Genetic selection during the past decade has increased the average number of pigs farrowed/litter from 10.2 to 11.1. With this emphasis on larger litter sizes has come a higher proportion of low-birth-weight pigs. It is now well established that pigs with low birth weights are less likely to survive until weaning, grow slower and are fatter at market weight.

The impact of these low birth weights on reproductive capabilities has not been extensively studied. However, Canadian research has shown that at 7 days of age, the number of Sertoli cells in the testicles is lower in low-birth-weight boars compared to their high-birth-weight counterparts. Because the number of Sertoli cells established before puberty determines adult sperm production, low-birth-weight boars may have poorer adult reproductive performance.

A study of 37 Yorkshire-Landrace crossbred, low- vs. high-birth-weight boars was conducted to assess their reproductive characteristics after they reached puberty.

The study, conducted at the Virginia Tech-Tidewater Agricultural Research and Extension Center in Suffolk, VA, showed that by approximately 11 months of age, 78% of the boars had been successfully trained for semen collection. The birth weights of boars successfully trained for semen collection (29) averaged 3.67 lb. at birth, while eight boars considered to be “untrainable” averaged 2.8 lb. at birth — even though body weights at training were similar.

Semen was collected weekly from trained boars for eight weeks and semen was analyzed using a computer-assisted sperm analysis system. Statistical analyses using Pearson correlation coefficients (R) showed that sperm concentration (R = 0.42), total sperm/ejaculate (R = 0.38), and number of artificial insemination (AI) doses/ejaculate (3 billion sperm/dose; R = 0.37) were all positively correlated with birth weight. The relationship between birth weights and AI doses appears in Figure 1.

Of the 29 boars collected, a subset of seven boars that weighed less than 3 lb. at birth was classified as low-birth-weight boars and another subset of nine boars that weighed over 4 lb. at birth were classified as high-birth-weight boars.

Sperm concentration was greater for high-birth-weight boars (387.6 million sperm/mL) vs. low-birth-weight boars (285.1 million sperm/mL), as was total sperm/ejaculate (94.1 billion vs. 76.2 billion) and number of AI doses/ejaculate (31.0 vs. 25.2).

The results of this experiment indicate that birth weight is a predetermining factor impacting reproductive potential in adult boars, with implications for seedstock suppliers to the commercial swine industry.

Researchers: Mark J. Estienne and Allen F. Harper, Virginia Tech, Blacksburg, VA. Contact Estienne by phone at (757) 657-6450 (ext. 408) or e-mail mestienn@vt.edu.

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