Impact of pullet growing strategies on sexual maturation and reproductive efficiency

Modern broilers grow quickly because they have tremendous genetic potential due to intensive selection for increased growth rate and efficiency (Zuidhof et al., 2014). The parents, broiler breeders, carry this genetic potential and when fed ad libitum, easily become overweight (Heck et al., 2004), compromising reproductive performance (Renema and Robinson, 2004; Chen et al., 2006) and reducing welfare (Mench, 2002). Thus, broiler breeder hens are typically feed restricted during rearing, and to a lesser extent during the lay period. However, despite increased genetic potential, breeder-recommended body weights (BW) have remained virtually unchanged over the last 40 years (Renema et al., 2007). Thus, the severity of feed restriction has increased. Severe feed restriction decreases nutrient availability in the body, like fat, which may affect body composition and might delay the onset of lay in modern broiler breeder hens. When fed precisely to a breeder-recommended BW by providing multiple meals per day, broiler breeder pullets partitioned energy to lean rather than adipose tissue. Thus, they were very lean, and under suboptimal lighting conditions, in particular, many did not undergo sexual maturation in a timely manner (van der Klein et al., 2018). Thus, it is likely that BW recommendations are becoming too severe, and new optimal growth trajectories are needed.

Laying hens have the opposite feed intake problem compared with broiler breeders. For many layer lines, it is challenging to get the birds to eat enough to sustain their health, welfare and productivity. It is well known that feed composition and feed intake affect pullet growth and development (Kwakkel et al., 1995; Traineau et al., 2015). As with broiler breeders, it is equally important to ensure that the pullet is in optimal metabolic and physiological condition around the time of sexual maturation. However, rather than restricting feed intake, optimal diet formulation is likely a more desirable approach to achieve optimal condition prior to the laying period. This feeding strategy is more complex and requires more a specialized system.

At the University of Alberta, recent precision feeding studies with broiler breeders and layers have been conducted with the objective of managing nutrient intake such that the pullets achieve optimal body condition to enter into lay and sustain production of eggs of the highest quality. This paper describes different precision feeding approaches for broiler breeders and layer pullets. Both had the same goal of achieving optimal body condition around the time of sexual maturation to maximize reproductive efficiency. The broiler breeder studies focussed on optimizing the growth trajectory, while the layer study examined mild growth restriction in combination with dietary energy levels during rearing. The objective of these studies was to evaluate the impact of pullet growing strategies on sexual maturation and reproductive efficiency.

Two concurrent broiler breeder precision feeding experiments were conducted. Both consisted of a set of 12 growth trajectories in 2 x 6 factorial arrangements. A total of 576 day-old Ross 308 broiler breeder females were randomly assigned to various BW trajectories, with 24 birds per growth trajectory. Birds on all treatments were housed together in 3 large floor pens, and the various treatments were applied to each individual bird (experimental unit). Birds in both experiments were managed using a precision feeding system (Zuidhof et al., 2017), which enables the implementation of different growth curves in free-run birds. In both studies, the BW trajectories were designed using a 3-phase Gompertz model (Zuidhof, 2020) to fit the breeder-recommended target (Aviagen, 2021) as follows:

where BWt was body weight (kg) at time t (wk); gi was the amount of gain (kg) occurring in phase i; bi was the rate of maturing of phase i; and Ii was the inflection point of phase i (wk).

In experiment 1 (Exp1), growth trajectories had 2 levels of early growth: standard, where g1 was equivalent to phase 1 growth estimated from the breeder-recommended target; or a 20% increase, 20% shift from g2 (pubertal growth) to g1 (pre-pubertal growth); and six I2 (timing of pubertal growth spurt; PGS) ranging from 15 to 23 wk, 21 wk being the breeder-recommended target I2 (Figure 1). Experiment 2 (Exp2) growth trajectories had two rates of b2 (pubertal rate of growth; PR): standard or 50% faster; and six early growth levels, shifts of growth from g2 to g1 ranging from -10 to 40% of breeder-recommended g2 (Figure 2).

After a training period, individual feeding in the precision feeding system started at 14 d of age. Feed intake and BW were recorded, and feed conversion ratio (FCR) was calculated for the rearing period (0 to 21 wk of age). Photostimulation occurred at 21 wk of age with 11L:13D increasing one hour of light per week until 13L:11D (30 lux). Dissections were done at 21 wk of age, with one bird per growth curve being dissected. Five birds per growth trajectory were dissected at the sexual maturation time point, the day of the first egg being laid. In both dissections, abdominal fat pad weight was recorded.

Analysis of variance was performed for FCR (two-way ANOVA for both trials) and analysis of covariance for abdominal fat pad and total egg production variables with early growth as a discrete source of variation and pubertal growth spurt as a continuous variable (Exp1). For Exp2, pubertal growth rate was a discrete source of variation and shifts to earlier growth were continuous. The MIXED procedure of SAS (Version 9.4, SAS institute Inc., Cary, NC) was used.

Feed conversion ratio (0 to 21 wk of age) was not affected by the timing of the pubertal growth spurt nor pubertal growth rate during the rearing phase. However, in both experiments FCR increased as second phase growth was shifted to the first phase (P < 0.05), reflecting the greater cost of maintenance of a larger body through the rearing period. At 21 wk of age, abdominal fat pad weight increased by 0.22%, and at first egg 0.09%, per week that the pubertal growth spurt was advanced (P < 0.05; Exp1). In Exp2, the abdominal fat pad at 21 wk of age was not affected by the growth trajectories, but was affected at first egg; the 50% faster pubertal phase growth rate (I2) yielded greater abdominal fat content (1.87 vs. 1.37%; P < 0.05). Egg production to 58 wk of age increased as the timing of the pubertal growth spurt was advanced in Exp1. For every week that the pubertal growth spurt was advanced, the number of eggs increased by 3.2 (P < 0.05). No difference in total egg production was observed in Exp2 to 58 wk of age.

Feed conversion ratio of pullets during the rearing period increased mainly by increased early growth (shifted from the pubertal to the prepubertal phase). The earliest growth is typically the leanest growth. Stimulating growth in this manner increased maintenance energy costs, but did not increase egg production; this approach is not recommended.

When feed restriction was relaxed during the pubertal phase, specifically when the pubertal growth spurt was advanced, birds increased fat deposition, and also increased total egg production. During the pubertal phase, the reproductive system develops and extra fat deposition during this time appears to be a key to increasing egg production. Further analysis will be conducted to determine if the increased egg production response is based on physiological signalling molecules originating from fat tissue (e.g. adipokines) or nutrient supply (e.g. lipoproteins) for egg production. In addition, an economic analysis will be conducted to determine the revenue for producers, but also on the supply chain level. As an initial conclusion, advancing the timing of the pubertal growth phase of broiler breeders is recommended because it did not affect FCR, but increased egg production to 58 wk of age.

Lohmann Brown-Lite pullets were randomly assigned to one of eight treatments in a completely randomized 2 feed restriction (FR) × 4 dietary metabolizable energy (ME) factorial arrangement. The two FR levels were Meal Every Visit (MEV) and Restricted feeding (RES). Birds assigned MEV were permitted to consume feed ad libitum when they entered the station, while RES birds were subjected to the lower range of the breeder-recommended target BW trajectory and were only permitted to eat from the feeder if their real-time BW was less than the target BW. There were three levels of dietary ME: Low (10.89 MJ/kg; 2,600 kcal/kg), Standard (Std; 11.72 MJ/kg; 2,800 kcal/kg), or High (12.56 MJ/kg; 3,000 kcal/kg). A fourth treatment (Choice) allowed birds to choose from amongst the diets. Diets were isonitrogenous. Birds from all treatments were comingled in two floor pens from 0 to 50 wk of age. A multi-feeder precision feeding station (Zuidhof et al., 2022) was used which permitted appropriate feeding of comingled treatments. Each bird received its allocated ration individually, allowing each bird to be its own experimental unit.

Photostimulation occurred at 18 wk of age. Data collection from the multi-feeder stations included the identity of each bird, and time-stamped BW, diet provided, and feed intake. Based on this data, flock uniformity and energy intake were calculated. Flock uniformity was measured using the coefficient of BW variation (CV). Egg production data was collected using a nest box system which was equipped with radio frequency identification readers which identified the nest occupant, the egg weight, and the time of lay. All data were analyzed as a 3-way ANOVA using the MIXED Procedure in SAS (Version 9.4. SAS Institute Inc., Cary, NC, 2012), with FR, dietary ME, and age as main effects. Tukey’s multiple range test was used to compare treatment means. Differences were reported where P ≤ 0.05, and trends noted where 0.05 < P ≤ 0.10.

There was an interaction between FR and dietary ME on body weight. Birds on the MEV treatment had greater BW than RES birds. MEV birds on Low and Std diets had greater BW than MEV birds on High and Choice diets (1,437 and 1,427 vs 1,399 and 1,400 g, respectively, P < 0.001). RES birds on the Std diet had the lowest BW (1,341 g). Overall, MEV birds had a greater CV (lower uniformity) than RES BIRDS. However, the effect of FR on CV depended on dietary ME. MEV birds fed the Choice diet had the greatest CV (least uniform) and RES birds on the High ME diet had the lowest CV (10.32 vs 3.68 %, respectively, P < 0.001). In terms of energy intake, in general, MEV birds had greater energy intake than RES birds (0.94 vs 0.91 MJ/d, respectively, P=0.009). However, the effect of dietary ME level on ME intake depended on age. At 20 wk of age, birds fed the Low diet had greater energy intake compared to birds fed the High and Choice diets (1.12 vs 0.95 and 0.97 MJ/d, respectively, P < 0.001). There was also an interaction between FR and age. At 20 and 21 wk of age, birds fed MEV had greater energy intake than birds fed RES (1.09 vs 0.94 MJ/d, respectively, P < 0.001). Total body fat was lower in RES birds (P < 0.001).

There was no effect of dietary ME on total number of eggs laid to 50 wk of age. There was a trend for birds fed MEV to lay more eggs than birds fed RES (194 vs 188 eggs, respectively, P=0.08). For egg weight, there was an interaction between FR and dietary ME. RES birds fed Std diets had the greatest egg weight, while MEV birds fed the Std diets had the lowest egg weight (58.1 vs 55.5 g, respectively, P < 0.001). RES birds fed the Low diet had a greater egg weight than MEV birds fed the Low diet (57.6 vs 56.1 g, P < 0.001). MEV and RES birds assigned Choice had greater egg weights than MEV and RES birds fed the High diets (58.0 and 57.4 vs 56.4 and 55.9 g, respectively, P < 0.001).

Precision feeding stations successfully reduced the BW CV, which could facilitate flock management. However, feed restricted birds with lower BW tended to have lower egg production. However, feed restricted Std birds produced heavier eggs than unrestricted Std birds. Further economic analyses considering weight-specific egg prices will determine ultimate profit routes for producers.

In both broiler breeders and laying hens, there was a correlation between total body fat reserves around the time of sexual maturation and reproductive efficiency. In broiler breeders, advancing weight gains too early decreased efficiency, while strategically advancing the timing of the pubertal growth phase increased body fat and reproductive success. In laying hens, feed restriction of pullets decreased body fat reserves and tended to reduce egg production, while pullet phase dietary ME had little impact on reproductive efficiency. Giving laying hens a choice of dietary ME led to the poorest egg production outcome. Therefore, full feeding of layer pullets on a standard diet is recommended. Further analysis of underlying physiological mechanisms and the economics of production will add further insights on the sustainability of these recommendations.

ACKNOWLEDGEMENT: Funding for this research was provided by Results Driven Agricultural Research (Edmonton, Alberta, Canada), Aviagen (Huntsville, Alabama, USA), Canadian Poultry Research Council (Ottawa, Ontario, Canada), Canadian Broiler Hatching Egg Producers Association (Ottawa, Ontario, Canada), Canadian Hatching Egg Producers (Ottawa, Ontario, Canada), Egg Farmers of Canada (Ottawa, Ontario, Canada), Egg Farmers of Alberta (Calgary, Alberta, Canada), Trouw Nutrition (Eindhoven, NL), and TopSector Consortium, led by Laura star (Aeres University, NL). Special thanks to Xanantec Technologies, Inc. (Edmonton, Alberta, Canada) for providing in-kind support for the design, manufacture, and maintenance of the precision feeding systems used in these studies.