When this data was modeled to quantify the changes in performance associated with increasing weaning age, researchers found it useful to express these benefits on a change-per-pound-of-weaning-weight basis. As a result, the importance of weaning age and weaning weight for subsequent performance is well understood.
Since then, many swine production systems have increased their weaning age to improve weaning weight, postweaning growth, efficiency of growth, pig welfare and economic return. However, litter size has also increased during this time — the result of improved genetics, better sow nutrition, feeding practices and health management. The increased lactation days may also be contributing to the improved reproductive performance.
Improved ovulation rates and embryonic survival have occurred without any measurable change in the uterine capacity of sows. This has caused some concern about the possibilities of reduced birth weights. While the relationship of birth weight and subsequent growth is fairly well understood, most studies have involved a relatively small number of pigs using only two or three birth weight categories. Other economically important traits, such as mortality, that may be influenced by birth weight have not been adequately described. Few studies have evaluated the effects of both litter size and birth weight on the subsequent performance of pigs.
Therefore, the objective of this study was to evaluate the relationship of piglet birth weight and the size of the piglet's litter-of-origin with subsequent preweaning performance using a large population of pigs on a commercial sow farm in Kansas.
A total of 2,204 PIC 327-sired pigs born alive during a 22-day period were identified with a numbered ear tag at birth. Each sow was assigned a body condition score (BCS; scale: 1 = very thin to 5 = very fat), and the total pigs born, number born live and dead, and the gender and birth weight of each live piglet was recorded within 18 hours of parturition and before the size of litters was equalized.
During lactation, all pigs fostered, removed or found dead were weighed and recorded. No creep or supplemental feed was provided.
Each pig was weighed and assigned a BCS (1 = emaciated, 2 = thin, 3 = full-bodied) at weaning. Weaning was spread over six days during a 19-day period. Average age at weaning was 25 days.
For data analysis, individual birth weight was used to assign pigs to one of four birth weight categories (≤ 2.3 lb., 2.4 to 3.3 lb., 3.4 to 4.3 lb. and ≥ 4.4 lb.; see Table 1 ), and the number of total born in each pig's litter-of-origin was used to assign pigs to one of three total born categories (≤ 11, 12 to 14 and ≥ 15; see Table 2 ).
Pigs of heavier birth weight categories were associated with a decreased number of total born and born live (Table 1 ). Also, preweaning average daily gain (ADG), weaning weight, weaning BCS and preweaning mortality were improved for pigs of heavier birth weight categories.
Average birth weight decreased as total born per litter increased. Conversely, as birth weight category increased, the number of total born and live born decreased.
Preweaning ADG (0.38 lb./day for ≤ 2.3 lb. birth weight to 0.59 lb./day for ≥ 4.4 lb birth weight), weaning weight (11.6 lb. to 19.5 lb.), weaning BCS (2.69 to 2.93), and preweaning mortality (24.2% to 4.6%) were improved for pigs of heavier birth weight categories.
Preweaning ADG (0.51 lb./day, 0.50 lb./day, and 0.50 lb./day, respectively) and weaning weight (16.3 lb., 15.9 lb., and 15.8 lb,. respectively) were only modestly improved for pigs from the smallest total born category compared to the two larger categories.
The birth weight of pigs from the smallest total born category (≤ 11) was greater than that of pigs from the largest total born category (≥ 15; Table 2.).
Sows of the largest total born category had an increased BCS after parturition compared to the other two categories. As expected, the litter total born, as well as live born and number born dead, increased with greater total born categories. Also, preweaning ADG and weaning weight were greatest for pigs from the smallest total born category (≤ 11) compared to the two larger categories. Preweaning mortality tended to be greatest for pigs from the 12 to 14 total born category.
Although larger litters resulted in a greater number of low-birth-weight pigs, the number of heavier pigs also increased.
In this study, the greater number of low-birth-weight pigs from larger litters was responsible for the reduced performance, but these litters also produced more pigs that were heavier than 2.3 lb. and 3.3 lb. (Figure 1  and Figure 2 ). Therefore, the growth differences among pigs from the three litter size categories were relatively small.
In conclusion, these data indicate that low-birth-weight pigs had poorer growth performance and higher mortality preweaning, especially those weighing 2.3 lb or less at birth. Although larger litters had a greater number of low-birth-weight pigs, these litters also produced a greater number of live pigs with a birth weight greater than 2.3 lb. Litters with 15 or more total born produced the greatest number of live pigs that were heavier than 3.3 lb at birth. In addition to increasing litter size, maximizing the reproductive and economic efficiency of swine requires the identification of methods to improve birth weight and performance of the lightest pigs born.
Researchers: J. R. Bergstrom; M. L. Potter; M. D. Tokach; S. S. Dritz, DVM; J. L. Nelssen; R. D. Goodband; and J. M. DeRouchey, all of Kansas State University; and Steve C. Henry, DVM, Animal Hospital, PA., Abilene, KS. Contact Bergstrom by phone at (785) 532-1277 or e-mail [email protected] .
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