Figure 1. IGF-I levels in milk.
K.M. Irvin*, J.K. Friesen, S.M. Neal,+
D.R. Owens*, and B.C. Irvin*
Animal Science Department* and
Agricultural Technical Institute+
Relationships among sow productivity traits, IGF-I, protein and fat levels in sow's milk, and dam weights from breeding to weaning were evaluated. Tenth generation litters in a selection experiment for improved sow productivity were included in the study. Select-line litters had higher SPI values and were larger at birth, at 21 days, and at weaning. Litter birth weights were larger in the select line, as were weights at 21 days and at weaning. Select-line sows had lower dam weaning weights, but there were no differences in dam breeding or prefarrowing weights.
Reproductive performance is an economically important trait in the pork industry. Sow productivity can be measured with the Sow Productivity Index (including number born alive and adjusted 21-day litter weight). Sow productivity also is reflected in several other traits included in the prenatal and postnatal environment the sow provides for the growth and development of the pigs. Component traits of the sow productivity index have changed as a result of selection. The increase in 21-day litter weight may be due to quality and/or quantity changes in the milk. Possible quality changes may be related to IGF-I, protein, or fat levels. This experiment evaluated differences in select and control lines for sow productivity and relationships among sow productivity traits, IGF-I, protein and fat levels in sow's milk, and dam weights from breeding to weaning.
Selection for improved sow productivity using the Sow Productivity Index (SPI) continued for ten generations. The Sow Productivity Index equals 6.5 times the number of pigs born alive plus 21-day litter weight adjusted for parity and number nursed. Select and control lines were maintained to allow the measure of effects of selection. The experiment was conducted at the Western Branch of OARDC at South Charleston.
Litter size was standardized to the approximate average litter size shortly after birth. Milk samples were collected from sows at farrowing and days 7, 14, and 21 postpartum. Oxytocin was administered to initiate milk letdown. Multiple teats were sampled to obtain a 20 cc sample of milk. The concentrations of protein were obtained using a dye-binding assay. The assay was used with immunoglobin G protein standard. Protein level (milligrams/milliliter) was measured from each milk sample at birth and days 7, 14, and 21. Milk samples were acid-ethanol extracted and neutralized with Tris-HCl prior to RIA to promote the removal of IGF-I binding proteins. Samples were incubated with IGF-I and chromatographed through Sephadex G-200 columns with the nanograms/milliliter recorded. Fat analysis followed the Babcock procedure. Milk samples of 17.5 ml were used, with 12 cc of sulfuric acid added. After mixing, samples were centrifuged for 5 minutes, water added, centrifuged for 3 minutes, and placed in a hot water bath (140F). Fat was measured at the top of each flask to yield a fat percent. Dams were weighed as they entered the breeding facility. A dam prefarrow weight was obtained when each dam entered the farrowing facility at 110 days of gestation. Dams also were weighed immediately after parturition and at weaning.
Results are discussed regarding three areas: 1) results of selection for SPI, 2) responses to selection in changes in dam weights, and 3) responses related to IGF-I, protein, and fat content in the milk. Results are illustrated in Figures 1 through 4.
Figure 1. IGF-I levels in milk.
Figure 2. Protein levels in milk.
Figure 3. Fat levels in milk.
Figure 4. Dam weights.
The select line SPI was significantly larger than the control line SPI (161.4 vs 138.6). This indicated that selection for SPI resulted in genetic response. The number of pigs born alive in a litter for the select line was 1.43 pigs more than the control line number born alive (P < 0.05). The adjusted 21-day litter weight was larger (P < 0.01) for the select line than the control line. Therefore, SPI increase is attributed to significant, positive improvements in the component traits of the index. The select-line sow litters had greater litter weight at birth and more pigs at 21 days but were no different from the control line in stillborn or mummified pigs. There were no differences in dam weights when comparing select- and control- line sows except at weaning. At weaning, control- line sows weighed more than select-line sows (P < 0.05). This indicated that the select-line sows used more body reserves in producing more pigs and larger litter weights. The extra production of select-line sows reduced weight at weaning. These results are illustrated in Figure 4.
Figures 1 through 3 illustrate patterns and differences in IGF-I, protein, and fat levels in the milk. Milk samples were taken at birth and days 7, 14, and 21 of lactation.
IGF-I concentrations were much higher at birth. By day 7 levels had decreased dramatically, and had decreased less dramatically by days 14 and 21. Patterns were similar for select and control lines. The only significant difference was noted at day 14 where select-line IGF-I was lower than control-line level (P < 0.01).
Figure 2 illustrates protein levels in milk for select and control lines at birth and days 7, 14, and 21. Patterns were similar for select and control lines. Highest protein levels were noted at birth, with dramatic decreases by day 7 and gradual decreases by days 14 and 21. Significantly different protein levels (P < 0.01) were noted at day 21, with select-line sows producing lower protein levels than control-line sows.
Fat levels appear in Figure 3. Highest fat levels were at birth, with gradual decreases at days 7, 14, and 21. Patterns were similar in select- and control-line sows, with the only significant difference (P < 0.05) being with control-line sows having higher fat levels than select-line sows at day 14.
Contents of IGF-I, protein, and fat follow similar patterns in comparing select- and control- line milk samples. These data provided preliminary results with base line levels. However, the genetic selection line differences were not sufficient to conclude that the differences in the levels of the components of the index can be attributed to milk content of IGF-I, fat, or protein.