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Replacement Heifers: Impacting Fertility Through Nutrition

It may seem odd to have a discussion about replacement heifer nutrition during preg-check season but it is something I think about every time I palpate an open heifer, and especially if open rates start creeping upwards of 10% (the Western Canadian average). Although genetics play an important role in fertility and age at puberty (think scrotal circumference in bulls) we can certainly influence heifer fertility through nutrition.

Heifer development consists largely of choosing the right heifers and managing them nutritionally to reach puberty early.

  • Higher weaning weights generally correlated with early puberty [born earlier, better ADG (average daily gain), and faster maturing]
  • Target 55-60% mature body weight at breeding. Although some heifers begin cycling at lower body weights, this target ensures most heifers will be cycling at breeding
  • First service conception rate is best in heifers that have cycled 2-3 times prior to the initiation of breeding season

The influence of nutrition on heifer reproduction is thought to begin long before the calf is even born and those effects persist throughout her life. Recent work in fetal programming suggest that dam nutrition can impact the future health and reproductive capacity of the heifer. Like humans, heifer calves are born with all of the follicles they will ever ovulate, and these develop in the first 60 days of gestation. Undernutrition of the dam during this time has been shown to reduce both the quality and quantity of follicles in the daughter. Nutrient restriction of the dam, especially in the 1st and 3rd trimester, has been shown to reduce pregnancy rates in daughters.

**Currently, it is considered not detrimental to restrict nutrients in a cows’ diet during the 2nd trimester as a way to reduce winter feeding costs, but as more fetal programming work is done, these recommendations may change.**

  • Milk production by the cow will have the largest influence on calf growth rate and weaning weight during the summer months
  • Creep feeding is an effective way to increase calf weight at weaning, but there is a risk of over-fattening heifers, which affects udder development and can limit future milk production
  • A low level implant such as Ralgro has been shown to be safe in heifers if given at greater than 30 days of age and prior to weaning. Outside of this window there is a LARGE risk that an implant will reduce fertility of replacement heifers

Post-weaning provides an opportune time for heifer development through nutrition. The pattern of growth during this time seems to have little impact on puberty, but a consistent, steady growth rate is best. It is a risky practice to hold heifers back to save money and depend on compensatory gain pre-breeding. We also do not want a ration so nutrient dense that nutrition is reduced when heifers move from dry lot to the pasture in the spring.

Energy

  • Target a moderate ADG. Heifers targeted to gain 1.5 lb/day had earlier puberty and better conception rate than lower gaining heifers
  • ADG > 1.5 lb/day can result in over-fat heifers
  • Economically, separating heifers into 2 different weight classes can save feed costs
  • The use of ionophores to improve Gain:Feed and growth rate can reduce age of puberty by 15-30 days. It also reduces intake and saves feed costs

Protein

  • Protein is the costliest aspect of any ration, and although growing heifers require more than mature cows (12-17 % Dry Matter vs 7-11 % Dry Matter), over supplementation can negatively affect reproduction by increasing blood nitrogen levels

Minerals

  • Copper deficiency can reduce fertility, as can molybdenum toxicity. Soils that are neutral or alkali pH and high moisture can increase molybdenum uptake by plants
  • Along with a balanced ration, a balanced mineral program is needed for proper growth and development of all calves

Post-weaning is where much of the cost of a replacement heifer is incurred (cost of winter feeding, yardage + opportunity cost of not selling her in the fall). In order to pay for herself, the heifer must get pregnant, and so investment in a nutrition program is paramount.

If you are interested in determining the cost of retaining replacement heifers, a useful tool is available at:  http://www.wbdc.sk.ca/pdfs/economics/Replacement_Calculator.xlsx

Written by: Dr. Brittany Wiese

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