Exploring genetic selection for feed efficiency

By Argenis Rodas-Gonzalez

Editor’s note: Argenis Rodas-Gonzalez is an associate professor in the Department of Animal Science at the University of Manitoba. He can be contacted at argenis.rodasgonzalez@umanitoba.ca.

Topigs Norsvin Canada boars at a Manitoba facility being prepared for manure sampling to measure feed efficiency.

Profitable, cost-effective swine production requires healthy animals, higher performance, increased feed efficiency and making the best use of available nutrients.

Feed efficiency can be described as the biological basis of how pigs consume and metabolize feed to grow and produce an edible product. This is a complex trait based on a pig’s ability to store and use nutrients from feed, such as fats, carbohydrates, and proteins. Skeletal muscle plays a significant role in using and storing those nutrients. Most of the efforts to improve feed efficiency have been focused on formulating balanced diets through investigating nutrient composition and genetic selection for carcass leanness.

Feed efficiency can be expressed as a feed conversion ratio (FCR), which is defined as feed intake over body weight gain. A low-FCR value means animals consume less feed per unit of gained body weight, making them highly efficient; in contrast, a high-FCR value means animals consume more feed per unit of gained body weight, making them less efficient. There are other expressions of feed efficiency, such as residual feed intake and lean feed conversion, but these will depend on additional knowledge required for informed decision-making.

Feed efficiency factors vary

Manure samples were analyzed to determine nutrient digestibility.

A common approach to improve feed efficiency is to supply appropriate nutrient requirements for animals; however, the improvement could be limited and depends on the ingredient composition of the feed, which can vary in nutrient digestibility.

On the other hand, purebred pigs are selected for feed efficiency at the nucleus level, and genetic selection to improve feed uptake continues to be a challenge. At Topigs Norsvin, selecting genetics based on the estimated breeding values for a given feed conversion ratio (EBV-FCR) has proven to be an effective strategy to maximize feed efficiency and improve growth performance and carcass composition. However, it is not clear if nutrient digestibility in pigs could be altered by genetic selection for feed efficiency.

Studies have shown that diverse feed efficiency groups resulted in lines of pigs showing both differences in growth performance and digestibility of nutrients, which happens in the gut.

For example, in Yorkshire breeding lines fed with high-energy diets, the digestibility of dry matter, total energy, absorption of nitrogen and other nutrients like fibre are similar in both high- and low-efficiency groups, based on residual feed intake (RFI) values. But when these groups are fed with low-energy diets, the high-efficiency group has greater digestibility of dry matter, total energy, nitrogen and fibre compared to the low-efficiency group; however, low-energy diets reduce growth performance of both efficiency groups.

In contrast, other studies have reported no differences in digestibility among divergent feed efficiency pigs, despite high-efficiency pigs presenting a faster growth and leaner carcasses. Thus, based on some contradictory results, uncertainty exists with selection for feed efficiency and its effect on growth performance, nutrient digestibility and carcass characteristics, especially within Large White lines.

Overcoming the feed efficiency dilemma

CT scanning was used to determine in-vivo carcass traits.

My team at the University of Manitoba includes researchers Ethendhar Rajendiran, Gustavo Mejicanos, Laura Beens and Ankita Saikia from the Department of Animal Science. We are working in collaboration with Topigs Norsvin Canada to respond to certain feed efficiency questions that have stumped breeders.

Our project is funded by Topigs Norsvin Canada, the Canadian Agricultural Partnership and the Ag Action Manitoba Research and Innovation program. The investigation is planning to evaluate 2,000 genotyped boars from two Large White dam and sire lines separated into low- and high-feed-efficiency groups based on EBV-FCR values within the breeding line.

This study investigates the variation in growth performance, apparent total tract nutrient digestibility (ATTD) and in-vivo carcass traits in finisher boars selected for low or high feed efficiency. Digestibility is determined by testing manure nutrient levels using an indigestible marker, while in-vivo carcass traits refer to the lean yield of the whole pig, determined using Computer Tomography (CT) scanning. Boars are selected at 23 weeks of age based on their feed efficiency value and fed a mixed corn-soybean meal-based diet using feeding stations that record individual feed intake and body weights.

Preliminary findings indicate that the sire line was heavier, with a final weight of more than 9.69 kilograms, growing more rapidly and more efficiently than the dam line, gaining more than 130.69 grams per day with a feed conversion ratio of less than 190 grams of feed consumed to gain one kilogram of body weight.

In addition, sire line presented greater loin depth, at more than 6.92 millimetres, and thinner fat depth, at less than 2.60 millimetres. However, there was no significant difference in nutrient digestibility between lines. On the other hand, regardless of the genetic line, and compared to the low-efficiency pigs, the high-efficiency pigs consumed less feed, at less than 284.97 grams per day, were more efficient, with a feed conversion ratio of less than 260 grams of feed consumed to gain one kilogram of carcass weight, had thinner fat depth at less than 3.05 millimetres, along with greater loin depth more than 6.92 millimetres compared to low-efficiency pigs.

The high-efficiency pigs were better able to digest crude protein and tended to show higher phosphorus and calcium digestibility compared to low-efficiency pigs. Based on the estimated protein deposition and the average feed intake one week before sampling, the amount of digestible lysine content met the requirements for the average pig. Nevertheless, high-efficiency pigs had a reduced feed intake, implying that they might have experienced a shortage of lysine in the diet. Thus, high-efficiency pigs offer favourable growth performance, heavier and leaner carcasses. However, it is still unclear whether the improved digestibility of crude protein was due to a lysine shortage or differences in genetic merit for feed efficiency.

Forward thinking for progressing efficiency

The University of Manitoba and Topigs Norsvin are also exploring the pig gut microbiome to identify the favorable microorganisms that contribute to feed digestibility, along with determining feed digestibility by using near infrared spectroscopy, and in-vivo leanness by CT scan. The findings may allow us to better understand the relationship between the gut microbiome, nutrient digestibility and genetics, leading to the improved and more effective selection of pigs for breeding programs.