FEEDING OPTIMAL AMINO ACIDS LEVELS TO LACTATING SOWS

Adapted in part from Kim et al. (2005), Pig News and Information 26:89N-99N. The author acknowledges his collaborators: Dr. David H. Baker, Dr. Robert A. Easter, Dr. In K Han, Dr. Walter L. Hurley, Dr. Fei n. Dr. Ronald D. Mateo, Dr. Kevin R. Pond, Dr. Hans H. Stein, and Dr. Guoyao Wu (alphabetic order) for their kind support and help.

Protein and Amino Acids for Lactating Sows
Proteins consist of specific sequences of amino acids (AA) that are essential nutrients required for tissue protein synthesis and other vital metabolic functions in animals (Reeds and Hutchens, 1994). Unlike lipids and carbohydrates, excess AA cannot be stored in the body and are subject to irreversible loss via oxidation and production of urea. Thus, AA need to be provided daily in the diet. Feed ingredients vary greatly in AA content. Available evidence shows that AA profiles in dietary proteins are important determinants of rates of tissue protein synthesis. Because AA are needed for maintenance and growth and also for reproduction and lactation, pregnant and lactating sows have high requirements for dietary protein and AA to support fetal growth and milk production.

Ideal Protein for Sows During Lactation
Amino acid intake and metabolism during lactation is directed primarily to milk protein synthesis. Even during a catabolic state, sows synthesize milk protein by mobilizing their body proteins to provide free AA, which renders the lactation period physiologically and nutritionally unique and distinct from other stages of a pig's life cycle. Lactating sows often lose body protein to support milk production. An excessive protein loss could result in a deficiency of one or more AA, ultimately impairing normal reproductive performance for subsequent parities, including delayed return to estrus, reduced litter size, or even reproduction failure (Kirkwood et al., 1987). Thus, a feeding strategy for lactating sows should focus on minimizing body protein loss by providing high-quality dietary proteins. Modern sows are highly prolific but have a low appetite. It is therefore a challenge to animal nutritionists to increase the sow's feed intake during lactation. Increasing dietary protein content and improving the quality of dietary protein are two practical feeding strategies for lactating sows.

The AA requirements of modern high-lean-type sows have been investigated in recent years, which reflects the significant genetic improvement of sows (Kim and Easter, 2003; Kim et al., 200Ia). Several studies also have been conducted to investigate AA needs for mammary gland growth (Kim et al., 1999abc; Kim et al., 2000b; Nielsen et al., 2002). Improving mammary gland growth has a direct effect on increased litter weight gain (Kim et al., 2000a).

The mammary gland is a major tissue for lactating sows because of its metabolic importance in synthesizing and secreting milk. Milk production is directly related to litter weight gain and litter weaning weight. Of note, lactating mammary glands continue to grow as lactation progresses. We have reported that 1.0 gram/ day (g/d) of Lys (or 7.0 g/d of essential AA) is incorporated into mammary tissue protein for sows with 10 nursing pigs (Kim et al., 1999a). However, nonsuckled extraneous mammary glands undergo substantial involution during the first 7 to 10 days of lactation (Kim et al., 2001b). Trottier et al. (1997) measured the amount of essential AA taken up by mammary glands (188.5 g/d), secreted as milk proteins (139.5 g/d), and retained in mammary glands (49.0 g/d). Among the 49.0 g/d retained essential AA, 7.0 g/d (14 percent) were accreted as mammary tissue and the remaining 76 percent was either transformed to other nonessential AA and nitrogenous substances or oxidized for energy (Richert et al., 1998; Table 1).

Kim et al. (1999b) demonstrated that mammary gland growth can be affected by dietary AA and energy intakes during lactation. Mammary gland growth in lactating sows was found to be maximized when sows consumed 55 g total lysine and 16.9 Meal ME per day during lactation (Figure 1).

The quantity of AA accreted in mammary tissue is affected by litter size. Lysine (or essential AA) accretion was increased by 0.13 g/d (or 1.20 g/d) for each one pig per litter increase during a 2l-d lactation (Kim et al. 1999c). Nielsen et al. (2002) measured the amount of Lys (or essential AA) taken up by mammary glands of sows with different litter sizes (1.92 g Lys/d or 7.65 g essential AA/d). Considering the Nielsen et al. (2002) data, AA needs for each additional pig in a litter would be higher than actual tissue AA accumulation (Table 2).

Milk production is relatively unaffected by modest dietary protein restriction because sows have a remarkable capacity to mobilize body protein to support AA needs for milk protein synthesis (Revell et al., 1998). However, severe protein restriction during lactation will decrease milk production (Jones and Stahly, 1999; Knabe et al., 1996). Sows can respond, however, to a high protein diet with an increase in milk production (King et al., 1993). Attempts to increase milk production by feeding a high protein diet must consider the balance of AA, especially the most limiting essential AA (Copper et al., 2001). Dietary protein fortification via a stomach cannula did not increase milk production, although it did decrease maternal tissue loss during lactation (Pluske et al., 1998). The concentrations of protein-bound AA in sow's milk are similar between days 7 and 21 of lactation (Wu and Knabe, 1994) and are shown in Table 3 (Kim and Wu, 2004).

Mature sow's milk contains 5.2 percent crude protein (Tilton et. al., 1992; Renaudeau and Noblet, 2001). However, the crude protein consists of significant amounts of urea (6.0 mmol/L defatted milk) and ammonia (1.4 mmol/L defatted milk) nitrogen (Wu and Knabe, 1994). Thus, true protein levels in milk should be determined by quantifying individual AA (Kim and Wu, 2004). To estimate the intake of AA from milk, it is necessary to properly express concentrations of AA in milk (e.g., g/L of whole milk; Table 3) and the amount of milk produced (Figure 2).

Available evidence shows that the profiles of AA in plasma (Wu et al., 1999) differ markedly from the AA patterns taken up by mammary glands or those in sow's milk (Trottier et al., 1997; Kim et aI., 2004b; Table 4). These differences in AA patterns reflect different rates of AA transport by mammary tissue, and also extensive transformation of AA (synthesis and catabolism) in mammary tissue (O'Quinn et al., 2002). When sows do not receive adequate amounts of dietary AA, maternal tissue proteins (particularly skeletal muscle proteins) are mobilized to support milk production. Excessive maternal protein mobilization often results in reproduction failure for the next parity (Jones and Stahly, 1995). Establishing nutrient requirements for lactating sows, therefore, is not limited to maximizing milk yield for nursing pigs. It also extends to maintaining an optimum body condition for subsequent parities (NRC, 1998; Kim and Easter, 2003). Amino acid mobilization occurs from various maternal tissues at different rates (Escobar, 1998; Kim and Easter, 2001). Muscle is the major AA donor during food deprivation or inadequate provision of dietary protein, but the reproductive tract contributes the largest portion of its own AA (Kim and Easter, 200 I). Dourmad et al. (1998) reported that high-producing sows need at least 55 g/d of dietary Lys for minimal weight loss, and this requirement is the same as the Lys need for maximal mammary gland growth (55 g/d) suggested by Kim et al. (I 999b). However, maintaining sows in an anabolic state during lactation may not, alone, be sufficient for improving sow fertility (Zak et al., 1998). Thus, a primary target should be minimizing body protein loss from the sow.

Kim et al. (2001a) suggested an ideal dietary AA pattern for lactating sows based on the concept that different AA patterns are needed for tissue protein, milk protein, and dietary protein. These correlated profiles would affect the final dietary AA pattern needed for sows during lactation. Some essential AA in milk protein are higher than those released from tissue mobilization and in a common corn-soybean meal based diet. Amino acids needed for mammary gland growth may also affect the ideal dietary AA pattern. Considering these factors, the ideal dietary AA patterns for lactating sows likely change dynamically while responding to the expected maternal protein loss during lactation (Table 5).

Body condition and the expected levels of AA mobilization are important factors that must be considered in designing diets for lactating sows. This dynamic ideal protein concept would allow for a more precise estimation of AA needs for lactating sows. For sows possessing a low voluntary feed intake and substantial tissue mobilization during lactation (primiparous and second parity sows), Thr is a critical AA, whereas valine becomes increasingly important for sows having a high feed intake and limited tissue mobilization (multiparous sows) during lactation (Kim et al., 2001a). However, Lys is the primary limiting AA in both cases. To apply the dynamic ideal protein pattern and the order of limiting AA, lactation diets can be designed for individual sows based on their expected levels of tissue mobilization during lactation.

Soltwedel (2005) and Soltwedel et al. (2003, 2005) recently evaluated the limiting order of AA in a corn-soybean meal lactation diet for sows losing about 25 kg body weight during a 21-d lactation. This would correspond to about a 30 percent level of tissue mobilization as calculated by Kim et al. (2001a): 30 percent of the AA output in milk being provided by tissue protein breakdown in the sow. Soltwedel et al. (2003, 2005) diluted (with starch) a 17.2 percent CP com-soybean meal diet (0.90 percent Lys) to about 10 percent CP and 0.55 percent ileal digestible Lys. Thus, the low protein, low Lys diet contained the same AA profile as a typical corn-soybean meal lactation diet. Using plasma urea nitrogen as an indicator of AA limitation, Lys was found to be first limiting, and Thr was second limiting in this diet. Val was tested in the Lys and Thr-supplemented diet, but no response was obtained. These results provided convincing empirical evidence that the AA profile in a typical corn-soybean meal lactation diet has Lys and Thr as its first and second limiting AA, and Val is not third limiting, under conditions where substantial weight loss and protein depletion are occurring during lactation. The implications of this research are that the ideal ratio of Thr to Lys does not exceed 0.63, and the ideal ratio of Val to Lys is lower than 0.81 for lactating sows losing 25 kg body weight during a 21-day lactation. Recently, Kim et al., (2004) demonstrated that first-parity sows fed diets containing AA that are ideally balanced for their needs had improved lactation performance (as indicated by both increased litter weight gain and decreased body weight loss) when compared with those fed common corn-soybean meal diets (Figure 3).

-Submitted by Sung Woo Kim

References available on request