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A fresh look at gilt management and nutrition

By Simon Tibble, pig nutrition specialist – SCA Iberica and Gerhard Pretorius, technical executive: Monogastric – Meadow Feeds (Tables available on request)
Due to genetic advancement, sow productivity is continually increasing and can be benchmarked against any one of the following criteria: the production of 30 plus piglets per sow per year,2,5 tonnes of meat per sow per year or 60 piglets weaned during a sow’s productive life. The aim of any pig unit should be to achieve optimum production at the lowest input cost possible.
As with any high achieving enterprise, the foundations need to be strong and in the case of long-term sow productivity, it means a regular supply of uniform gilts in optimal body and physiological condition at point of first service to ensure optimum productivity and longevity.
Sow productivity
As little as 25 years ago, it was relatively easy to feed replacement gilts in a commercial piggery. Gilts would have been fed from 30kg body weight to first service on a standard finisher diet and usually a single diet was offered to dry and lactating sows. The dry sow diet has hardly changed over the last twenty years, although impending EU legislation on loose housed gestating sows will certainly change this scenario. The demands placed upon a lactating sow, often expected to rear litters of 13 plus piglets, have required a dramatic re-evaluation of the nutritional requirements and management practices of the modern hyper-prolific sow. The question is, how do we prepare the gilt to meet these expectations?
Sow productivity has evolved continually year on year, as indicated in Table 1, with a clear 10% increase in the number of piglets produced per sow per year, but perhaps more importantly for the relevance of this article, the sow replacement rate has increased by a staggering 40% in the same time period. A recent report from Ireland also stated that sow replacement rate increases by between 1,5-2% per annum.
These figures, however, also need to be viewed in context; the top 10% of producers in a BPEX study (UK) are achieving 28 plus pigs per sow per year with a replacement rate of nearly 60%, while many individual operations across the globe already reports production levels of 32 plus piglets per sow per year.
Foxcroft (2007) reported that an increase in replacement rates may potentially negatively impact on herd performance by increasing the number of replacement gilts required. Consequently the number of gilts in the rearing phase will also have to increase, but with the same accommodation and quarantine facilities it will result in over-crowding and provoke lower growth rates. Under these circumstances, gilts will take longer to reach puberty and destabilising the sow herd parity on the farm. Since gilts have lower immunity levels, a destabilised sow herd may increase the risk of potential disease outbreaks.
Moore, in a National Hog Farmer article in 2004, also reported a higher incidence of Mycoplasma, PRRS and a decrease in growth rates resulting from a destabilised sow herd parity profile. This would also lead to an increase in the number of rejected productive “gilts”, the replacement of sows in their first and second cycles, up to 40-45%. The two major causes of these gilt replacements are reproductive performance (35-45%) and locomotive problems (15-25%).
This highlights the fundamental importance of a correct and clearly defined gilt nutrition program to minimise the causes of sow rejection.
Determining recommendations for optimal gilt condition
The interaction between weight, age and body condition (lean to adipose [fat] tissue ratio) are the key factors that determine the optimal time for first service.
The selection for a leaner more efficient slaughter pig with a higher mature body weight, has had consequences on the reproductive ability of the modern sow. Sows of larger mature body weights (Jagger 2008) generally have higher lean growth and less back fat at any given age if compared to sows of a lower mature body size. The sows will therefore continue to deposit significant amounts of lean tissue up to their third parity, which should be taken into consideration when designing the nutritional programme for the gilt and subsequent lactation cycles.
How do we define optimal gilt 
condition?
What is the key criteria for optimal gilt condition at first service? For many years it was believed that fat reserves at first service, was a key factor in optimising gilt fertility and longevity in the breeding herd.
Table 2 clearly shows that those gilts that had a P2 of 20mm at first service produced the highest number of piglets over five parities.
The industry adopted a series of key criteria for replacement gilts (Table 3) based upon this and other supportive research during the last 20 years.
However, it would be interesting to evaluate how many of today’s gilts, representative of the modern hyper-prolific sow, actually meet these criteria.
A review of the extensive research during the last ten years using genetics, representative of today’s breeding female, has indicated that perhaps some of the criteria presented in Table 3 may no longer be valid for the current genetics. The next session of this article will highlight the latest research that suggests a new, more aligned set of criteria for the modern gilt.
1.    Back fat thickness
From the reviewed literature it has become apparent that the relationship between back fat and life time productivity is perhaps not as important as previously perceived. Both Williams (2005) (Figure 1) and Gill (2007) (Figure 2) failed to show any relationship between back fat level and sow productivity.  They concluded that the original studies investigating back fat were perhaps confounded by gilt weight.
As part of a large commercial study, Williams et al. (2005) monitored the performance of 1 674 pre-pubertal gilts to evaluate the effect of body composition at first service on lifetime productivity.
Williams et al (2005) clearly illustrated in Figure 1 that back fat at first service had a minimal impact upon sow productivity throughout three parities. As a result of these studies, Prairie Swine Centre conducted a collaborative study, which indicates a clear relationship between body weight and eye loin mass (an indirect measure of protein mass), at 100 days of age. No relationship between back fat depth and body weight at first service or between changes in back fat and body weight over three parities was illustrated. Williams et al (2005) and Foxcroft (2007) concluded that although minimal levels of back fat are desirable from a welfare perspective, measuring back fat is not a meaningful measure of sow body condition in terms of longevity and lifetime performance.
Gill (2007) summarised the question regarding back fat thickness (as indicated in Figure 2) by concluding that the focus should be on “fitness” and not “fatness”. Although the role of back fat levels should not be ignored, it is not as relevant with modern genetics as previously perceived. In fact the level of body protein seems to be more important for reproductive performance.
2.    Body weight at first service
If back fat does not have such a significant role as previously thought, then how important is gilt body weight and or protein mass at first service?
Body weight can be greatly influenced by genetics. A fast grown gilt of approximately 160kg will have a higher feed cost as a result of an increase in maintenance nutrient requirements, while on the other hand a restricted fed smaller gilt of about 135kg with lower maintenance nutrient requirements will have poorer life time productivity.  So how do you determine the optimal gilt body weight?
It is generally accepted that a gilt at parturition should have an ideal body weight of 175 to 190kg. These gilts should have sufficient body reserves to avoid excessive weight loss during first lactation. They must be able to wean a litter with an average litter body weight gain of approximately 35 to 40kg.  However, to get to the ideal weight at parturition, the optimal body weight for a gilt at 1st service should be 135 to 150kg.
This is supported by the research of Williams et al (2005), who clearly illustrated in Figure 3 that gilts weighing less than 135kg at first service, produced fewer piglets over three parities which implies that a minimum lean body mass is critical at first service for optimum lifetime performance.
3.    Oestrous number
The third criteria for optimal gilt condition is oestrous number (Figure 4). Reviewing the latest information, it appears that there is very little difference between serving a gilt at second oestrous, assuming it has a minimum body weight of 135 kg, or third oestrous in reproductive performance. However, from an economic perspective, the additional twenty-one non production days cannot be justified.
Servicing gilts at first oestrous is generally avoided, as it is usually difficult to detect and synchronise oestrous. From Alberta Swine Research Centre information, Foxcroft (2005), reported that when comparing litter mates, gilts of the same age and weight serviced on their second oestrous, showed a significant increase in number of pigs  born alive ( 11,1 vs 10,0 , P<0,05) over those serviced on their first oestrous.
4.    Body weight gain
The question is then raised, what is the correct growth rate for the modern gilt?  This topic has been studied by several researches. Beltranena (1991) recommended a daily lean weight gain of 550 gram per day, which is in agreement with the work of Hughes (2007) who clearly demonstrated that lean body weight gain less than 550 gram per day, increased the percentage of gilts showing delayed puberty while body weight gain between 600 to 800 gram per day had little effect on the age of first appearance of puberty.
5.    Timing of boar exposure
Patterson (2001) showed that in modern genetics one of the most important factors affecting gilt reproduction performance is the timing of the first exposure to the boar, assuming that the body weight gain is within the range of 600 to 800 gram per day, and the gilt, 136 to 140 days of age.
6.    Feed intake
If growth rate is a reflection of feed intake, what feeding programme should be used for the modern gilt, ad-libitum or restricted fed?
Restricted feeding may present potential problems by impacting on the hormonal axis, interacting with gonadatrophin releasing hormone influencing the release of both, lutenising hormone and progesterone, which can affect ova maturation and follicle size negatively (Van Wettere, 2007).
As illustrated in Figure 5, follicle size was significantly affected by growth rate, while an increased number of large follicles (3-6mm) were produced with an increase in growth rates.
This was also supported by Chen et al (2011) who recently offered gilts one of two feeding regimes, a low plane of nutrition vs a high plane of nutrition, during the period of early follicle development. He observed that there were a greater number of gilts on the high feed intake that had follicles larger than 3,5mm. Ovulation rate was independent of follicle size. However, in the low feed intake group, only those gilts that had follicles larger than 3,5mm achieved a similar ovulation rate to those gilts on the higher feed intake.

Recommended criteria for optimal gilt performance

In conclusion, the latest research recommends a growth rate of 600 to 800 gram per day from birth to the beginning of boar stimulation to achieve the appearance of puberty and first service between 135 and 155kg body weight.
If locomotive problems are a significant cause of gilt rejection on a particular farm, then it may be necessary to restrict feed to the animals to control body weight, but it must be taken into consideration that the onset of puberty will subsequently be delayed. The losses caused by the delayed onset of puberty can easily be out-weighed by the reduced number of rejections in the number of first lactation sows due to leg problems.
In this situation, knowing that modulating feed intake may have negative effects on reproductive performance, researchers are in agreement that increasing feed intake 14 days before insemination by flush feeding, 2.5 times maintenance nutrient requirements, will normalise the ovulation rate caused by restricted feeding.
From the reviewed literature, it is now possible to determine a revised set of criteria that are perhaps more reflective of current genetics (Table 4).
The criteria are listed in order of priority and clearly back fat has a less important role to play than previously thought, while gilt weight and age are the key drivers of a successful gilt development program.
Role of nutrition
With the change in genetics it has become important that the nutritional programmes during the rearing and early parities of the gilt, meet the nutrient requirements of the developing gilt to minimise the risk of sow rejections due to reproductive and/or locomotive failures later in life. Nutritional programmes have to be developed to fulfil the recommended criteria in the first part of the article, ensuring gilt productivity and longevity beyond four parities.
As indicated earlier, the modern gilt will continue to deposit lean tissue up to the third parity and therefore the nutritional programme must meet the requirements for protein and amino acids, energy, calcium and phosphorus, for bone development and vitamins and trace minerals to optimise fertility.
1.    Feeding programmes
Historically gilts have been raised on finisher diets or gestation diets, neither of which are formulated to meet the nutrient requirements of the developing gilt.
A finisher diet is designed for fast growing animals with high lean meat deposition. Excessive growth rates in gilts may, however, lead to future production problems such as osteochondrosis and leg weakness, increasing the gilt replacement rate within the herd.
In contrast a gestation diet is designed for a sow that has finished growing and will not meet the amino acid, nor the mineral requirements of the developing gilt.
The University of Teagasc, Ireland (unpublished data), has recently conducted a trial to evaluate the ideal nutritional programme for gilts. The trial comprised of 100 gilts selected at 55kg live body weight, to determine the effect of three different gilt nutritional programmes on gilt performance (Table 5).
The objective of the trial was to determine the benefit of a specific gilt developer programme, designed to meet the nutritional requirements of the rearing gilt, compared to standard finisher and gestation sow programmes.
It is important to take note that the specialised gilt diets were fortified by organic trace minerals such as zinc, copper and manganese.
The normal gilt performance parameters, as well as the additional parameters, such as locomotory ability, joint abnormalities and bone density at 12 weeks, were recorded. Gilts were slaughtered at 12 weeks of age.  Table 6 clearly indicates that a gilt developer programme reduced lameness significantly, as well as claw lesions, claw size and surface lesions on the cartilage of elbow joints, compared to the other treatment groups.
2.    Vitamins and minerals
As previously indicated, a major cause of gilt rejections in the first two parities, representing 25% of total sow herd rejections, is caused by locomotive problems as defined by lameness, osteochondrosis and claw health. This could be associated with poor mineral supplementation during the rearing period.
Lameness furthermore has an indirect consequence on reproductive performance by negatively affecting the production and release of reproductive hormones, apart from the obvious direct causes such as poor lactation, feed intake and physiological changes associated with infection and inflammation.
Although there is limited research on the effect of vitamin and trace mineral supplementation on gilt development, those associated with fertility and immunity are recommended to be supplemented in a gilt developer programme.
Additional vitamin E and biotin is required to improve hoof development and immune function, while vitamin B6 and folic acid are involved in embryo survival and reproductive performance. Supplemented zinc and manganese assists with the formation and maintenance of cartilage and bones.
3.    Organic trace minerals
The role of organic minerals in a gilt developer programme requires further investigation, since data from large pig integrations in both the USA and Spain, indicated a positive effect of the use of organic trace minerals (Zinc, Copper and Manganese) in gilt diets (Table 7). The added cost of such mineral sources should, however, be evaluated based on return on investment (ROI).
4.    Mycotoxins
Several mycotoxins can influence gilt performance. Zearalone has oestrogenic effects, T2 has potentially negative effects on feed intake, Ochratoxin is involved with the development of gastric ulcers and Aflatoxin can have an immunosuppressive effect. The above illustrated the importance of including an effective mycotoxin binder in all gilt diets.

5.    Early life nutrition

Although the objective of this article is not focused on early life nutrition of the gilt but rather on the effect on the development of the gilt from 30kg body weight for optimal lifetime reproductive performance, it is interesting to note that gilts, unlike other mammals, are not born with a predefined number of egg nests on the ovariums. The final number of potential ova in gilts are only determined at about three to four weeks after birth. This could allow for potential future nutritional intervention (Morbeck, 1993) to improve reproductive performance in terms of ova numbers and fertilisation. To the contrary, these advancements may well bring even more challenges for piglet viability up to 70 days of age.
North Carolina State University in a recent field survey (Flowers, 2008), also observed that gilts suffering diarrhea in the first weeks of life, showed delayed onset of puberty, had lower farrowing rates, as well as a lower number of piglets born alive, indicating that early life nutrition and management has a definite impact on lifetime performance.

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