Reproduction in Domestic Animals

Nutritional Interactions and Reproduction 131mammals is made up of complex mixtures of proteins,fats and simple and complex carbohydrates as well astrace elements, vitamins and other micronutrients, andall of these components have the potential to influencereproduction, either directly or indirectly. The currentconsensus suggests that dietary energy is a very importantcomponent of the diet with respect to nutritionalinfluences on ovarian function. The short-term administrationof glucose or other energy-yielding substratescan increase ovulation rate in ewes (Nottle et al. 1988;Teleni et al. 1989; Downing et al. 1995a,b; Vin˜ oles et al.2005). However, there are other published reports inwhich the increased supply of glucose did not increaseovulation rate in ewes (Iglesias et al. 1996) and furthermore,in the male, the published data suggest that theincreased spermatogenesis seen in response to feeding alupin grain supplement is not caused by an increasedavailability of glucose (Blache et al. 2002).From the above remarks it follows that, whenexamining the effects of nutrition on ovarian functionand folliculogenesis, there is an obvious need for a moredetailed description of both animals’ diets and metabolicstates than is customary.Sources of metabolic glucose that affect ovarian functionGlucose for the generation of ATP is derived from eitherdietary sources or from gluconeogenesis, either with orwithout accompanying glycogen synthesis. Monogastricspecies derive their glucose principally from dietarysources rather than from gluconeogenesis, whereas theruminant species reduce their dietary glucose in theanaerobic conditions of the rumen and derive theirglucose almost exclusively by gluconeogenesis, fromdiet-derived long-chain fatty acids and short-chainvolatile fatty acids. Compared with monogastrics, this isprobably a highly significant functional difference withrespect to the effects of nutrition on ovarian functionbecause ruminants, animals with highly efficient gluconeogenesisrarely become hypoglycaemic or, for thatmatter, hyperglycaemic. For example, dairy cows with ahigh genetic merit for milk production do not becomehypoglycaemic at the peak of lactation, although they canbe in severe negative energy balance. Indeed, hypoglycaemiais usually only seen in ruminants in the most extremesituations such as pregnancy toxaemia (twin lambdisease) in sheep and in lactating first-calf beef heifersthat are still growing. Hypoglycaemic beef heifers have anextended period of post-partum anovulation associatedwith reduced LH pulsatility, because the hypoglycaemiainhibits the GnRH pulse-generating system. The situationwith monogastric species appears to be quite differentbecause they depend on the diet for supplies of glucoseand have a limited capacity for gluconeogenesis comparedwith ruminants. Monogastric species readilybecome hypoglycaemic when deprived of dietary energyand the consequence is a reduced GnRH ⁄ LH pulsefrequency and thus anovulation.Seasonality and Ovarian FunctionSeasonal patterns of reproduction are relatively commonamong the mammals including the domesticlivestock – sheep, goats, horses and, under somecircumstances, cattle and pigs show seasonal rhythmsof ovarian function that are driven primarily by photoperiod.These patterns and the processes that underpinthem, especially in sheep, have been extensively investigatedand there are a number of detailed reviews on thesubject (e.g.: Yeates 1949; Hafez 1952; Karsch et al.1984; Malpaux 2006).Interactions between nutrition and seasonalityPhotoperiod and nutrition both exert major influenceson reproduction so it seems axiomatic that seasonalrhythms in ovulation will be influenced by nutrition. It ishighly probable that photoperiodic and nutritionalinputs to the hypothalamus interact at the level of theGnRH neuron (Ho¨ tzel et al. 2003; Blache et al. 2007),yet there are relatively few published investigations ofthe interaction between nutrition and photoperiodism infemales. There are few controlled studies with ewesinvestigating the relationship between nutrition and thepattern of seasonality, although there are reports ofalterations in the seasonal pattern of cyclic ovulatoryactivity associated with nutritional conditions in previousseasons (Hunter 1962; Smith 1965; Oldham et al.1990). In one study carried out over 13 months (Huletet al. 1986), ewes at pasture had a lower proportionovulating in the months of anoestrus (May, June andJuly) compared with ewes in a drylot that weresupplemented with alfalfa hay (Fig. 1). The pasturefedewes had a higher proportion of anovulatory ewes inFebruary suggesting that they were entering anoestroussooner. Similarly, in August, the pasture-fed ewes had alower proportion of anovulatory ewes, again suggestingthat they were slower to resume ovarian cyclicity at thestart of the new breeding season. In another study, theseasonal pattern of ovarian cyclicity was determined inRasa Aragonesa ewes maintained at two levels of bodycondition (Forcada et al. 1992; Forcada and Abecia2006). This study showed that the duration of anoestruswas reduced by approximately 2 months in the ewesEwes ovulating (%)1009080706050403020100FebPastureDry lotMayJun JulTime of the yearsAugSepFig. 1. The proportion of ewes ovulating in February (enteringanoestrus), May to July (during anoestrus) and August and September(the start of the next breeding season) in two groups of fine-wool ewes.One group was managed on rangeland and the other group wasmanaged with alfalfa hay in drylot. The within month treatment effectswere significant for all months except September. Data taken fromHulet et al. (1986)Ó 2008 The Authors. Journal compilation Ó 2008 Blackwell Verlag