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Journal of Animal Science - Animal Genetics

Lifetime reproductive performance and survival analysis of mice divergently selected for heat loss1

 

This article in JAS

  1. Vol. 92 No. 2, p. 477-484
     
    Received: July 29, 2013
    Accepted: Nov 18, 2013
    Published: November 24, 2014


    2 Corresponding author(s): mnielsen1@unl.edu
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doi:10.2527/jas.2013-6974
  1. A. S. Bhatnagar and
  2. M. K. Nielsen 2
  1. Department of Animal Science, University of Nebraska-Lincoln, Lincoln 68586-0908

Abstract

Divergent selection for heat loss was implemented in mice creating maintenance high (MH) and low maintenance (ML) lines and an unselected control (MC) in 3 independent replicates. Mice from the ML line have improved feed efficiency, due to decreased maintenance energy requirement, but there is potential for a correlated decline in reproductive performance and survivability. Number fully formed (NFF), number born alive (NBA), number weaned (NW), litter weaning weight (LWW), pup weaning weight (PWW), fraction alive at birth (FAB), fraction alive at weaning, and birth interval were recorded at every parity on 21 mating pairs from each line × replicate combination cohabitated at 7 wk of age and maintained for up to 1 yr. Traits were summed over parities to evaluate lifetime production. Pairs were culled due to death or illness, no first parity by 42 d cohabitation, 2 consecutive litters with none born alive, 3 consecutive litters with none weaned, 42 d between parities, or average size of most recent 2 litters less than half the average of first 3 litters. Survival probabilities were produced and evaluated for each line and used to calculate mean number of parities using a Markov-chain algorithm assuming a maximum of 4, 6, 8, 10, or 12 parities or 1 yr. Line was insignificant for all litter traits while NFF, NW, and FAB decreased with parity (P < 0.05) and PWW tended to increase (P < 0.07). The MC mice had higher lifetime NW, LWW, and PWW (P < 0.04). Birth interval showed that MH mice had increasingly larger intervals while remaining the same in ML mice (P < 0.01). In the survival analysis, MC mice had the greatest survival rates overall, but ML mice had the greatest rates in the period up to 5 parities while MH mice had the greatest rates in later parities. This resulted in greater mean number of parities for ML mice up to maximum of 8 parities and higher means for MH mice when the maximum number of allowed parities was 10 or higher. Reproductive performance was not substantially affected by changing maintenance energy requirements. The ML animals appear to survive well in early parities and produce more parities when a low number of maximum parities is enforced, but this benefit declines in later parities and MH animals survive better and increase mean number of parities when turnover rates are low. Therefore, selection for low maintenance animals may be beneficial for systems desiring a short generation interval but less so for systems desiring longevity.



INTRODUCTION

Feed intake to meet maintenance energy requirements is the largest component of feed consumption and the largest economic input in livestock production systems (Ferrell and Jenkins, 1985; Noblet et al., 1993). Therefore, selection to reduce maintenance requirements without affecting output (reproduction, growth, etc.) could be a beneficial avenue to improve feed efficiency. Energy intake that is not stored as product is released as heat. Heat loss contains genetic variation and can be used as an indicator of maintenance energy requirements (Nielsen et al., 1997a,b; Williams and Jenkins, 2003).

Nielsen et al. (1997b) demonstrated that maintenance energy requirements could be altered by selection for heat loss measured by direct calorimetry. Selection over 16 generations in mice established a maintenance high (MH) and maintenance low (ML) line, which differed significantly in heat loss as well as feed intake per unit of body weight, with the ML line consuming less feed than the MH line (Nielsen et al., 1997a,b). Selection was relaxed for 26 generations and then renewed for 9 generations, once again showing further response in heat loss and feed intake (McDonald and Nielsen, 2007). However, although ML mice tended to have higher conception rates, ML mice had smaller litter sizes (McDonald and Nielsen, 2007; Nielsen et al., 1997a). The ML mice also tended to have smaller litter weaning weights (LWW) due to poorer milk production (McDonald and Nielsen, 2006). Decline in productivity and reproductive performance may be a correlated response to selection for reduced maintenance energy requirements. Stayability may be reduced in ML mice if culling for poor reproductive performance is applied.

The objective of this study was to use these mouse lines to imitate a livestock system and test the hypothesis that reducing maintenance energy requirements negatively affects productivity, reproductive performance, and survivability, which could diminish the benefit in improved feed efficiency.


MATERIALS AND METHODS

Experimental Animals

All animal procedures were approved by the University of Nebraska-Lincoln Institutional Animal Care and Use Committee. Animals used in this study were sampled from lines of mice divergently selected for heat loss, as an indicator of maintenance energy requirments (MH = maintenance high, ML = maintenance low, and MC = unselected control) and have been previously described by Nielsen et al. (1997b). Briefly, heat loss per unit of metabolic body weight (kcal∙kg-0.75∙day-1) was measured on individual males 9 to 11 wk of age by placing them in direct calorimeters for 15 h overnight. Selection occurred in 3 replicates, creating 9 independent lines. Initial selection lasted for 16 generations and then selection was relaxed for 26 generations although independence of the lines was maintained. Selection was then resumed for 9 generations, based on the same selection criteria. In the present study, 21 mating pairs were selected from each of the 9 line × replicate combinations from generation 70, resulting in 189 total pairs of mice. Pairs were cohabitated at 7 wk of age and maintained together for the duration of the study unless culled. Mice were housed in plastic cages with wire lids and had ad libitum access to water and feed (Teklad diet 2019: 19% crude protein, 9.0% crude fat, 2.6% crude fiber, and 3.3 kcal of ME/g; Harlan Teklad, Madison, WI). Rooms housing animals were subjected to a 12:12 h light:dark cycle and ambient temperature was maintained at 23.5 ± 1.0°C.

Measuring Reproductive and Maternal Performance

Litter traits were recorded for each pair at every parity. Number fully formed (NFF) and number born alive (NBA) were recorded within 24 h after birth. Litters were weaned at 21 d after birth, when number weaned (NW) and LWW were recorded. Pup weaning weight (PWW) was calculated as LWW/NW, fraction alive at birth (FAB) as NBA/NFF, and fraction alive at weaning as NW/NBA. Each trait was also summed across parities for each pair to obtain lifetime performance measures. Birth interval was recorded as the number of days between consecutive parities and was recorded for all parity intervals.

Culling Criteria

Several culling criteria designed to be similar to criteria used in livestock production systems were established. Culling criteria were purposely made more lenient to ensure enough pairs would survive into later parities for accurate analysis. Pairs were culled due to death or illness of either member. If the male died, the female was maintained for 21 d (normal gestation length is 20 to 21 d in mice) to determine if she was pregnant. In the case of pregnancy, the female was allowed to deliver and wean her litter; if open, she was culled. Additionally, pairs were culled due to poor reproductive performance. If no first litter was produced 42 d (2 full gestations lengths) after cohabitation, the pair was considered reproductively unsound and culled. Pairs were also culled if they produced 2 consecutive litters with none born alive or 3 consecutive litters with none weaned. If the birth interval between consecutive parities was longer than 42 d, then the pair was culled. Litter size was deemed too small when the average of the most recent 2 litters was less than half the average of the first 3 and the pair was culled. Otherwise pairs were maintained for 1 yr. Culled animals were euthanized by CO2 asphyxiation.

Linear Models Analysis

Traits were analyzed using the GLIMMIX procedure of SAS 9.3 (SAS Inst. Inc., Cary, NC) with the following model:in which yijkl is the phenotypic record for each recorded trait, linei is the fixed effect of line (MH, ML, or MC), parityj is the fixed effect of the parity when the trait was recorded (1 to 11), repk is the random effect of replicate (rep; 1, 2, or 3), and eijkl is random error. Parity was treated as a repeated measure with pair nested within rep × line as the subject. An autoregressive heterogeneous variance component structure was chosen based on Akaike information criterion, corrected for finite sample size (Burnham and Anderson, 2002).

The following model was used to analyze lifetime performance:in which yijk is the phenotypic record for each summed trait, linei is the fixed effect of line (MH, ML, or MC), and repj is the random effect of replicate (1, 2, or 3). For both models, orthogonal contrasts were used to test for selection response (MH vs. ML) or asymmetry of response [(MH + ML)/2 vs. MC].

Birth interval was analyzed by linear regression with the following model:in which yijkl is the number of days between consecutive parities, β0 and β1 are the overall intercept and slope, respectively, rep0i and rep1i are the random effect of replicate on the intercept and slope, respectively, line0k and line1k are the effects of line on the intercept and slope, respectively, and X is the parity interval (parity 1 to 2, parity 2 to 3, etc.). Contrasts were used to test for a difference in slopes and intercepts between selection lines.

Survival Analysis

Survival analysis was performed using the LIFETEST and PHREG procedures in SAS 9.3 (SAS Inst. Inc., Cary, NC). Survival was measured in maximum number of parities recorded for the pair before culling; therefore, time was treated as a discrete measure. All pairs were culled before the end of 1 yr of cohabitation, so censoring of the data was not necessary. Data were analyzed over the entire study and in 2 periods (Period 1: ≤5 parities and Period 2: >5 parities). Survival functions were produced for each line using Kaplan-Meier estimates, defined asin which is the Kaplan-Meier estimator of survival to time t, di is the number of individuals culled at time ti , where t is parity (1 to 12), and ni is the total number of animals at risk of culling at time ti (Allison, 1997). Log-rank tests were used to determine differences in survival functions between lines.

Hazard functions were produced for each line using a Cox discrete hazard model:in which h0(t) is the baseline hazard at time t, β1 is the coefficient associated with line (MH, ML, or MC) and xi1 is parity number. The function hi(t) is defined as Pit/(1 – Pit), in which Pit is the conditional probability individual i is culled at time t, given that it has not already been culled (Allison, 1997). Log-rank tests of hazard ratios were used to compare lines.

A competitive risk analysis was performed to evaluate the risk of each culling criterion for all experimental animals and also within each line. Models were identical to the Cox discrete hazard model above with cause of culling incorporated as censoring. For example, when assessing the risk of culling due to death or illness, animals that were culled for other reasons were treated as censored (Allison, 1997).

Parity Equilibrium

Survival probabilities were used to estimate the parity distribution of a population of mice within each line, to compare mean number of parities produced by such populations and potential differences in replacement rates. This was achieved using Markov-chain methods described by Azzam et al. (1990). In short, a transition matrix (P) was created relating the probability a mating pair at a certain parity would be retained in the population for an additional parity or if they would be replaced by a parity 1 mating pair. A column vector, π, was defined as the proportion of mating pairs in the population at each parity after the parity distribution has reached equilibrium, assuming population size is constant. Proportions were found by simultaneously solving the set of equations π = Pπ. However, this equation does not have a direct solution in its current form. This was corrected by arbitrarily eliminating 1 equation and replacing it with Σiπi = 1. The set of equations can then be solved by Gaussian elimination. The average number of parities was then calculated by multiplying π by a vector of corresponding parities. Parity distributions were calculated assuming animals were maintained a maximum of 4, 6, 8, 10, or 12 parities or for 1 yr as was done in the study.


RESULTS AND DISCUSSION

Reproductive and Maternal Performance

Changes in litter size traits, across parities, are shown in Fig. 1. Parity did have a significant effect on some litter size traits, with NFF and NW both significantly decreasing in later parities (P < 0.01). As shown in Fig. 2, FAB also decreased in later parities (P < 0.02).

Figure 1.
Figure 1.

Number fully formed (NFF), number born alive (NBA), and number weaned (NW) by parities (* indicates significant effect of parity on trait [P < 0.05]). Error bars denote SEM.

 
Figure 2.
Figure 2.

Fraction alive at birth estimated as number born alive (NBA) divided by number fully formed (NFF) across parities. Error bars denote SEM.

 

In the repeated measures analysis, there was a line × parity interaction for LWW (P < 0.01), due to greater LWW in MH mice versus ML mice at parity 6 (P < 0.03) and asymmetry of response at parity 7 due to greater LWW in MC mice compared to the average of the 2 selection lines (P < 0.01). Otherwise, line was not significant for any reproductive or maternal performance trait measured at a single parity.

Previous studies in these populations showed MH mice producing more pups than ML mice and found a positive genetic correlation between number born and heat loss (McDonald and Nielsen, 2007; Nielsen et al., 1997a). However, in those studies, litter size was only recorded for a single parity and pairs were older (12 wk) when mated compared to the current study (7 wk). These previous studies postponed breeding until mice reached full maturity while this study bred animals closer to onset of puberty to more closely imitate conditions in livestock systems. In previous studies, the difference in litter size between MH and ML mice was small (1.6 pups) and there were small differences in litter size traits between selection lines at individual parities observed in this study, but these were insignificant when viewed in the context of lifecycle production. A previous study (McDonald and Nielsen, 2006) also found a tendency for MH mice to have heavier weaning weights compared to ML mice, which was not seen in the current study. In a study evaluating productivity and lifetime reproductive performance in mice, Newman et al. (1985a,b) found a decrease in litter size traits as parity increased, both at birth and weaning, analogous to the decrease in later parities in NFF, NW, and FAB seen in this study. Johnston et al. (2007) also did not find a relationship between litter size and basal metabolic rate in mice. In pigs divergently selected for residual feed intake (RFI), Barea et al. (2010) found a positive association between RFI and heat production. In the same population of pigs, the high RFI line had smaller numbers of total piglets born, born alive, and weaned, indicating an inverse relationship between heat production and reproductive performance (Gilbert et al., 2007).

Pup weaning weight showed a tendency to increase in later parities (P < 0.07), due to the corresponding decrease in litter size (Fig. 3). Newman et al. (1985b) evaluated lifetime productivity and reproductive performance in several lines of mice and found a similar increase in PWW in later parities as the current study.

Figure 3.
Figure 3.

Average weaning weight per pup estimated as litter weaning weight (LWW) divided by number weaned (NW) across parities. Error bars denote SEM.

 

For lifetime production, there was evidence of asymmetry of selection for NW, LWW, and PWW (P < 0.04). The MC mice weaned more pups over the span of their lifetime as well as producing larger total weaning weights and larger weaning weights per pup than the average for the 2 selection lines (Fig. 4). Line was not significant for any other lifetime reproductive traits, and MH and ML mice did not differ in any trait.

Figure 4.
Figure 4.

Lifetime number weaned (panel A), lifetime total weaning weight (panel B), and lifetime litter weaning weight/number weaned (panel C) per breeding pair for lines of mice selected for high (MH) or low heat loss (ML) or unselected control (MC). Error bars denote SEM.

 

Results of the linear regression analysis of birth interval are shown in Fig. 5. Slopes for regression of birth interval on parity were 0.58 ± 0.14, 0.38 ± 0.12, and 0.06 ± 0.15 d/parity for MH, MC, and ML mice, respectively. Slopes were different between MH and ML mice (P = 0.01) with birth intervals increasing for MH mice in later parities while remaining similar for ML mice. Intercepts were 23.16 ± 1.04, 25.23 ± 1.04, and 24.54 ± 0.99 d for MH, MC, and ML mice, respectively. Intercepts were not significantly different between MH and ML mice (P = 0.18). Other studies in mice have also found that birth interval increased over parities (Wallinga and Bakker, 1978; Newman et al., 1985a). In both of these studies, birth interval was measured in multiple, independent lines and there were significant differences in birth interval between the various lines indicating that genetic differences are at least partially responsible. In the study by Wallinga and Bakker (1978), the increase in birth interval was more pronounced in mice that had been selected for increased litter size that were subjected to continuous mating instead of interval mating where the male was removed just before parturition and returned after weaning. Authors proposed that this was due to the stress associated with simultaneous gestation and lactation, which is amplified with larger litters. Dams could not meet the energetic needs of gestation while lactating so pregnancy was delayed, resulting in a longer breeding interval. Somewhat similarly, MH mice have a greater energetic requirement for maintenance that impedes pregnancy during lactation because the dams could not maintain concurrent gestation and lactation. A study by Gilbert et al. (2012) provides evidence to support this theory in pigs. Dams selected for low RFI (that have also been shown to have lower maintenance energy requirements) were more able to divert energy to lactation by mobilizing body reserves than dams selected for high residual intake.

Figure 5.
Figure 5.

Linear relationship between birth interval and parity for lines of mice selected for high (MH) or low (ML) heat loss and the unselected control (MC).

 

Ultimately, selection for reduced maintenance energy did not result in substantial decreases in single-parity or lifetime reproductive performance as there were few significant differences between MH and ML animals. When considering lifetime production, however, MC mice are superior to either selection line, particularly in weaning traits. This is in part due to the increased longevity of the MC line found in the survival analysis. Additionally, for the MC line, MC mice had lower rates of inbreeding and thus accumulated inbreeding (F) than either selection line, which were equal in accumulated inbreeding (F = 0.38 vs. 0.46). Inbreeding has repeatedly been shown to negatively affect traits associated with fitness, including reproductive and survival traits in several species, including mice (Falconer and Mackay, 1996). Studies have reported reduction in litter size, litter weight, and survival traits associated with an increase in inbreeding (Bowman and Falconer, 1960; Beilharz, 1982; DeRose and Roff, 1999). Therefore, less accumulated inbreeding is likely at least partially responsible for the superior performance observed in MC mice. Because of the magnitude of the difference in feed intake previously observed in these populations, correlated response in reproductive performance is unlikely to outweigh benefits of reduced feed intake.

Survival Analysis

Survival probabilities for all lines are shown in Fig. 6. Control mice pairs had greatest survival rates at all time points, and the MH and ML lines showed different trends. The MH line appeared to have poorer survival rates during early parities while ML mice survived well in early parities and were lost at a greater rate in later parities. This relationship was the justification for analyzing the data in 2 periods as well as over the entire length of the study.

Figure 6.
Figure 6.

Probability of survival until next parity for breeding pairs of mice from lines selected for high (MH) or low (ML) heat loss and the unselected control (MC).

 

Hazard ratios, shown in Table 1, better quantify the relative risk of culling of 1 line to another. Over the entire study, MC breeding pairs had the smallest hazard and were less likely to be culled than pairs of either selection line (P < 0.03). There was no difference in hazard rates between MH and ML lines (P > 0.33). However, MH mice had the greatest risk of culling before achieving their fifth parity, higher than both ML and MC mice (P < 0.04 and P < 0.01, respectively). In pairs that produced greater than 5 parities, ML mice were more likely to be culled than either MH mice or MC mice (P < 0.04 and P < 0.01, respectively).


View Full Table | Close Full ViewTable 1.

Hazard ratios comparing lines1 for overall lifetime survival or for survival ≤5 parities or >5 parities

 
Period
Ratio Overall ≤5 parities >5 parities
MH:MC 1.622 ± 0.13 3.492 ± 0.14 1.42 ± 0.19
ML:MC 2.002 ± 0.11 1.36 ± 0.36 2.582 ± 0.11
MH:ML 0.82 ± 0.17 2.572 ± 1.14 0.552 ± 0.15
1MH = high and ML = low heat loss selection; MC = unselected control.
2Indicates a hazard ratio significantly different from 1 (P < 0.05).

Results indicate that MH mice are more likely to be culled early and ML later, but MC mice have greater survival rates overall. This outcome could have different implications, depending on the goals of the breeding system in question. For systems where smaller maximum parities are desirable, such as a nucleus population where a short generation interval is a priority, ML mice have less involuntary losses and thus enhanced overall reproductive efficiency in addition to their lower feed energy for maintenance. But this advantage erodes in systems allowing larger numbers of parities, where ML mice have greater rates of losses in later parities. Therefore, because nucleus operations are responsible for defining breeding objectives, reducing maintenance energy requirements may need to be balanced with longevity in breeding goals to prevent excessive losses of animals in commercial herds while still exploiting the improved feed efficiency in all segments of the system. Notably, the increased reproductive survivability of MC mice is responsible for the observed lifetime reproductive performance improvement, and the greater numbers of weaned pups makes the MC line superior over either selection line in reproductive performance. The improvement in feed efficiency seen in ML mice may still prove to be substantial enough to override the increase in output seen in MC mice.

Survival curves from the competitive risk analysis are shown in Fig. 7. Survival rates were similar in the early stages of the study, but a long birth interval and small litter size became the most likely reason for culling in later parities. Table 2 shows the hazard ratios between lines for each culling criterion. The MH and ML mice did not differ for any criteria but were more likely to be culled due to death or illness than MC mice (P < 0.05 and P < 0.09 for MH and ML versus MC, respectively). Additionally, ML mice were more likely to be culled due to small litter size than MC mice (P < 0.05).

Figure 7.
Figure 7.

Probability of survival until next parity of breeding pairs of mice culled due to death or illness (DI), 2 consecutive letters with none born alive (0BA), 3 consecutive litters with none weaned (0W), a birth interval longer than 42 d (42BI), and average of the most recent 2 litters was less than half the average of the first 3 (LS).

 

View Full Table | Close Full ViewTable 2.

Hazard ratios comparing lines1 for overall lifetime survival within each culling criteria

 
Culling criteria2
Ratio DI 0BA 0W 42BI LS FL
MH:MC 2.663 ± 0.18 3.86 ± 0.30 2.04 ± 0.45 1.47 ± 0.23 1.10 ± 0.29 1.30 ± 0.98
ML:MC 2.364 ± 0.21 2.00 ± 0.73 2.36 ± 0.37 1.21 ± 0.29 1.893 ± 0.17 2.55 ± 0.47
MH:ML 1.13 ± 0.47 1.94 ± 2.34 0.86 ± 0.68 1.22 ± 0.44 0.58 ± 0.20 0.51 ± 0.49
1MH = high and ML = low heat loss selection; MC = unselected control.
2DI = death or illness; 0BA = 2 consecutive litters with none born alive; 0W = 3 consecutive litters with none weaned; 42BI = birth interval longer than 42 d; LS = average of the most recent 2 litters was less than half the average of the first 3; FL = no first litter by 42 d cohabitation.
3Indicates a hazard ratio significantly different from 1 (P < 0.05).
4Indicates a hazard ratio with a tendency to be different from 1 (P < 0.10).

Parity Equilibrium

Table 3 shows the mean number of parities for each line when the maximum number of allowed parities was 4, 6, 8, 10, or 12 as well as over the entire study period of 1 yr. When all animals were culled at 4, 6, and 8 parities, the average number of parities was greater for ML animals compared to MH animals. However, the average number of parities was greater for MH animals compared to ML animals when animals were maintained for a maximum of 10 and 12 parities and over a 1-yr time period. Control animals generally produced a greater number of parities than MH or ML animals at all time periods. Two exceptions occurred: MC average was lower than ML when a maximum of 6 parities was allowed, and MC average was lower than MH when a maximum of 10 parities was allowed.


View Full Table | Close Full ViewTable 3.

Average number of parities by line1 assuming differing maximum number of parities allowed

 
Maximum MH MC ML
4 parities 2.395 2.466 2.461
6 parities 3.290 3.368 3.383
8 parities 4.153 4.282 4.193
10 parities 4.945 4.933 4.841
12 parities 5.583 5.710 5.431
1 yr 6.214 6.248 6.036
1MH = high and ML = low heat loss selection; MC = unselected control.

Similar to the results from survival analysis, the implications of these results depend on the strategy of the breeding program. The increased survival probabilities for ML mice in early parities resulted in a greater mean number of parities when animals are not retained for longer than 6 parities. Therefore, in breeding populations desiring a shorter generation interval, animals with low maintenance energy requirements not only offer the benefit of improved feed efficiency but will also remain in the breeding population longer and potentially provide increased total output compared to high maintenance energy or unselected animals. In livestock breeding programs where longevity is important, such as a commercial population, the smaller mean number of parities seen in ML mice may be detrimental as it implies that in a population of animals selected for low maintenance energy requirements, replacement animals will have to brought in more frequently than in a population of high maintenance energy or unselected animals, to replace those that are involuntarily culled. This may increase the input costs because replacement animals must be obtained and could erode the decreased input costs attributed to improved feed efficiency.

Implications

Selection for reduced heat loss and thus reduced maintenance energy requirements in mice has resulted in improved feed efficiency, which would be a desirable outcome in all livestock species. This study showed that reducing maintenance energy requirements did not negatively affect reproductive performance as there were few significant differences in litter traits between high and low maintenance lines of animals. Low maintenance animals have better survival rates in early parities, which could increase their efficiency in systems where smaller maximum parities are desirable. However, their survival rates decline in later parities, and high maintenance energy animals showed improved longevity in systems allowing larger numbers of parities. Therefore, breeding objectives should be designed to balance reducing maintenance energy requirements while maintaining longevity. Additionally, animals not selected for reduced maintenance energy requirements showed improvements in lifetime weaning traits and overall survivability, partially due to less inbreeding in the MC line. Integration of these results with feed intake and efficiency data is necessary to determine the effect of changing maintenance energy requirements on lifetime efficiency.

 

References

Footnotes


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