Factors affecting estrus and ovulation time in weaned sows with induced ovulation by GnRH administration in different seasons

Prostaglandin F2α treatment does not hasten ovulation in weaned sows

This study evaluated the efficiency of prostaglandin F2α (PGF) to hasten ovulation in weaned sows. In experiment I, weaned sows detected in estrus (0 h) received: no hormone (Control; n = 56); 0.5 mg PGF IM at 0 h and 2 h (PGF0; n = 56); or 0.5 mg PGF IM at 24 h and 26 h (PGF24; n = 55). In experiment II, weaned sows that did not express estrus signs until 72 h after weaning (0 h) were assigned to: no hormone (Control; n = 45); 10 µg buserelin acetate IM at 0 h (Buserelin; n = 43); 0.5 mg PGF IM at 34 h and 36 h (PGF; n = 44); or 10 µg buserelin acetate IM at 0 h plus 0.5 mg PGF IM at 34 h and 36 h (Buserelin + PGF; n = 45). In experiment I, no effect of PGF on the interval treatment onset to ovulation was observed (P > 0.05), and no treatment effect was observed on the relative or cumulative proportion of females that ovulated post-treatment onset (P > 0.05). In experiment II, treatment onset to ovulation interval was shorter for Buserelin group than for PGF group (P < 0.05), and a higher cumulative percentage of Buserelin treated sows ovulated up to 48 h compared to PGF and Control groups (P < 0.01), with no differences from Buserelin + PGF. Treatments did not affect total number of piglets born in both experiments (P > 0.05). In conclusion, PGF did not hasten ovulation timing or affect litter size in weaned sows.

Supplementing a carbohydrate-rich diet from late lactation to insemination increased glucose and insulin levels in weaned sows, leading to improved subsequent piglet birth weight

Premating maternal nutrition is crucial for postweaning follicle growth, thereby influencing piglet birth weight in subsequent litters. The present study investigated the impact of supplementing a carbohydrate-rich premating diet in sows on metabolic hormones, subsequent piglet birth weight and reproductive performance. Sows were distributed into three groups, control (n=42) received standard diets; treatment I (n=41) received the same diets supplemented with 500 g of a carbohydrate-rich premating diet from weaning until insemination; treatment II (n=42) received the same diets supplemented with 500 g of a carbohydrate-rich premating diet from 7 days before weaning until insemination. Blood samples were taken from sows around weaning to measure serum insulin-like growth factor-1 and insulin, and blood glucose after feeding. The study found that sows on a carbohydrate-rich diet (treatment II) had higher postprandial glucose (P<0.05) and insulin levels (P=0.06) than others. This diet did not affect overall reproductive performance, but it did increase piglet birth weight and reduce the number of low-birth weight piglets compared to the control (P<0.001) and treatment I groups (P<0.05). Supplementing a carbohydrate-rich premating diet for 7 days before weaning until insemination enhanced postprandial glucose and insulin concentrations in weaned sows. This dietary intervention led to improved subsequent piglet birth weight and reduced the proportion of low-birth weight piglets.

Managing prolific sows in tropical environments

Litter size in modern sows has been dramatically improved in recent decades by genetic selection for highly prolific sows. In a tropical environment, the average total number of pigs born and number born alive are reported to be as high as 17.2 and 15.1 piglets per litter, respectively. Therefore, the new production target in many herds aims to achieve 30-40 pigs weaned per sow per year. Despite the improvements in litter size, the mean preweaning piglet mortality rate remains high, at between 10% and 20%, in major pig-producing countries. A sufficient daily feed intake by lactating sows is important for high milk production as sow milk yield is the limiting factor for piglet growth rate. Heat stress, which can occur when the ambient temperatures rise above 25°C, is one of the major problems that decreases daily feed intake and compromises milk yield. Therefore, it is necessary to encourage high feed intakes to achieve high milk yields. However, even with high nutrient intakes, productivity can be constrained by intestinal barrier function, limiting digestive ability, and allowing potential pathogens and/or toxins to become systemic. This is more likely greater under tropical conditions because of heat stress, exacerbating sow fertility problems. Underpinning sow herd performance, including responses to environmental challenges, is the selection of appropriate gilts, for example, selection and management for early puberty, thus presumably selecting the more fertile gilts and the correct management of lactation to improve the number of weaned piglets are some of the key factors for future reproductive efficiency of the farm under tropical conditions.

Maternal Magnolol Supplementation during Pregnancy and Lactation Promotes Antioxidant Capacity, Improves Gut Health, and Alters Gut Microbiota and Metabolites of Weanling Piglets

Maternal nutrition exerts a profound effect on the postnatal performance of offspring, especially during the weaning period. The multifunctional bioactive component magnolol (MAG) has shown promise as a dietary supplement. This study aimed to explore the effects of maternal MAG supplementation on the antioxidant capacity, gut health, gut microbiome, and metabolome composition of weanling piglets. Fifty pregnant sows were randomly divided into two equally sized groups, the control group and the group supplemented with 100 g/t MAG during the gestation and lactation periods, and 7 days postweaning, the pups were euthanized. The microbiome and metabolome features of weanling piglet colons were compared. Our results revealed that maternal MAG supplementation modified the serum redox status of weanling piglets by decreasing malondialdehyde concentration and increasing superoxide dismutase activity and total antioxidant capacity. Moreover, the decreased indicators of diarrhea were accompanied by improved gut barrier function, in which serum diamine oxidase concentration was decreased, and expressions of zona occludens-1, claudin-1, and intestinal alkaline phosphatase were increased in the colon of weanling piglets from sows supplemented with MAG. Further analysis of the gut microbiota indicated that maternal MAG supplementation significantly increased the relative abundance of beneficial bacteria in the colon of weanling piglets, including Faecalibacterium prausnitzii and Oscillospira. Metabolome analysis identified 540 differential metabolites in the colon of piglets from MAG-fed dams, of which glycerophospholipid classes were highly correlated with progeny gut health and key beneficial bacteria. Our findings indicated that maternal MAG supplementation can improve the oxidative status and gut health of weanling piglets, possibly due to alterations in the gut microbiota and metabolites.

Season and temperature do not affect cumulative live birth rate and time to live birth in in vitro fertilization

Objective

To explore whether season and temperature on oocyte retrieval day affect the cumulative live birth rate and time to live birth.

Methods

This was a retrospective cohort study. A total of 14420 oocyte retrieval cycles from October 2015 to September 2019. According to the date of oocyte retrieval, the patients were divided into four groups (Spring(n=3634);Summer(n=4414); Autumn(n=3706); Winter(n=2666)). The primary outcome measures were cumulative live birth rate and time to live birth. The secondary outcome measures included the number of oocytes retrieved, number of 2PN, number of available embryos and number of high-quality embryos.

Results

The number of oocytes retrieved was similar among the groups. Other outcomes, including the number of 2PN (P=0.02), number of available embryos (p=0.04), and number of high-quality embryos (p<0.01) were different among the groups. The quality of embryos in summer was relatively poor. There were no differences between the four groups in terms of cumulative live birth rate (P=0.17) or time to live birth (P=0.08). After adjusting for confounding factors by binary logistic regression, temperature (P=0.80), season (P=0.47) and duration of sunshine(P=0.46) had no effect on cumulative live births. Only maternal age (P<0.01) and basal FSH (P<0.01) had an effect on cumulative live births. Cox regression analysis suggested no effect of season(P=0.18) and temperature(P=0.89) on time to live birth. Maternal age did have an effect on time to live birth (P<0.01).

Conclusion

Although season has an effect on the embryo, there was no evidence that season or temperature affect the cumulative live birth rate or time to live birth. It is not necessary to select a specific season when preparing for IVF.

GnRH agonists: Updating fixed-time artificial insemination protocols in sows

Protocols for fixed-time artificial insemination (FTAI) in swine reproduction can help increase genetic improvement and production efficiency. Different gonadotropin-releasing hormone (GnRH) agonists have been developed to gain better control of follicular development, timing, and ovulation quality; therefore, they have been extensively used in FTAI protocols. This literature review resumes the most important characteristics of the physiology of follicular development and ovulation in sows, followed by a discussion about the hormonal alternatives available to induce ovulation (human chorionic gonadotropin, hCG; porcine luteinizing hormone, LH and GnRH agonists). Also, ovulation induction failures with GnRH agonists are described. Finally, current FTAI protocols with GnRH agonists are resumed and discussed. FTAI with GnRH agonists has proven to be an efficient, successful reproductive protocol that can be implemented in pig farms due to better knowledge of an endocrine system that regulates follicular development and ovulation and increased availability of several GnRH agonists that allow more efficient reproductive swine programs.

Gilt reproductive performance in a tropical environment after oestrus synchronization and fixed-time artificial insemination

The present study was performed to evaluate the reproductive performance of gilts inseminated using fixed-time artificial insemination (FTAI) protocol. A total of 408 Landrace × Yorkshire crossbred gilts were included in the experiment. Gilts at 8 months of age were randomly allocated into three groups: control AI (n = 192), treatment 1-TAI (n = 117) and treatment 2-FTAI (n = 99). Gilts in the control AI group were inseminated 2-3 times during standing oestrus at 0, 12 and 24 h after the onset of oestrus. Gilts in the treatment 1-TAI group were orally administered 20 mg per day of altrenogest for 18 days and then inseminated 2-3 times during standing oestrus by conventional AI. Gilts in the treatment 2-FTAI group were synchronized like gilts in treatment 1-TAI group but then GnRH (10 μg of buserelin) was administered 120 h after the end of altrenogest treatment and fixed time artificial inseminated twice at 24 and 32 h after GnRH irrespective of the presence of oestrus or not. Conception rate of gilts in treatment 2-FTAI (87.9%) was similar to the treatment 1-TAI (94.9%) and control AI (83.3%) (P > 0.05). Conception rate in treatment 1-TAI (94.9%) was higher compared to control AI group (83.3%, P = 0.040). Farrowing rate of gilts in treatment 2-FTAI (83.8%) was similar to treatment 1-TAI (89.7%) and control AI (76.0%) (P > 0.05). Farrowing rate of treatment 1-TAI (89.7%) was higher than control AI gilts (76.0%, P = 0.033). In treatment 2-FTAI, the conception and farrowing rate of the nine gilts that were inseminated even if they were not detected in oestrus (all during warm season) was 44.4% and 44.4%, respectively. Regular return to oestrus was similar between groups (9.4%, 0.9% and 4.1% for control AI, treatment 1-TAI and treatment 2-FTAI, respectively, P > 0.05). The total number of piglets born per litter in treatment 1-TAI group was higher than control AI (13.1 ± 0.2 versus 11.6 ± 0.2, respectively, P < 0.001) and treatment 2-FTAI groups (12.2 ± 0.3, P = 0.019). The number of piglets born alive was higher in treatment 1-TAI (12.1 ± 0.3) compared to treatment 2-FTAI (11.3 ± 0.2) and control AI group (11.2 ± 0.3). The percentage of stillbirth and mummified foetus were not different between groups (P > 0.05). The present study indicated that fixed-time AI in gilts can be successfully performed by administration of altrenogest for 18 days, GnRH at 120 h after altrenogest withdrawal and then double fixed-time AI at 24 and 32 h after the administration of GnRH. Fertility metrics such as conception rate, farrowing rate and litter performances using this method were similar to gilts inseminated at oestrus with conventional AI.

Effects of kisspeptin-10 on the reproductive performance of sows in a fixed-time artificial insemination programme

Kisspeptin (KP) is a major positive regulator of the hypothalamo-pituitary-gonadal axis and affects female reproductive cyclicity in mammals. It offers an attractive alternative strategy to control reproduction in fixed-time artificial insemination (FTAI) protocols. We aimed to evaluate the effects of different doses of kisspeptin-10 (KP-10) on sow reproductive performance in FTAI protocols. One hundred ninety-eight weaned sows were divided into three groups at random. A FTAI-GnRH group of sows (n = 98) received 100 µg (2 mL) gonadotropin-releasing hormone (GnRH; gonadorelin) by intramuscular injection at 96 h after weaning (t = 0 h); FTAI-KPL (KPL: low-dose KP-10, n = 50), and FTAI-KPH groups of sows (KPH: high-dose KP-10, n = 50) received 0.5 or 1 mg KP-10 (2 mL) respectively at 96 h after weaning. Sows were checked twice daily for oestrus. Ultrasonographic evaluations were performed to determine the follicular diameter and time of ovulation; blood samples were collected immediately before injection (t₀ = 0 min) and at 15, 30, 45, 60, 75, 90 min, 24 and 48 h postinjection. Sows were inseminated at 112 and 132 h after weaning. The oestrus rates (96 vs 92%; 96 vs 88%) and weaning-to-oestrus intervals (98.9 vs 98.6 h; 98.9 vs 97.1 h) were not affected by treatment, but oestrus in the FTAI-KPL group was significantly longer than in the FTAI-GnRH group (38.7 vs 30.0 h; P < 0.05). The peak LH concentrations were 1.29 times greater than at t₀ = 0 in the FTAI-GnRH group, and 1.45 and 1.44 times greater than at t₀ = 0 in the FTAI-KPL and FTAI-KPH groups, respectively. Follicular diameters and pregnancy rates (86 vs 88%, 86 vs 80%, respectively) did not differ between the treatments. Moreover, the total numbers of piglets born and those born alive did not differ among the three groups. These findings suggested that 0.5 mg KP-10 given at 96 h after weaning could be used in FTAI programmes to manage batch farrowing in sows.

Equine chorionic gonadotropin pretreatment 15 days before fixed-time artificial insemination improves the reproductive performance of replacement gilts

Fixed-time artificial insemination (FTAI) technology uses exogenous reproductive hormones to regulate the sexual cycle and ovulation of sows without oestrus identification, which improves the sow breeding utilisation rate, reduces the number of non-productive days, and elevates the efficiency of pig farm management. In this study, we aimed to optimise FTAI procedures. Healthy 190-day-old and about 90 kg Large White × Landrace crossing breed replacement gilts (n = 166) which were of unknown reproductive status were randomly selected and divided into three groups: a control group (n = 62), an eCG-15D group in which the gilts were pretreated with equine chorionic gonadotropin (eCG) injection 15 days before starting FTAI (n = 50), and an eCG-20D group pretreated with eCG injection 20 days before starting FTAI (n = 54). All three groups were then subjected to the same conventional FTAI procedure. Pigs were orally administered Altrenogest (ALT, 20 mg per pig per day) for 18 days and then 42 h after ALT feeding was stopped, they were injected with 1 000 IU eCG followed by 100 μg GnRH 80 h later. The gilts were inseminated for the first time 24 h after gonadotropin-releasing hormone (GnRH) injection and then again 16 h later. After 42 h of ALT feeding, gilts in the eCG-15D group displayed a higher follicular diameter until artificial insemination (AI) than those from the other groups (P < 0.05). In addition, the ovulation times were the most synchronised in the eCG-15D group, with 100% of the gilts ovulating before the second AI on day 25 of FTAI. Furthermore, the gilts in the eCG-15D group achieved the highest pregnancy rate (92%), farrowing rate (90%), total pigs born (11.59), and pigs born alive (11.18). Together, the findings of this study demonstrate that reproductive performance can be optimised by pretreating gilts with eCG 15 days before conventional FTAI.

Timing and temperature thresholds of heat stress effects on fertility performance of different parity sows in Spanish herds

High temperature is an environmental factor that impairs sow fertility. In this study, we identified the critical weeks for heat stress effects on aspects of fertility performance, namely weaning-to-first-service interval (WSI) and farrowing rate (FR). We also examined the threshold temperatures above which the fertility performance deteriorated and whether there were any differences between parities regarding heat stress effects or thresholds. Performance data of sows in 142 herds from 2011 to 2016 were matched to appropriate weekly averaged daily maximum temperatures (Tmax) from weather stations close to the herds. Two types of ratios (i.e., ratio for WSI and odds ratio for FR) were used to identify the critical weeks for heat stress by comparing the respective measures for two sow groups based on Tmax in different weeks around weaning or service events. The ratios for WSI were calculated between groups of sows exposed to Tmax ≥ 27 °C or <27 °C in each week before weaning, with the Tmax cutoff value based on a recent review study. Similarly, the odds ratios for FR for the two groups were calculated in weeks around service. The weeks with the largest differences in the fertility measures between the two Tmax groups (i.e., the highest ratio for WSI and the lowest odds ratio for FR) were considered to be the critical weeks for heat stress. Also, piecewise models with different breakpoints were constructed to identify the threshold Tmax in the critical week. The breakpoint in the best-fit model was considered to be the threshold Tmax. The highest ratios for WSI were obtained at 1 to 3 wk before weaning in parity 1 and 2 or higher sow groups. The threshold Tmax leading to prolonged WSI was 17 °C for parity 1 sows and 25 °C for parity 2 or higher sows. Increasing Tmax by 10 °C above these thresholds increased WSI by 0.65, and 0.33 to 0.35 d, respectively (P < 0.01). For FR, the lowest odds ratios were obtained at 2 to 3 wk before service in parity 0, 1, and 2 or higher sow groups. The threshold Tmax leading to reductions in FR was 20, 21, and 24 to 25 °C for parity 0, 1, and 2 or higher sow groups, respectively. Increasing Tmax by 10 °C above these thresholds decreased FR by 3.0%, 4.3%, and 1.9% to 2.8%, respectively (P < 0.01). These results indicate that the critical weeks for heat stress were 2 to 3 wk before service for FR and 1 to 3 wk before weaning for WSI. The decreases in fertility performance in parity 0 to 1 sows started at temperatures 3 to 8 °C lower than in parity 2 or higher sows.

Single Fixed-Time Post-Cervical Insemination in Gilts with Buserelin

Current protocols for gilts recommend the deposit of multiple semen doses in the cervix each 12-24 h after estrus detection. Our objectives were: (1) to determine the effect of buserelin and a single fixed-time artificial insemination using the new post-cervical artificial insemination technique (FTAI-PCAI) on reproductive and productive performance in gilts, and (2) to compare this protocol with conventional estrus detection and double PCAI without hormonal induction. In the control group (C; n = 240), gilts were inseminated twice (8 and 12 h from estrus onset). Gilts in the treatment group (T; n = 226) received buserelin (10 μg, intramuscular) 120 h after altrenogest treatment (18 d) and one single PCAI 30-33 h after buserelin administration. The groups did not differ in reproductive and production performance (p > 0.05). The T group showed greater piglet birth weight and shorter estrus duration (p < 0.001). Delivery batch length differed significantly depending on the season (p < 0.05); the shortest length corresponded to autumn. Both groups only differed significantly in spring (p = 0.018), with a shorter length in the T group. This new FTAI-PCAI protocol with buserelin is recommended in gilts, helping with optimization of genetic diffusion, boars, and semen doses.

Factors influencing pre-ovulatory follicle diameter and weaning-to-ovulation interval in spontaneously ovulating sows in tropical environment

Follicle development and timing of ovulation are indicators of the reproductive performance of sows. The present study aimed to determine factors influencing pre-ovulatory follicle diameter and weaning-to-ovulation interval (WOI) in spontaneously ovulating sows in tropical climates with special emphasis on breed, parity and backfat thickness at weaning. In total, 80 sows were included in the study. Follicle development was determined by using transrectal real-time B-mode ultrasonography every 6 hr after standing oestrus. Weaning-to-oestrous interval (WEI), oestrous-to-ovulation interval (EOI), WOI and the diameter of graafian follicles were investigated in relation to breed, parity number (1, 2-3 and 4-7) and backfat thickness (low, moderate and high) of sows. Overall, WEI, EOI, WOI and the pre-ovulatory follicle diameter were 92.5 ± 21.6 hr, 64.3 ± 19.3 hr, 156.3 ± 29.1 hr and 10.3 ± 2.0 mm, respectively. Pre-ovulatory follicle size was smaller in primiparous sows compared with sows of greater parity, 4-7 (9.7 ± 0.51 and 11.7 ± 0.52 mm, respectively, p < .05). Weaning-to-ovulation interval was positively correlated with WEI (r = 0.75, p < .001) and EOI (r = 0.66, p < .001), but negatively correlated with size of the graafian follicle (r = -0.34, p < .01). Sows with a shorter WEI had a larger pre-ovulatory follicle diameter (at 64 hr after oestrus) (r = -0.37, p < .01). Sows with low backfat thickness had a WOI 23.4 hr longer than those with moderate backfat thickness (p < .05) and 17.6 hr longer than sows with a high backfat thickness (p = .140). The follicle diameter in primiparous sows with high backfat thickness (11.7 ± 1.1 mm) was higher than in those with low (8.9 ± 0.7 mm, p < .05) or moderate (8.6 ± 0.8, p < .05) backfat thickness. In conclusion, factors influencing follicle diameter and WOI in sows included parity number and backfat thickness at weaning. The impact of backfat thickness on follicle diameter, WEI and WOI was most pronounced in primiparous sows.

Weaned Sows with Small Ovarian Follicles Respond Poorly to the GnRH Agonist Buserelin

Simple Summary

This study evaluated the influence of mean ovarian follicle size and the season of weaning on the effectiveness of administering the GnRH agonist buserelin to synchronize ovulation in weaned sows. The results from 352 sows demonstrated that sows with small follicles (<0.5 cm in diameter) at treatment are poor responders, a condition more frequent among sows weaned in summer–autumn than in those weaned in winter–spring.

Abstract

The GnRH agonist buserelin (GnRH), used to synchronize ovulation in weaned sows, attains only 70–80% effectivity, owing to several reasons of ovarian origin. This study evaluated in particular whether mean ovarian follicle size at treatment and the season of weaning are among those influencing GnRH responsiveness. The experiment was carried out in a temperate-region farm with 352 sows of 1–6 parities weaned either in winter–spring (WS, 174 sows) or in summer–autumn (SA, 178 sows). The sows were randomized into two groups: GnRH (10 µg of buserelin acetate at 86 h after weaning, 172 sows) and control (180 sows). The ovaries were transrectally scanned from weaning to ovulation and the sows clustered according to their mean follicular size at treatment time: small (<0.5 cm in diameter), medium (0.5 to 0.64 cm) and large (0.65 to 1.09 cm). In total, 88.33% of the GnRH-treated sows ovulated, with 82% of them within the expected time window (120–132 h after weaning). In contrast, 95.45% of the unresponsive sows had small follicles at the time of treatment and were mostly weaned in SA (20.45%) than in WS (4.76%). In conclusion, the conspicuous presence of sows having small ovarian follicles at treatment time compromises the efficiency of the GnRH agonist buserelin to synchronize ovulation in weaned sows, which occurs more frequently in summer–autumn weaning.

Reproductive performance of weaned sows after single fixed-time artificial insemination under a tropical climate: Influences of season and insemination technique

We evaluated the reproductive performance of sows after single fixed-time AI under a tropical climate and investigated the influences of season and insemination technique on the efficacy of single fixed-time AI. After weaning, the sows were divided into CONTROL (n = 212) and FIXED-TIME (n = 212) groups. Sows in the CONTROL group were inseminated at 12 and 36 h after the onset of oestrus, while sows in the FIXED-TIME group were administered 10 μg of GnRH at 72 h after weaning and were inseminated 32 h later. Reproductive performance parameters, including total born, born alive, mummified foetuses and stillborn piglets per litter, piglet birth weight, variation of piglet birth weight within litter, regular return-to-oestrus and farrowing rate, were compared between the two groups. Season was classified into two groups: cool (n = 170) and hot (n = 254), and insemination technique was classified into two groups: conventional AI (n = 171) and intra-uterine insemination (IUI) with a reduced number of spermatozoa (n = 253). On average, regular return-to-oestrus (3.3 vs. 5.6%, P > 0.05) and farrowing rates (92.8 vs. 88.1%, P > 0.05) did not differ between CONTROL and FIXED-TIME groups. However, the total born and born alive piglets per litter in the FIXED-TIME were lower than in the CONTROL group (12.0 vs. 12.8 piglets/litter; P = 0.030 and 11.3 vs. 12.2 piglets/litter, P = 0.007). Interestingly, the number of total born piglets in the FIXED-TIME group was lower than in the CONTROL group only in the sows inseminated in the hot season (11.7 ± 0.32 and 12.9 ± 0.31, respectively, P = 0.005). Piglet birth weight, variation of piglet birth weight within litter, number of piglets at weaning and body weight of piglets at weaning did not differ between groups, irrespective of the season (P > 0.05). The total number of piglets born per litter in the FIXED-TIME group was lower than that in the CONTROL group in sows inseminated via IUI (11.7 ± 0.32 and 12.9 ± 0.32, respectively, P = 0.013), but not in sows inseminated using conventional AI (12.7 ± 0.42 and 12.5 ± 0.41, respectively, P = 0.772). Single fixed-time AI could be successfully performed in sows under a tropical climate, with a promising reproductive performance. However, a decreased litter size at birth after single fixed-time AI was observed when insemination was performed in the hot season. Moreover, single fixed-time AI using IUI with a reduced number of spermatozoa also decreased litter size at birth.