Effects of dietary crude protein on fertility: Meta-analysis and meta-regression

Abstract

Studies used to evaluate effects of dietary intervention on fertility may be subject to confounding because modification of one nutritional input requires a change in at least one other input. Meta-analytical modeling allows examination of a main intervention for a series of studies, but also an examination of a series of related effects through use of meta-regression. Effects of dietary crude protein (CP) on fertility were examined using this approach. We obtained 21 studies containing 32 comparisons that had pregnancy or conception data and met the eligibility criteria for meta-analysis of randomized controlled experiments providing information on diets used. Publications that contained data on prospective, randomized controlled experiments examining effects of dietary CP, either concentrations or degradability, or effects of a specific feed ingredient intervention on fertility were identified. Details on dietary formulation and diet intake were extracted from the publications, as were measures of urea in blood or plasma. Estimated fixed and random effects relative risks showed that risk of conception was lower in cows fed higher CP or more degradable CP diets (fixed effect (Mantel–Haenszel Relative Risk) = 0.91 (95% CI 0.84–0.98); P=0.019). This effect was homogenous (I² = 0) and not influenced by difference in blood urea N, duration of intervention, breed, parity, milk production or type of diet delivery. Significant associations among CP components of the diet and carbohydrate fractions supported the hypothesized potential for confounding, but only the amount of soluble CP eaten was a significant meta-regression covariate that reduced risk of conception. There was no evidence that the significant reduction in fiber or non-fiber carbohydrate (NFC) fractions of the diets associated with increased concentration of CP, soluble CP or rumen degradable fractions or soyabean products content of the diet influenced conception rates. Results support findings of experiments showing that increased intake of soluble CP reduced conception rates, and provides strong evidence that increased concentrations of CP or increased degradability of CP, within the ranges evaluated in the studies contributing to this meta-analysis, reduce the risk of conception in lactating dairy cattle.

Introduction

There have been quantitative (Ferguson and Chalupa, 1989, Westwood et al., 1998) and qualitative (Butler, 1998) literature reviews of the effects of dietary crude protein (CP) on reproductive performance indicating that increased dietary CP content or increased degradability of CP of the diet reduces fertility of dairy cattle. Despite the strength of the evidence showing negative associations between increased CP content or degradability and animal fertility provided in these reviews, there remains conjecture about this interaction (Evans and Patterson, 2007, Law et al., 2009). Observations which have lead to this conjecture include the better fertility of cattle fed on pastures that contain very high levels of CP (Westwood et al., 1998) and the possibility for confounding of observations because soyabean-based products used to increase the CP degradability of the diets may contain phyto-oestrogens that depress reproductive performance (Evans and Patterson, 2007).

Many of the studies examining the relationship between protein and fertility were conducted on limited numbers of dairy cattle (Jordan and Swanson, 1979, Canfield et al., 1990, Son et al., 1996) and many studies did not find a significant negative effect of increasing CP content or degradability on cattle fertility (Carroll et al., 1988, Canfield et al., 1990). Some studies had an outcome that was not consistent with an hypothesis that increased CP content or degradability lowered fertility of dairy cattle as assessed by conception or pregnancy rates (Edwards et al., 1980, Barton et al., 1996, Son et al., 1996).

Treatment designs used to evaluate effects of a dietary intervention on fertility are subject to confounding, because the modification of a nutritional input requires a change in at least one other input. A confounding variable, or confounder, is a variable which lies outside the causal relationship under study, such as CP content or degradability and cattle fertility, and is independently related to the study effect, and outcome. For example, an increase in the CP content of the diet may only be achieved by a reduction in carbohydrate, fat or mineral content. There are data suggesting that the fiber (Francos et al., 1977, Mayer et al., 1978, Bogin et al., 1982), starch, fat (Staples et al., 1998, Thatcher et al., 2006) and mineral contents (Rabiee et al., 2010) of diets influence fertility. Consequently, there is a need to examine whether there were differences in diets resulting from changes in treatment diets to alter CP content or degradability that could have confounded estimates of effects of dietary CP on fertility as identified in the individual studies.

Meta-analytical studies are being used in agriculture and veterinary science to resolve conflicts in the literature and to use existing data to address hypotheses which could not be addressed in previous studies (Lean et al., 2009). In the current meta-analysis, we examine the effect of increased CP content or degradability in the diet on cattle fertility; and potential for confounding to arise in evaluating the interaction between dietary CP and reproduction in dairy cattle, either through inclusion of soyabean products or through changes in carbohydrate, fat or mineral content, when experimental diets were altered to increase their CP content or degradability. The hypothesis that changes in carbohydrate, fat or mineral content caused by changes when experimental diets were altered to increase CP content, influenced fertility responses has not been previously examined. Similarly, there is no previous quantitative examination of the role of soyabean products and the potential of these to confound fertility responses in dairy cattle exposed to diets of higher CP content or ruminal degradability.