Strategic Supplementation of Young Cows for Reproduction
By Rachel Endecott, Extension Beef Specialist, Montana State University
Maintaining a yearly calving interval is imperative for a beef cow to remain a profitable calf producer in the herd, and can be a demanding task for young range beef cows. Even with supplementation, young cows experience a period of negative energy balance and weight loss before and after calving, and their response to supplementation may vary from year to year. Poor reproductive performance of first–and second–calf cows is a challenge faced by cow-calf producers in the West.
Poorer–than–expected cow response to supplementation may be partially due to low availability of glucose (blood sugar) and impaired absorption of available glucose into tissues. Cows absorb little glucose from their diet and rely on their body to produce glucose from other precursors. If precursors are absent or in limited supply, glucose availability and absorption may be impaired, which might result in a shift in nutrient partitioning toward milk production at the expense of body weight gain and return to cyclicity.
Increasing the supply of glucose precursors in the diet may shift nutrients back toward postpartum weight gain and reproduction. Propionate, a volatile fatty acid, is a primary precursor for glucose production. We investigated responses of 2–and 3–year–old postpartum cows to different amounts of propionate salt added to range protein supplements.
Experimental Approach
Supplements were fed twice weekly at a rate of 2.5 lb per day for 70 days postpartum. Supplemental crude protein (CP) was approximately 50 percent ruminally degraded protein (RDP; degraded mostly in the rumen) and 50 percent ruminally undegraded protein (RUP; passes through the rumen relatively undegraded) and were fortified with minerals and vitamin A. All supplements (30 percent CP) were based on wheat middlings, cottonseed meal, and feather meal, with increasing proportions (0, 80, or 160 g/d) of propionate salt (NutroCAL™, Kemin Industries, Inc., abbreviated as RUP0, RUP80, and RUP160).
Outcomes and implications: What does this mean?
A supplement by year interaction was observed for days to first estrus (Table 1). Cows fed RUP0 and RUP160 took longer to return to estrus in 2004 than in 2003, while cows fed RUP80 returned to cyclicity in similar days postpartum regardless of year. Cows fed RUP80 were able to overcome year–to–year variation in this crucial reproductive measurement.
Table 1. Supplemental propionate and year interacted to affect days to first estrus of 2–and 3–year–old postpartum cows.
| Year | ||
|---|---|---|
| Supplement | 2003 | 2004 |
| RUP0 | 56 d | 78 d |
| RUP80 | 66 d | 65 d |
| RUP160 | 60 d | 7 d |
Milk production for cows fed RUP80 was lower than for the other two supplement groups, but did not impact calf weaning weight (Table 2).
Cows fed moderate amounts of glucose precursors partitioned nutrients away from milk production and exhibited more consistent return to estrus. Strategic supplementation with a combination of glucose precursors may be best suited to shift nutrient partitioning in young postpartum range cows grazing dormant forage, and may serve as a tool to enhance cow longevity and sustainability of extensively managed ranches.
Table 2. Production responses of 2–and 3–year–old postpartum cows fed supplements with increasing amounts of propionate salt as a glucose precursor.
| Supplement | |||
|---|---|---|---|
| Response | RUP0 | RUP80 | RUP160 |
| Fall Pregnancy Rate | 100% | 100% | 91% |
| Calving Interval | 367 days | 361 days | 369 days |
| Cow weight change —calving to breeding | −4 lb | 2 lb | −11 lb |
| Milk Production (60 days postpartum) | 18.3 lb | 15.9 lb | 19.2 lb |
| Calf weaning weight | 530 | 524 | 519 |
Are the benefits worth it?
Feed costs for the supplementation period of this study were $21.09, $30.27, and $39.89/cow for RUP0, RUP80, and RUP160. To compare benefits of supplementation strategies, results of a previous, similar experiment and the present study were combined (4 years of data). In all years, RUP80–fed cows produced less milk and cycled earlier. Both groups of cows had equivalent calf growth (2.3 lb/day of age) and estrous cycle fertility. A financial comparison (Table 3) was calculated to predict hypothetical results of two 100–cow herds fed either RUP0 or RUP80 for 70 days postpartum. Additional feed inputs for the year included free–choice mineral and prepartum supplement. RUP0 calves were assumed to be 205 days old at weaning, and all calves were valued at $1.00/lb at weaning. Even though feed costs for the year were higher for cows in the RUP80 group, their calves had potential to be heavier at weaning because cows fed RUP80 bred back sooner than cows fed RUP0. This resulted in an increase in income of $17.82/cow when RUP80 was compared to RUP0.
Table 3. Hypothetical financial comparison of two 100–cow herds fed either RUP0 or RUP80 supplements. Feeding RUP80 resulted in a predicted increase in income.
| Supplement | Pregnancy rate (%) | Days to first estrus | Predicted lb calf weaned/cow exposed | Yearly feed cost ($/cow) | Predicted income difference ($) |
|---|---|---|---|---|---|
| RUP0 | 91.4 | 98 | 431 | $32.41 | — |
| RUP80 | 93.1 | 89 | 458 | $41.59 | $17.82 |