Key Points in the Nutrition of Breeding Bitches

INTRODUCTION

Reproduction is a biological process that allows new organisms to be created and thus ensures the preservation of a species. Although there is scientific evidence of the benefits of early spaying and neutering, breeding dogs remains a common practice.

Ideally, breeding domestic dogs should be limited to responsible owners with pets in optimal health and with comprehensive reproductive management: preventive medicine, estrus detection, natural mating or artificial insemination, pregnancy diagnosis and monitoring, whelping care, monitoring of lactation and weaning, preventive medicine for the litter and proper nutritional management, among others.

Given the breadth of this topic, this article focuses on the nutrition of the pregnant and lactating bitch and its impact on the health of the litter.

NUTRITIONAL MANAGEMENT OF BREEDING BITCHES

The goal of a proper nutrition program for a breeding bitch (gestation and/or lactation) is to maintain her weight, minimize body weight loss from gestation-lactation through weaning (<5%)(1), reduce the risk of dystocia, increase litter survival, ensure adequate, good-quality milk production, appropriate birth weight, a strengthened immune system, a good vaccine response in the puppies and adequate litter growth and weight, to name a few.

To design a proper nutrition program for a pregnant and/or lactating bitch, we will briefly review these two physiological processes and their effect on the bitch's nutritional requirements.

CANINE GESTATION

Gestation or pregnancy in dogs is the biological period between fertilization and whelping (2). In most cases, the exact day of ovulation and fertilization is unknown, so canine gestation is considered to last 57-72 days, with an average of 63 days (1,3,6). For practical purposes, canine gestation is divided into 21-day periods called thirds, so embryonic development in dogs is considered to have 3 stages (4):

• Ovulatory (days 2-17). During this period, ovulation, mating, fertilization, cell division and implantation of the morula in the uterine tissue take place.
• Embryonic (days 17-35). The stage during which organogenesis takes place—the process in which the embryonic layers are modified to build the embryo's organs (5)—along with morphogenesis, the biological process by which the embryo acquires the shape corresponding to its species (5). Up to day 35, the puppies' body weight represents only 2% of their birth weight (1).
• Fetal (from day 35 until birth). During this stage, embryonic development is completed and the fetuses prepare for birth. Three-quarters of fetal growth takes place during this last third (from day 40), when the puppies gain up to 70% of their birth weight (6,7).

Given the variety of sizes among dog breeds, bitches may have estrous cycles every six months or once a year. Litter size (1-14 puppies) is usually related to the bitch's size: small bitches tend to have smaller litters, but with puppies of greater relative body weight (7). Considering these variations, it is important to individualize and closely follow up each case.

CANINE LACTATION

The Royal Spanish Academy defines lactation as the first period of a mammal's life, during which it feeds exclusively on milk. Before gestation, mammary tissue is composed of fatty and connective tissue and, to a lesser extent, glandular tissue. During gestation, as a result of several hormones (estrogens, progesterone and prolactin), the mammary gland increases in size and its glandular cells become secretory, milk-producing cells.

Milk production capacity depends on several factors, including the bitch's weight before mating, nutritional management during gestation, hereditary factors, mammary gland anatomy (inverted nipples, mastitis, functional glands) and litter size (6,7). The latter is defined by the number of puppies suckling and activating the physiological processes of milk production. During the first week of lactation, milk production represents 2.7% of the bitch's body weight, peaking around weeks 3 and 4 (6) at up to 8% of her body weight (1).

The length of lactation varies. In general terms, weaning (when the puppies begin eating solid food) starts from week 5, which coincides with the decline in the bitch's milk production capacity. It is advisable to reduce the bitch's nutritional intake gradually so that milk production decreases progressively, reducing the risk of mastitis (6).

BODY WEIGHT OF THE BREEDING BITCH

Body condition. The Body Condition Score (BCS) is a tool developed over time by international veterinary authorities that assesses body morphology and palpation of bony structures to estimate the animal's muscle and fat tissue. There are 1-5 and 1-9 scales (Table 1), in which the ideal body condition is considered to be 3 and 5, respectively.

A body condition above ideal (overweight) in a breeding bitch predisposes her to reduced ovulatory capacity, silent heat, smaller litters, prolonged anestrus, limited milk production and dystocia (6), among other reproductive disorders. For this reason, to optimize fertility, an overweight bitch should undergo an appropriate weight loss program before being considered for breeding.

Body weight during gestation and lactation. Bitches are estimated to gain only 15-25% of their body weight from mating to whelping, depending on litter size. This weight gain is mainly due to the weight of the puppies themselves, the placenta and placental fluids (7). After whelping, they usually weigh 5-10% more than at the time of mating, due to the growth of mammary tissue, extracellular fluid and genital tissue (1). Remember that the goal is to minimize weight loss at the end of the reproductive cycle.

KEY NUTRITIONAL REQUIREMENTS DURING GESTATION AND LACTATION

a) Water. Water is a commonly underestimated nutrient; however, its importance in many chemical reactions and in maintaining the body's overall homeostasis, as well as its effect on milk production, should not be overlooked. During gestation, water intake will be that of an adult at maintenance; during lactation, however, water intake is high, since water is used not only to replace fluids lost through urine, feces and respiration (6), but is also vital for milk production. Water intake is mainly affected by the environment, the moisture content of the food, physical activity and the animal's physiological state; therefore, all animals, and pregnant and lactating bitches in particular, should have free access to clean, fresh water (6).

b) Energy. Energy is not a nutrient in itself but a property of food, derived from three nutrients: proteins, fats and carbohydrates. Energy is expressed in kilocalories (Kcal) or kilojoules (Kj). Animals need energy to maintain their metabolism. Insufficient energy intake predisposes to poor performance (7).

Energy requirements during gestation. As mentioned above, canine gestation is divided into three stages with gradual embryonic development, which is directly reflected in the breeding bitch's energy requirements. During the ovulatory phase and the first two-thirds of gestation, the bitch's nutritional requirements are those of an adult at maintenance (1), so no changes to her diet are needed as long as she eats a complete, balanced and highly digestible food. In the last third of gestation, fetal tissue increases exponentially (1), leading to a considerable rise in the bitch's nutritional and energy requirements (7) (an increase of around 30-70% depending on litter size (1,6)).

Energy requirements during lactation. Nutritional and energy requirements during lactation are undoubtedly the most demanding of a bitch's adult life (6), nearly matching the requirements for growth or for high-performance dogs. The bitch's energy requirements during lactation are directly related to the volume and energy content of the milk she produces (7).

There are several methods for determining the energy requirements of lactating bitches; it is the responsibility of the attending veterinarian to adopt the one best suited to each bitch's particular conditions.

c) Protein. Proteins are molecules made up of long chains of amino acids. There are 20 amino acids that, in combination, give rise to countless proteins, each with its own properties. Proteins perform several functions, such as metabolic regulation (enzymatic, endocrine) and forming cell structures and muscle fibers, which makes them necessary for tissue repair and growth. Protein is a source of essential amino acids that animals cannot synthesize on their own but need to synthesize other proteins in the body.

It is important to note that not only the amount of protein matters, but also its quality. A highly digestible protein will be reflected in the availability of the amino acids needed to meet the animal's nutritional requirements.

Protein requirements during gestation. The protein requirements of bitches from mating to the end of the second third of gestation are similar to those of an adult dog at maintenance (15-30% of dry matter). At the end of gestation, protein requirements rise by 40-70% compared with those of an adult dog at maintenance. Protein deficiency during gestation can lower birth weight, increase mortality within the first 48 hours of life and reduce the puppies' immune response (1).

Protein requirements during lactation. Because of the puppies' growth rate during lactation, canine milk must be highly nutrient-dense, especially in protein and fat. The bitch needs to eat a food with an adequate nutritional profile to ensure milk quality (6), and the volume of milk produced will define her nutritional requirements.

d) Fat and essential fatty acids. Fat is not only a stored energy source for the animal's body; it also improves the absorption of fat-soluble vitamins, provides essential fatty acids that the body cannot synthesize and adds palatability (pleasant taste) to food (7). Offering foods rich in Omega 3 fatty acids during gestation and lactation improves the puppies' retinal function.

Fat requirements during gestation. As mentioned above, the bitch's requirements do not change until the end of the second third of gestation; however, during the last third, a diet providing 8.5%-20% fat on a dry matter basis is recommended, depending on the breed and litter size (1).

Fat requirements during lactation. Choosing a diet that provides up to 20% fat to meet the lactating bitch's energy requirements, with the resulting increase of up to 30% in milk fat content, benefits the puppies, since at birth they have limited energy reserves and this nutrient must be supplied through their mother's milk (1). The concentration of DHA (docosahexaenoic acid) and EPA (eicosapentaenoic acid) in milk depends directly on the intake of these nutrients in the mother's diet, so it is advisable to choose foods formulated with ingredients rich in these fatty acids, such as fish oil.

e) Carbohydrates. Carbohydrates are the source of glucose, the primary energy molecule in the bodies of omnivores (7). Carbohydrate deficiency during gestation predisposes to weight loss in the mother, reduced food intake, lower birth weight, reduced neonatal survival and a higher risk of stillbirths (1).

Carbohydrate requirements during gestation. Glucose is known to be the main source of the energy needed for fetal development. Pregnant bitches have high carbohydrate requirements at the end of gestation. Feeding a carbohydrate-restricted diet to a pregnant bitch predisposes her to hypoglycemia and ketosis (increased levels of ketone bodies in blood and urine).

Carbohydrate requirements during lactation. If for any reason a low-carbohydrate diet is fed to a lactating bitch, the caloric density of her milk may decrease (from 3-3.5% to 2%) (1), directly affecting the energy supplied to the puppies.

f) Micronutrients. Vitamins and minerals are considered micronutrients because they are required in very small amounts; however, a deficiency has a considerable effect on animal health.

Role of minerals during gestation and lactation.

• Calcium (Ca) and Phosphorus (P). During the last third of gestation, requirements for these minerals increase by up to 60% due to the rapid growth of the fetal skeleton. Calcium deficiency during lactation can cause eclampsia.
• Copper (Cu). Copper plays an important role in connective tissue formation and in hematopoiesis (blood cell formation (5)).
• Iron (Fe). At the end of gestation, the bitch has high iron requirements because the puppies store large amounts of this mineral in the liver, in addition to her own requirement to compensate for the high iron concentrations in colostrum; however, this requirement returns to normal once the transition to mature milk begins. Iron deficiency may increase the predisposition to poor litter growth and to low hemoglobin concentrations and hematocrit levels in puppies fed iron-poor milk.
• Zinc (Zn). This mineral is vitally important during periods of significant tissue synthesis, such as gestation and growth.

In daily practice, veterinarians often recommend switching to a diet designed for puppies to meet these energy needs, since these products usually have greater nutrient and energy density and high digestibility. This benefits the bitch by compensating for her reduced intake capacity due to the abdominal space taken up by the gravid uterus (1,6). Choosing a complete and balanced diet avoids the need for vitamin and mineral supplements, since the food per se contains all nutrients in the right amounts to ensure adequate nutrition (Table 2).

NUTRITIONAL MANAGEMENT

With all the information described in this article, we can now define the nutritional management of the breeding bitch.

• Determine the reproductive stage. The first step is to establish where the bitch is in her reproductive cycle. Her nutritional requirements will increase according to her physiological state (gestation or lactation).
• Analysis of the current diet. The veterinarian should analyze the current diet and determine whether the nutrient concentration of the formula meets the specific requirements of the animal in question.
• Choice of diet. Review the diets available on the market and choose the one that provides the necessary nutrients for the bitch's reproductive stage. An important point to consider is cost: we should choose a food that not only meets the bitch's nutritional needs but also fits the owner's budget, since we need to be sure the bitch will keep eating that food until the end of lactation. Mix feeding (combining dry and wet food) can be beneficial, given that canned food has a considerable moisture content and serves as an indirect source of hydration, providing water essential for milk production.
• Calculation of energy requirements and recommended daily amount. By regulation, all pet foods must state the recommended daily amount according to the animal's weight; manufacturers usually provide this information through Feeding Guides. In the particular case of puppy foods, most products include two Feeding Guides: one for growing puppies and another for pregnant and lactating bitches. If the veterinarian prefers to perform their own nutritional calculations, there are several formulas for doing so; two examples were mentioned in previous paragraphs. Once the calculation is done, the recommended daily amount can be determined (Table 2). It is important to remember the limited digestive capacity of pregnant bitches. Monitoring the bitch's weight and body condition (Table 1) is essential, as it will allow us to determine whether the amount of food offered needs to be increased or reduced.

• Change of diet. Remember that any change of diet should be made gradually, preferably over a period of 7-10, to reduce the likelihood of mechanical diarrhea caused by changes in the gut microbiota.
• Feeding regimen. This refers to the recommended number of meals per day, which the veterinarian defines according to the owner's availability. No ideal method has been developed to date, but it is important to consider the limited stomach capacity of the pregnant bitch, the importance of not overfeeding to avoid unwanted weight gain, and of not promoting weight loss due to the nutritional overdemand of reproduction. Offering small amounts of food several times a day or ad libitum is usually beneficial for the pregnant bitch (6).
• High-density feeding period. Given that the main goal of a nutrition program for breeding bitches is to provide all nutrients and maintain the bitch's ideal weight and body condition throughout gestation and until the end of lactation, the veterinarian will determine the period during which the bitch should eat puppy food. The usual practice is to start this transition in the last third of gestation and continue until the litter is weaned. Medical check-ups throughout gestation and monitoring of litter growth during lactation are very useful for the attending veterinarian.

CONCLUSION

Many aspects must be considered when a bitch that is about to be bred, or already in the process, arrives for a consultation. The veterinarian faces an important challenge, since a bitch's reproductive management is a comprehensive program that must undoubtedly include nutrition. This article is a brief summary of the nutritional aspects the veterinarian should consider to define proper nutritional management for the pregnant bitch, improving the quality of life not only of the bitch but of the entire litter.

Nutritional management of a breeding bitch should:

• Use a complete and balanced diet, that is, one that provides the energy and nutrients needed for the bitch's maintenance and to meet the nutritional demands of the puppies in the womb and during lactation (6).
• Avoid excessive weight loss in the bitch.
• Avoid, as far as possible, an increase in the bitch's body weight.
• Provide a diet with higher caloric and nutrient content to offset the limited digestive capacity caused by the gravid uterus against the high energy and nutritional demands of reproduction.
• Be highly palatable to ensure the bitch eats the food.

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5. Diccionario Médico. (2020) Clínica Universidad de Navarra. https://www.cun.es/diccionario-medico
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7. National Research Council of the National Academies. (2006). Nutrient Requirements of Dogs and Cats . Washington, D.C.: The National Academies Press. pp 36,37, 53, 101-102, 119-120