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Pasture, Body Condition, and Reproduction

A cow's energy status during late lactation and the early part of the subsequent lactation is critical to her health and reproduction, as well as to the economics of the dairy farm. Pasture systems often result in lower BCS or greater loss of BCS during the first 1 to 2 months of lactation, and proper concentrate supplementation during lactation to restore body condition has immediate benefits of higher production as well as long term economic benefits.

In a previous article, we discussed the marginal profit response to supplemental concentrate feeding. That article focused on the milk and profit response to increasing amounts of concentrate feeding. In reality, the long-term responses to supplemental feeding in terms of body condition, herd health, and reproductive performance may be more important economically than the short-term response of milk yield. However, the returns to this “investment” of feeding supplemental concentrates are not immediate, but occur several months later.

Body Condition

Body condition scoring (BCS) of dairy cows began in the mid ‘80s and is now a widely accepted management practice on many dairy farms. The primary purpose of scoring body condition is to assess or estimate the dietary energy status and internal fat by an external body condition evaluation. Research has reported positive correlations between BCS and the internal fat depth over the ribs and lumbar vertebrae. The basic scoring system, which ranks cows from 1 (very thin) to 5 (very fat), has little value unless the data is used to make feeding and management changes. Scoring cows 4 to 5 times during the lactation cycle is recommended. Recommended times to score cows are at dry-off, at calving, twice during the first 2 to 3 months of lactation, and at least once during the latter part of lactation.

The ideal BCS at calving is about 3.25. This has been shown on research studies and from field experiences in the USA. Cows greater than 3.5 at calving time are at higher risk for increased incidence of metabolic problems, have lower feed intake post calving, and a lower milk yield. Cows below 3.0 at calving have less body flesh to mobilize, which often results in lower peak milk yield and less milk during the total lactation.

Pasture and Body Condition

In general, cows on pasture have lower BCS than cows managed under a non-grazing environment. We recently completed a study at Penn State University comparing high producing cows managed under a confinement system and fed a total mixed ration (TMR) versus a system of pasture plus concentrate with the concentrate fed at a rate of 1 lb/4 lb of milk. At the start of this experiment and the grazing season, the 45 cows on the study averaged 110 days in milk and 97 lb of milk. During the 21-week study, cows fed the TMR averaged 83.8 lb milk; whereas cows on the pasture system averaged 62.7 lb of milk (see Table 1). Cows fed a TMR ended the study at a BCS of 3.05, whereas cows on pasture plus concentrate lost BCS, and finished the trial at 2.61 or a difference of nearly 0.5 in BCS. Another group of cows was fed pasture and were supplemented with a “partial” TMR, and averaged 70.4 lb milk (Table 1). Even with adequate amounts of high quality pasture and quite high amounts of concentrate, high producing cows fed pasture and concentrate still lost BCS.

Table 1. Milk production and body condition of dairy cows with three different feeding systems.a
Item Pasture + Concentrate Pasture + TMR TMR
a Penn State research (unpublished, 2000).
Milk/cow/day, lb 62.7 70.4 83.8
Milk fat, % 3.13 3.35 3.30
Milk true protein, % 2.82 2.95 2.99
Body Condition Score (1 to 5)


Initial 2.81 2.89 2.86
Final 2.61 2.90 3.05
Change -0.20 +0.01 +0.19

One of the most frequent comments we hear from graziers is that cows are losing BCS, or are not gaining body flesh during the lactation. These are the situations where we are concerned about breeding efficiency and subsequent lactational performance during the next lactation if body condition is not restored in late lactation.

Restoring Body Condition

One of the goals in nutrition is to provide adequate dietary energy during mid- and late-lactation to restore body condition that was mobilized during early lactation. The body flesh or fat is a “deposit in the bank,” and the cow “withdraws from the bank” during the next lactation to produce milk. Research has shown the following relationships:

  • The loss of one BCS in early lactation provides adequate energy to produce about 1,200 lb of milk.
  • To “deposit” one BCS during the lactation requires the energy from about 600 lb of concentrates above the amount fed to produce milk.

What happens if adequate BCS is not restored during the lactation? Let’s compare a cow with an ideal BCS of 3.25 versus a cow with a 2.75 BCS at calving. The cow with a higher body condition will have body reserves to mobilize to produce about 600 lb more milk during the subsequent lactation. To restore her to a BCS of 3.25 vs. 2.75 requires the feeding of about 275 lb more supplemental concentrate. This 275 lb of supplement, which costs about $17 ($.06/lb) returns 600 lb more milk during the next lactation valued at $78 ($.13/lb), or a $61 higher return per cow per lactation compared to the cow that was fed inadequate supplement to restore body condition.

This example shows that with the efficiencies of energy deposition into body condition and the mobilization during the next lactation, the investment in extra supplemental concentrate during the latter half of lactation pays a good dollar return in milk during the subsequent lactation. In addition, the cows with a higher BCS at calving experience less body condition loss in early lactation and will be expected to have better reproductive performance, an additional dollar return.

Body Condition And Reproduction

The relationship of actual BCS and the loss of body condition to reproductive performance have been studied extensively. Declining energy balance and the accompanying loss of BCS is a major factor in the inhibition of ovarian activity during the first several weeks of lactation. Cows that lose greater than 1.0 BCS during the first 30 days of lactation take 15 days longer to first ovulation postpartum than cows that lose 0.5 BCS. The number of ovulatory cycles preceding first ovulation has in turn been shown to positively influence subsequent conception rate. In nearly all published studies, each 0.5 unit loss in BCS between calving and insemination has resulted in a decline in conception rate of 10 to 15%. Thus, cows that lose greater than 1.0 BCS tend to have lower conception rates. In addition, the cows that lose more body condition have lower lactation peaks, poor lactation persistency, and milk protein content.

Most of the relationships between body condition loss and reproduction performance are based on research and field experiences with confinement systems. There is no extensive research data base with pastured dairy cows, but observations and dairy farmer records suggest that these relationships exist with grazing dairy cows.

Summary

Clearly, how you manage the cows’ energy status during late lactation and the early part of the subsequent lactation is critical to their health and reproduction, as well as to the economics of the dairy farm. With the often lower BCS or greater loss of BCS during the first 1 to 2 months of lactation with a pasture based system, proper concentrate supplementation during the lactation to restore body condition clearly has long term economic benefits in addition to the immediate benefits of higher milk output. However, the milk:feed price ratio needs to be considered when deciding the amount of concentrates to feed.

In addition, the nutritional and management strategies during the dry and transition period also influence the body condition and energy balance during the subsequent lactation.

Published as pages 79-81 in proceedings from "Nutrition of Dairy Cows on Pasture-Based Systems" held March 31, 2003 in Grantville, PA.

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Pasture, Body Condition, and Reproduction

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Lawrence D. Muller
  • Professor Emeritus