18 months.

That demand has skewed the farmgate price for a dozen eggs, meaning farmers will receive up to 15p/doz less for medium eggs over large, creating a strong incentive for producers to try and produce more large eggs.

It has become a major issue for the egg sector, so much so that the British Free Range Egg Producers’ Association has campaigned for consumers to choose more mixed-weight and medium packs.

Beyond the simple fact that about 45% of eggs are medium or small, there are potential problems in pushing birds beyond their natural capacity for egg production. There is also the risk of a range of health issues. At the less severe end is the potential for increased stress, feather loss or higher feed consumption, according to Crediton Milling poultry specialist Will White and head of poultry at the firm Russel Crang.

Extreme cases where birds have been encouraged to lay large eggs can result in high rates of second-quality eggs and issues like prolapsed oviducts and secondary issues like egg peritonitis.

That’s why farmers must take a balanced approach to manage egg size. He suggests taking advantage of advice from nutritionists, genetics company reps and vets.

Before anything else is considered, if a flock is to perform at or above its breed standard for egg size, it needs to be in good health.

Here are four areas to consider when thinking about egg size.


Hens start to lay after reaching sexual maturity at about 19 weeks, and egg size naturally increases as birds age, in particular in the first 10-12 weeks of lay as they approach peak production at around 25-26 weeks. For example, a Lohmann Brown Classic has a rated cumulative egg weight of 42.9g at 20 weeks, rising to 63.5g at week 72.

Poultry genetics firms work hard to create breeds of bird that offer – as much as possible – consistent results when it comes to egg numbers and sizes based on market requirements.

Most breeds serving the UK market have historically been geared at producing a large number of medium eggs, and so are not necessarily suited to the demand for more large eggs. “In the ideal world you follow exactly what the breeding company recommends,” says Mr White. “As soon as you step outside of their best advice, which is where birds want to be naturally, you have the potential to run into problems.”

The trouble is that it takes time to breed new characteristics into birds, so genetics companies have been playing catchup with the market.

Despite that, most brands that serve the UK market have now introduced a hen geared towards producing larger egg sizes.

For example, the Hy-Line Brown Plus is marketed at producing eggs up to 2g heavier than the Hy-Line Brown.


A farmer who wishes to target egg weight as a priority must let his pullet rearing company know before his birds have been hatched, Mr White says. It is during rear that management can have the most significant influence on egg size later on when birds are in lay.

The basic principle to delay the onset of egg production. This allows birds to mature more slowly and develop a higher bodyweight. “That way, egg sizes are larger when they come into lay and remain larger throughout the life of the flock,” he explains.

This is typically done by adjusting lighting regimes. Pullets are given 20 hours of light a day and reduced over the first few weeks. If the lighting is lowered more slowly, birds will develop at a slower pace.

For example, the H&N Brown breed guidelines suggest a gradual reduction from 23 to 13 hours light over the first 6 weeks in the rearing shed to encourage larger eggs.

At the other end, birds after transfer at 16 weeks should be gradually exposed to longer lighting cycles to slow the onset of lay further and allow time for proper muscular and skeletal development.

Diet in early lay

Hens are fed a high-spec diet in their first weeks in the laying house. “At 16 weeks you’ve got a bird with a range of demands – maintenance, further growth and the beginning of egg production,” Mr White says.

As a result, diets are formulated with a higher quality protein profile and better specification than in the rearing ration, and calcium levels are ramped up to help with skeletal growth and build up a reserve before egg-laying commences.

Birds typically increase their body weight by about 50% in their first 10 weeks on the laying farm, demonstrating the importance of a good diet.

From there it is a careful balance to get egg size right, explains Mr White. “You don’t want egg size to get too far from the natural growth of the breed target curve.” At this stage, it’s vital to monitor birds’ condition and bodyweights and be ready to adjust diets if necessary. It’s important to note there is no fixed guidance for when to switch diets, and decisions should be taken on a flock-by-flock basis.

If egg size is deemed to be becoming too high, then hens will be switched to a ration with the same energy, but a different protein profile. Lysine and methionine are two micro-ingredients that have a significant impact on controlling egg size and will be reduced gradually over the life of the hen.

But it’s essential to take care, as adjusting the amino acid profile can also cause egg numbers to drop, Mr Crang adds. “You’ve got to be careful when you step down the diets to address egg size, that you don’t lose production.

“In a similar vein, calcium doesn’t make diets more palatable, so you have to be careful with its content. The birds can only absorb so much.”

End of Lay

As birds age, egg size naturally increases, but shell quality can diminish. However, producers are hoping to keep birds beyond 72 weeks, which until recently was the standard. Therefore, maintaining bird health becomes even more important, says Mr Crang.

A range of nutritional supplements can support shell quality. Oyster shell as a quality source of calcium has been used for many years. More recently, a variety of specialist products have become available, such as Pidolin, from Agrosom, or products in Alltech’s Total Replacement Technology (TRT) range.

Will White

Will is an integral part of our team with a broad knowledge of the feed industry. He has earned a Diploma in Nutrition from Harper Adams, spent 2 days a week for 6 months working at Blackdog Free Range eggs, passed his HGV test and has worked in the mill and warehouse. Now a full time Poultry Feed Specialist, Will is currently undertaking a distance learning degree in poultry nutrition. He is an industrious character and becomes deeply involved in his customers enterprises, enjoying it so much that he is now a free range egg producer in his own right.
In his spare time Will enjoys hunting, point to points and table tennis, but most of his time is taken with his young family.