Changes in body condition score from calving to first insemination and milk yield, pregnancy per AI, and pregnancy loss in lactating dairy cows: A meta-analysis

Changes in body condition and body weight naturally occur during lactation and can influence future lactation and fertility outcomes depending on management during late lactation and during the dry period of the dairy cow. Body weight alone is not a good indicator of body reserves because body weight is affected by factors such as parity, stage of lactation, frame size, gestation, and breed [1]. Although scoring body condition measures the amount of adiposity, it is subjective in nature so determining both the inter-assessor reliability and intra-assessor consistency of BCS evaluation is important. Variation among three experienced and one less experienced assessor using a 5-point scale [2] was evaluated in Holsteins and the assessors either agreed or deviated by no more than 0.25 BCS units in more than 90% of the evaluations [3].

Assessing BCS is an important tool because of its relationship to milk yield, reproductive traits, health, and disease. Relationships between BCS (and its postpartum change) and metabolic disease and health have been addressed in individual studies [[4], [5], [6]] and in extensive reviews [1,7,8]. In general, over conditioned cows at calving and BCS loss thereafter increased risk for infectious, clinical, and subclinical metabolic disease, uterine disease, mastitis, and retained placenta. Primiparous Canadian Holstein and Ayrshire cows with a genetically high BCS conceived earlier in lactation with a greater chance to become pregnant [9]. In general, genetic selection for production traits resulted in greater postpartum loss of body condition and a failure to repartition significant amounts of energy toward body reserves until later in lactation or when lactation ceased [10]. Body condition scoring has its limitations because the relationship of BCS and body fat becomes weak at the bottom of the BCS scale in which sc fat content is limited and the decrease in BCS at these levels indicate muscle protein loss and not loss of internal fat depots [8,11].

Prepartum BCS and its changes during early lactation are associated with inactive ovaries, postpartum anestrus, greater likelihood of failure to conceive by 150 d in milk, greater incidences of retained placenta and metritis, and more culling [5,6,12,13]. Loss of BCS during the dry period was a predisposing factor associated with health disorders and reduced productive and reproductive performance in Holstein cows [14]. Postpartum loss in BCS was associated with increased milk and less BCS loss during the first month of lactation, whereas changes in BCS during the dry period, BCS at dry-off, duration of the dry period, and change in BCS during the first month of lactation were more strongly associated with milk yield than health problems [15]. In general, cows with more body fat reserves mobilized more fat for milk fat synthesis and under conditioned cows at calving produced lower-fat milk without affecting milk yields [6].

Previous meta-analyses [16,17] addressed the relationships between reproduction or milk yield and BCS in dairy cows. Body condition in those studies were scored on different scales and required transformations to a common scale of 0–5, different from that standard 1–5 scale used in North America. Traditional meta-analyses offer quantitative advantages to qualitative review articles because they attempt to analyze results (not the original experimental unit outcomes) from all available studies in the literature. Traditional meta-analyses present, however, their own challenges. Results are estimated using means and some measure of variation that are unbalanced and factor effects are far from being independent. “This leads to unique statistical estimation problems similar to those observed in observational studies, such as leverage points, near collinearity, and even complete factor disconnectedness, thus prohibiting the testing of the effects that are completely confounded with others” [18]. Moreover, from a statistical standpoint, individual studies are blocks and their effects must be considered random because the inference being sought is to future unknown studies [19].

Information in the literature regarding the effects of BCS on pregnancy per AI and pregnancy loss are limited to few analyses in dairy cows exposed to presynchronization of estrous cycles and inseminations after estrus or ovulation synchronization. The objective of this report was to examine the effect of BCS at calving, at first AI, and prebreeding change in BCS using the same defined scale (1–5) at 0.25-unit cut points on pregnancy outcome, pregnancy loss, and milk yield in cows inseminated at first service after detected estrus or ovulation synchronization.

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