In Pursuit of a Better Broiler: Growth, Efficiency and Mortality of 16 Strains of Broiler Chickens

Abstract

To meet the growing consumer demand for chicken meat, the poultry industry has selected broiler chickens for increasing efficiency and breast yield. While this high productivity means affordable and consistent product, it has come at a cost to broiler welfare. There has been increasing advocacy and consumer pressure on primary breeders, producers, processors and retailers to improve the welfare of the billions of chickens processed annually. Several small-scale studies have reported better welfare outcomes for slower growing strains compared to fast growing, conventional strains. However, these studies often housed birds with range access or used strains with vastly different growth rates. Additionally, there may be traits other than growth, such as body conformation, that influence welfare. As the global poultry industries consider the implications of using slower growing strains, there was a need for a comprehensive, multidisciplinary examination of broiler chickens with a wide range of genotypes differing in growth rate and other phenotypic traits. To meet this need, our team designed a study to benchmark data on conventional and slower growing strains of broiler chickens reared in standardized laboratory conditions. Over a two-year period, we studied 7,528 broilers from 16 different genetic strains. In this paper, we compare the growth, efficiency and mortality of broilers to one of two target weights (TW): 2.1 kg (TW1) and 3.2 kg (TW2). We categorized strains by their growth rate to TW2 as conventional (CONV), fastest slow strains (FAST), moderate slow strains (MOD) and slowest slow strains (SLOW). When incubated, hatched, housed, managed and fed the same, the categories of strains differed in body weights, growth rates, feed intake and feed efficiency. At 48 days of age, strains in the CONV category were 835-1264 g heavier than strains in the other categories. By TW2, differences in body weights and feed intake resulted in a 22 to 43-point difference in feed conversion ratios. Categories of strains did not differ in their overall mortality rates.

In pursuit of a better broiler: A comparison of the inactivity, behavior, and enrichment use of fast- and slower-growing broiler chickens

“…CONV were two weeks younger than the other categories at TW 1 (34 vs. 48 days of age). However, CONV birds were also lighter than the expected target weight of 2.1 kg (~1.8 kg at d34) and lighter than FAST and MOD (~2.3 kg) birds at day 48 (although similar to SLOW birds, 1.9 kg, at d48; Torrey et al, 2021). This lighter weight may have been a consequence of consuming feed formulated for slower-growing strains or being processed 2 and 3 days prior to the breeders' projections for reaching 2.1 kg; given the magnitude of the project, both adjustments were necessary for logistical reasons.…”

“…This suggests that aspects other than overall growth rate may impact behavior, such as temperament or strain-specific growth curves. For instance, strain H had a lower ADG than other MOD strains from approximately 17-35d (Torrey et al, 2021), which corresponded to the time period when strain H inactivity differed from that of other MOD strains. Future analyses using other data collected as part of this large-scale study will examine whether outlier strains differ in other characteristics that may explain the limited within-category behavior differences.…”

“…This lighter weight may have been a consequence of consuming feed formulated for slower-growing strains or being processed 2 and 3 days prior to the breeders' projections for reaching 2.1 kg; given the magnitude of the project, both adjustments were necessary for logistical reasons. At TW 2 (~3.2 kg), CONV had a similar BW to FAST and MOD, but were heavier than SLOW (Torrey et al, 2021), suggesting that these mismatches in anticipated body weight was mainly an issue at the lower target weight. Nevertheless, given these weight differences, discussion of inactivity and behavior will focus on results from the age analyses.…”

“…At the same age, platform use generally mimicked all enrichment use, suggesting that platforms were the most used enrichment; this may have been driven by a preference for elevated platforms over other types of enrichments, or by virtue of the platforms being large enough to simultaneously accommodate more birds than any other enrichment. At TW 2, when categories generally had similar body weights (Torrey et al, 2021), enrichment use followed growth rate, with faster-growing categories using enrichments less than slower-growing categories; this suggests that physical ability rather than body size itself limited platform use, particularly since access necessitated mobility: walking up an inclined ramp. Similarly, Rayner et al (2020) noted that faster-growing broiler strains were observed on top of rectangular straw bales less frequently than slower-growing strains.…”

“…To do so, we developed a large scale, multi-outcome study designed to evaluate the effects of both age and body weight on the welfare outcomes of 2 conventional and 12 slower-growing broiler strains. Differences in efficiency and mortality are available in Torrey et al (2021). This paper presents the results of a sub-section of the larger study, with a focus on behavioral measures of welfare.…”