The first lactation is a critical period for the gilt. The outcome of the first lactation will dictate whether or not the gilt remains in the herd or is culled. Culling of gilts following a poor lactation is common practice and represents an economic drain for producers. Preparing the gilt well for a successful first lactation will also impact lactation performance in subsequent parities. The gilt is faced with biological challenges that one should recognize in order to optimize a successful lactation. The gilt is still growing substantially at time of breeding, and throughout pregnancy and lactation. Her growth is not only confined to the skeletal structure, muscle mass and internal organs but also to the mammary system. Udder health, conformation and growth of the gilt are critical determinant of a successful first lactation. Another critical determinant is the feed intake capacity. In gilts, feed intake capacity is relatively limited, again because her internal organs, including the stomach, are smaller than her multiparous counterparts. For the first time in the life, the gilt will enter into a demanding phase requiring a rapid three-fold increase in feed intake to support her nursing litter. In this article, these two critical determinants of a successful first lactation will be discussed in relation to the biological constraints of the gilt, and how we can apply knowledge of these constraints to help and prepare the gilt to optimize milk yield in the first lactation.
Maximizing feed intake during lactation
- Restrict feed intake during gestation
There are a number of factors that impact the gilt’s voluntary feed intake during lactation. The first being her body condition during gestation. It is well known that the over-conditioned gilt has a much lower overall daily feed intake during lactation, in particular, during the first week. The female pig has naturally evolved to roam (and thus move around) and to forage during pregnancy, with a remarkable capacity to accumulate body fat. Increased feed intake and fat accretion during the gestation period in many mammals is an essential evolutionary mechanism which is associated with reduction in appetite not only near parturition but also several days postpartum. A low appetite “allows” the female pig to prioritize nest building and nursing over eating, in particular during the critical phase of colostrum availability which will gradually change to milk over 72 hours. She will lose a substantial amount of weight during lactation, a natural process in itself, and wean her pigs as late as seven weeks. Long lactation periods permit full regain of appetite, body weight, and reproductive tract health for re-breeding. Within a matriarchal society, long lactation periods optimize socialization and positive behavioral development. Although this article is not meant as a review of the evolutionary basis behind the female Sus scrofa, recognizing some of the biological constraints is essential to “help” and prepare the gilt to enter into a system vastly divergent from what her genes were designed to encode for. In fact, we demand just the exact opposite. First, restricting feed during gestation is critical, and more so in gilts compared to sows. The over-conditioned gilt is far more susceptible to dystocia (difficult and extended length of farrowing) than her multiparous counterpart. Over-conditioning and decreased activity (Bufford et al., 2014) are both interrelated with increased rate of dystocia and constipation (Oliviero et al., 2010) and lower lactation feed intake. Dystocia is associated with higher rate of piglet mortality at birth and within the first seven days of lactation (Bufford et al., 2014; Malmkvist et al., 2006).
Over-conditioned gilts that do not experience dystocia per se will be less willing to stand and eat compared to a well-conditioned gilt and will tend to eat and produce less milk, and have more fat in their mammary tissue. Therefore it is important to set body condition goals for gilts entering into the breeding herd and to monitor them closely during the gestation period. The most effective, objective method to monitor body condition of gilts is to weigh them and measure back fat thickness during the mid-gestation period. Although the body condition scoring method is available, it is a subjective approach.
- Increase fiber intake during gestation
In addition to restricting caloric intake during gestation, increasing the concentration of crude fiber in the diet will benefit the gilt considerably. Increasing crude fiber from 3.8 (which is typical of a lactation diet) to 7.0% decreases the rate of constipation by 75% and may alleviate constipation by as much as 80% in severe cases (Oliviero et al., 2009). Increasing bowel movement is important in gilts because long transit of fecal matter in the large intestine increases risk for bacterial endotoxin production that in turns interfere with prolactin, the hormone needed to initiate and maintain lactation. Increasing dietary fiber also stimulates water intake by nearly 10 L per day in early lactation. More water intake and bowel movements also mean more active gilts. In addition to decreasing constipation, increasing fiber in gestation diet and feeding this diet through early lactation increases bulk and therefore stomach volume and feed intake capacity. In fact, gilts fed a gestation diet containing 11% crude fiber compared 2.8% crude fiber ate as much as 2 kg more feed per day during lactation (Quesnell et al., 2009).
Therefore, it is advisable to include a good source of fiber, for example wheat bran, and ensure at least 7% crude fiber in the gestation diet. Thus, keeping the gilts on gestation feed through the end of gestation and throughout the early days of lactation will be beneficial instead of switching to the lactation feed, which typically contains less fiber, prior to farrow. Depending on the production systems, access to straw in later period of gestation is an excellent approach to stimulate foraging and fiber intake, as well as encouraging nesting behavior and as such, increasing gilt mobility prior to farrow.
- Increase feeding frequency during lactation
If the management allows, feeding gilts three times per day during lactation with smaller quantity of feed will stimulate feed intake compared to twice per day. Frequent feedings also decrease feed spoilage. If economics allows, ad lib or self-feeders are desirable. Sows using self, wet-dry feeders whereby water nipple is integrated within the feeder, over conventional hand fed, dry feed feeders, increase their voluntary feed intake by 0.7 kg per day during lactation and piglet average daily gain (Peng et al., 2006).
Maximizing mammary gland use
Of all agricultural species, the female swine is the only litter bearing one, which makes them unique in terms of how they regulate their milk production and interact with their progeny. The sow is equipped with 14 to 16 “complex” mammary glands. There are termed “complex” because each one is composed of two “simple” glands and two lactiferous ducts draining into two separate teat openings. Each simple gland is composed of small alveoli packaged in lobules, resembling grape bunch as depicted in the cross section of an actual sow mammary (Figure 1). Milk is freshly synthesized by epithelial cells that line the interior surface of each alveolus. The mammary tissue (referred to as “mammary parenchyma”) of sows is composed of 80% of epithelial (milk producing) cells and 20% of stromal cells (supportive and connective tissue cells). And, unlike other livestock species, the pig mammary gland has a very limited milk storage capacity because both gland and teat cisterns are relatively small. Consequently, alveoli fill up with milk about every 50 to 70 minutes, and must be emptied just about 50 to 70 minutes by the piglets. Pigs are designed to nurse just about every hour and full use of all of the available functional glands is crucial to sustain mammary growth during the first lactation and likely over subsequent lactation cycles.
- Preventing teat injury and selecting for udder conformation
Just as a dairy cow udder conformation is critical to milk production, the gilt udder must be equipped with 14 or 16 equidistant, well defined functional teats with no inverted nipples. Unlike other livestock species, each teat belongs to one piglet. Consequently, for each malfunctioning teat, there is one piglet out of luck because piglets do not share teats. For example, in the picture (Figure 2) taken during a research project at Michigan State University, a small litter of eight piglets was left on a sow equipped with 12 healthy teats and highly functional glands at farrow.
Within 72 hours of farrow, the four posterior glands (depicted by the white circles on the picture) were noticeably reduced in size, and by mid lactation, the glands had involuted completely, leaving eight teats and glands for eight piglets. Incidence of teat injuries can be reduced with proper gestation flooring, farrowing crate flooring, and hind hoof claw trimming and delaying teeth clipping. Depending on the farrowing crate flooring, if needed, rubber mat positioned under the sow udder helps reduce the incidence of teat injury. If at all possible, it is a good practice to check on the gilts during gestation for any teat trauma or injury so that treatment can be provided accordingly to increase the chance of full recovery and teat functionality in time for lactation.
- Stimulating mammary cellular activity and growth
During pregnancy, the mammary glands begins to grow at an increasing rate starting around mid-gestation (~ day 60). At the end of gestation, the amount of protein in mammary tissue is just over 600 grams which is nearly as much as half of the protein content of litter size of 12 piglets at birth (NRC, 2012). While gilts tend to have fewer piglets at birth compared to multiparous sows, it is wise not to restrict the number of piglets with the goal of “sparing” the gilts. If the gilt was well bred, selected for her underline, managed and fed correctly during gestation, she is ready to nurse. With this in mind, if a gilt has 12 healthy teats and functioning mammary glands, then 12 piglets should be nursing her. There are no available published data per se directly demonstrating a negative impact of an unsuckled gland during the first lactation on subsequent milk yield potential of that gland, but there are several data that we can infer upon indicating this may be the case. During lactation, the suckled mammary glands continue to grow, doubling in the number of cells as measured by DNA concentration between day 5 and 21 of lactation in the gilt (Kim et al., 1999). Therefore suckling stimulates mammary gland growth. Sucking also stimulates mammary cellular activity, as depicted by the ratio of RNA to DNA, which also doubles from pre-farrow (~ day 110) to day 17 of lactation (Figure 3).
Figure 4 demonstrates that mammary glands at the end of involution period (~seven days post-weaning) that are suckled during lactation are quite larger than the unsuckled glands. Hence, they have more mammary tissue available presumably for redevelopment during the next pregnancy. So it is very likely that glands that are suckled in a first lactation have enhanced productivity in the next lactation compared with glands that are not suckled.
Adding one or two piglets to an existing litter to maximize the uses of functional glands by cross-fostering from another gilt or sow should be done well within 24 hours following birth if at all possible. When milk fills up the tiny alveoli, increase in pressure signals oxytocin release for milk let down. If there is no suckling, further increase in pressure over time will signal the beginning of mammary gland involution. Figure 5 presents the impact of leaving a gland unsuckled for 24 hours from day 1 to day 2 of lactation on piglet average daily gain, and the continued effect thereafter up to day 28 of lactation. The data suggest that the gland is never quite “rescued” if the fostering is not accomplished rapidly. There are of course many logistical issues that may prevent fostering as early as one would like, nonetheless, knowing how impactful this may be allows setting a better bench mark.
Gilts and sows in the United States are weaned between day 19 and 21, a period during which mammary cellular activity is at its greatest. The impact of weaning during this period compared to a later day, such as 28, on mammary “preparedness” and growth during the subsequent lactation cycle is unknown. A continuous farrowing system could lend itself to weaning first parity sows a week later in order to stimulate more stomach volume for the gilt, and maintain mammary cell activity and growth.
Another important factor that impact gilt lactation more so than sow lactation is ergot contaminated grains in areas that uses barley, rye, wheat and oats in lactating gilt and sow diets. Ergot is a general term that applies to all species of the fungus Claviceps. There are different ergot alkaloid compounds depending on the grain, but they all share similar mechanism whereby they inhibit prolactin secretion, and consequently, depress milk yield. Concentrations near 3 mg/kg dihydroergosine, an alkaloid produced by sorghum ergot (which corresponds to about 0.3% ergot) decreases prolactin concentration in gilts by approximately 75% relative to that of multiparous sows. Multiparous sows tolerate much higher levels of ergot in the feed. This may be of concern for the cereal grains produced and fed on the farm and not inspected.