Investigations on breeding boars to contribute to a functional feeding strategy

Artificial insemination (A.I.) is a rapid growing industrial activity. In 1987 about 45 % of the Dutch breeding sows were fertilized by means of Artificial Insemination. One of the factors influencing the efficiency of an A.I. Centre is reproductive output of the breeding boars. A good reproductive output from a boar means a good libido, a high number of produced sperm cells and a good fertilizing capacity of these sperm cells. Within boars these characteristics vary due to all kinds of environmental conditions.

One of the factors which will influence the reproductive characters is nutrition. There are few data in literature regarding the nutritive needs of boars to optimize reproductive characters. In this thesis some aspects of the energy and protein requirements of breeding boars are investigated.

In chapter I a literature review is given concerning the influence of energy and protein intake on the reproductive characters, libido, number of produced sperm cells and fertilizing capacity of the sperm cells of breeding boars. Scarce literature data on breeding boars shows a contradiction in the required level of protein in the diet to optimize reproduction. Some authors state that protein levels far above the usual level in commercial sow diets will have a positive effect on the number of produced sperm cells when high mating frequencies are applied. Other authors state that a normal commercial sow diet contains enough protein; in their experiments, however, boars were kept on a low semen collection frequency.

Furthermore it is shown from literature on rams and bulls that reproductive characters can be influenced by the level of energy intake. Feeding male animals at or below a defined maintenance requirement would result in a decreased number of produced sperm cells. In experiments with boars these effects on semen production could not be confirmed. This results in the fact that many authors recomment a low feeding level for breeding boars.

The contradiction in literature regarding the effect of high protein levels in the diet on reproductive parameters in boats was the reason to conduct an experiment on the effect of a high protein intake on sperm production in boars on a high and low semen collection frequency. This experiment is described in chapter II. Results show that at low as well as at high semen collection frequencies no effect of protein levels above that of a commercial sow diet (12.56 MJ metabolizable energy, 14.5 % crude protein, 0.68% lysine and 0.44 % methionine plus cystine) can be expected on semen production characteristics.

In Dutch A.I. Centres a 20% decrease of the number of produced sperm cells is seen at the end of the winter period. Energetic undernutrition induced by low ambient temperatures might explain reduced semen production in this season. For this reason, the variation in energy requirements due to the variation in environmental temperature is studied. Literature on non-lactating sows shows a large effect of low ambient temperatures on the energy metabolism. At low ambient temperatures a large portion of the daily energy intake is used for thermoregulatory processes.

In chapter III the effect of ambient temperature on the energy metabolism of breeding boars is investigated. Breeding boars in this experiment had a lower critical temperature (LCT) of 20°C, and extra thermoregulatory heat requirement (ETH) of 16 kJ/ kg ^0.75/°C per day. Boars above the LCT showed a protein and fat gain of 103 and 41 g/day, respectively, while the same boars at the same feeding level but housed at 10°C had a protein gain of 8 g/d while they lost 156 g/d of body fat. From this experiment it was concluded that low ambient temperatures have a large influence on the energy metabolism of boars. In a feeding strategy allowances for low ambient temperatures should be included.

In chapter IV it is shown that the thermal demand of a breeding boar shows a clear daily pattern. The LCT was lower at night (about 18°C) compared to the day (about 21°C). The ETH however was higher at night (22 kJ/kg ^0.75/24h/°C) than during the day (14 kJ/kg ^0.75/24h/°C) At low ambient temperature this compensates the advantage of a lower LCT.

Boars need feed for maintenance, growth and reproductive processes . From chapter III and IV it was clear that, due to environmental temperature, the amount of energy required for maintenance could vary considerably. There was no literature available on the energy requirements for reproduction and whether or not this would be influenced by the semen collection scheme applied.

In chapter V the influence of the semen collection scheme on the energy metabolism of A.I. boars was investigated. Measurements on the energy metabolism were done with boars adapted to a constant semen collection frequency of 1 or 3 times a week. Also measurements were done after changing the collection scheme of adapted boars from 1 to 3 times a week or 3 to I time a week. Effects of semen collection scheme on the energy metabolism of breeding boars are small. On semen collection days boars produce about 18 kJ/kg ^0.75/day extra heat compared to non-semen collection days. Total heat production per week was equal or even lower in boars at high semen collection frequencies. Boars on high semen collection schemes more than compensate their extra energy requirement due to the higher mating frequency when compared to boars on low semen collection schemes. Probably boars on a high scheme rest more between mating. Effects of different semen collection schemes and adaptational reactions are too small to incorporate them into in a feeding strategy for boars.

The literature review in chapter I showed that feeding rams or bulls at or below maintenance resulted in a lower semen production. In literature on boars these effects were not found. Experiments on boars were done with few boars per treatment group or feeding levels well above maintenance. Therefore it was not clear whether differences in feeding strategy could result in differences in semen quality and quantity.

In chapter VI the effect of feeding level on semen quality and quantity of breeding boars is investigated. Boars were fed ad libitum (H=5.74 kg/d), a medium (M=3.62 kg/d) or a low feeding level (L=1.92 kg/d) for twelve weeks. The adoption of these feeding levels resulted in a significantly different number of produced sperm cells. In the last two weeks of the experiment the M and H fed boars ejaculated 46 and 69% more sperm cells, respectively, compared to the L fed boars. After the 12 weeks H boars were fed on a L feeding level (HL boars) and L boars were fed on a M feeding level (LM boars). M fed boars were kept on the same feeding level (MM boars). After 8 weeks the differences between the HL and MM boars and LM and MM boars were not significant anymore. Quality parameters of the ejaculate were not influenced by the adopted feeding levels.

There seems to be a clear effect of feeding strategy on semen quantity of breeding boars. Therefore it is important to try and find an optimal feeding strategy for boars.

In the general discussion a feeding strategy for boars on basis of maintenance requirements, requirements for growth and requirements for reproduction is described on basis of sow data from literature and data from this thesis. This results in a basic feeding allowance for boars on A.I. Centres of 2.7-3.1 kg (depending on liveweight) of the above mentioned commercial sow diet. An extra allowance of 100 g of the diet per °C below 20°C per day should be given to account for extra thermoregulatory heat production in the cold. This extra allowance is not of ten given to breeding boars at Dutch A.I. Centres. This may explain the decrease in semen production at the end of the winter.