Cows sired by proven AI sires produced 1400 kg more herd lifetime actual milk and were $148 more profitable when compared to daughters of non-AI sires. Despite economic advantages of AI over NS, NS remains popular in many dairy herds.
Reproductive performance studies do not show a clear advantage for either NS or AI. In herds with low PR related to poor estrous detection, TAI or NS are viable options.
Both breeding systems require strict attention to management compliance to optimize reproductive performance. Natural service breeding programs are expensive when both direct and indirect costs are considered.
Economic analysis in this study found TAI to be less expensive than NS, as well as allowing for immediate submission and a more rapid PR of all animals at the designated waiting period.
Natural service (NS) remains a component of many breeding programs, commonly used after unsuccessful AI attempts (Caraviello et al., 2006; Champagne et al., 2002; De Vries et al., 2005; NAHMS, 2002; Smith et al., 2004). Dairy producers using NS believe human errors in estrus detection are avoided when bulls are used, resulting in more cows are bred by NS than AI. Natural service breeding programs are thought to be cheaper and easier to manage than an AI program.
The ability of NS bulls to impregnate cows is influenced by nutritional, environmental and management factors. Bulls do not always maintain reproductive soundness during their use for NS, adding uncertainty to the reproductive program.
Pregnancy rate is the product of estrus detection and conception rate. A high PR at the end of the postpartum voluntary waiting period means more cows pregnant earlier in lactation, maximizing farm income (Risco et al., 1998). Poor estrus detection is a major factor that contributes to low PR on many dairies. The use of NS and timed artificial insemination (TAI; cows are AI at a fixed time without being detected in estrus) are two options to minimize problems with estrus detection. This paper compares reproductive performance between TAI and NS breeding systems.
NS vs. AI at detected estrus
Seasonal fertility effects in dairy herds was evaluated for AI and NS under field conditions using Dairy Comp 305® (Valley Agricultural Software, Tulare, CA) (Niles et al., 2002). Heat stress reduced PR for cows bred by either AI or NS; no difference in PR was found between NS and AI bred cows during the cool season. Herds with poor estrous detection achieved higher PR with NS (Niles et al., 2002). Herds classified by percentage of NS usage ( BS1) 0 %, BS2) 1 to 20 %, BS3) 21 to 89 %, and BS4) 90 to 100 %) were evaluated for reproductive performance (Smith et al., 2004). Calving interval was shorter in herds using mainly NS (BS4). However, herds using a combination of AI and NS or mostly NS had longer dry periods than herds using all AI. Days dry and the percentage of dry periods greater than 60 d were less for herds that used all AI breeding. Overall efficiency, percentage of cows in milk and herd milk yield, was greatest for all AI herds, declining as percentage of NS increased. Over an 8 year time period, the use of NS bulls did not provide meaningful advantages or disadvantages in PR (De Vries et al., 2005). In contrast to the previously cited studies, a California study found that AI cows had higher pregnancy rate across all days in milk (DIM) (Overton and Sischo, 2005).
NS vs. timed AI: a field observation
Timed AI (TAI) and resynchronized TAI were used to improve herd PR in a large commercial dairy herd extensively utilizing NS (Thatcher et al., 2006). The 2000 cow herd experienced approximately 1200 calvings per year. Bulls used for NS underwent a breeding soundness exam and entered the NS program only if classified as satisfactory (Chenoweth, 1992). Breeding soundness exams were repeated every 6 months and bulls that graded unsatisfactory were replaced. Pens contained one bull for every twenty cows; pen bull:cow ratios were adjusted monthly based on non-pregnant cow numbers. Bulls were rested for 14 d after 14 d of cow exposure. Cows more than 127 dim and diagnosed non-pregnant were entered in a timed AI program.
245 cows averaging 356 DIM were enrolled in a timed AI program from January 11 to June 21, 2006. Timed AI was a presynchronization ( 2 injections of PGF2α 14 d apart), followed by Ovsynch (GnRH 14 d after the 2nd PGF2α injection of presynchronization, followed by PGF2α 7 d after the first GnRH injection, then a second GnRH injection 2 d after PGF2α with a timed insemination between 16 to 20 h after the second GnRH), and a resynchronized timed AI with the Ovsynch protocol repeated in cows diagnosed open by ulstrasound at 32 d after AI. Resynchronization was repeated twice, resulting in three possible timed AI. Cumulative PR based on ultrasound diagnosis at 32 d was 56 % for the first three TAIs. At 60 d after insemination, 37.8 % of inseminated cows were diagnosed pregnant via rectal palpation. Pregnancy losses between d 32 and 60 were substantial at 32.5%, showing this group was indeed sub-fertile. Nevertheless, 37.8 % of cows not pregnant through NS conceived with timed AI. This field experience found that timed AI resulted in 38 % of sub-fertile cows becoming pregnant.
Direct comparison of timed AI vs. NS
Reproductive performance was recently compared between two different breeding systems without estrous detection; TAI and NS (Lima et al., unpubl.). One thousand fifty five lactating Holstein dairy cows from a single farm in north central Florida were randomized at 42 ± 3 d postpartum into two groups, TAI (n=543) and NS (n = 512). Cows were blocked by parity (primiparous and multiparous). TAI utilized the Ovsync protocol.
Cows in the NS group received PGF2α (25 mg; Estroplan®, Pfizer Animal Health, New York, NY) at d 42 ± 3 and 56 ± 3 and moved to a bull pen at 70 ± 3 d postpartum. Movement of cows into the bull pen 14 d after the last PGF2α treatment (70 ± 3 d postpartum) was performed to synchronize estrus and bull breeding close to 80 d postpartum, similar to the TAI group.
Cows diagnosed pregnant were re-confirmed 28 d later to identify pregnancy loss. The BCR in the NS herds was one bull per twenty open cows. Bulls were rested for 14 d after 14 d of cow exposure. All cows underwent a body condition score evaluation (BCS) at 70 + 3 d postpartum prior to being introduced with bulls (NS group) or receiving the GnRH injection (TAI group).
Results from this study agree with previous work (Niles et al., 2002) that overall PR drops for both AI and NS during periods of heat stress (summer). Median times to conception were 109 d (95 % CI = 104 to 125) for TAI and 116 d (95 % CI = 115 to 117) for NS. There was no difference between the treatment (NS vs. TAI) conception curves. However, the rate when cows became pregnant during the first cycle at the end of the VWP between NS and TAI was different. For NS cows, PR to the first service cycle 21 days after bull exposure (70 to 91 DIM), was 33.0 % (cool and warm season) representing a PR of 1.57 cows / day. Conversely, for the TAI group, with a first service PR of 37% (cool and warm season) during the 10 days of the OvSynch and TAI protocol (70+3 to 80+ 3 DIM), the PR was 3.7 cows per day. One quarter of all pregnant cows conceived for NS at 84 DIM (95% CI=83to 86) and 81 days for TAI (95% CI = 80 to 82).
We attribute this finding to the TAI management and not necessarily better fertility from TAI. In the NS group, pregnancy is dependent on the ability of the bull to identify cows in heat, breed, and impregnate them on a daily basis. When compared to NS, more cows are synchronized to be bred at a given acute service period in the TAI group. For the TAI group, it took five services for cows to become pregnant up to 223 DIM. In contrast, the NS group cows had more opportunities to be bred (at least 7 services) due to daily bull exposure and cows recycling every 21 to 23 days.
Body condition score at first service and parity to overall PR affected pregnancy. Cows with a BCS < 2.75; had lower odds to conceive (AOR= 0.73; 95 % CI= 0.56 to 0.09; (PR=32.33%) compared to cows with a BCS > 2.75 that had PR=38.85%. Primiparous cows had greater odds to conceive (AOR = 1.91; 95 % CI = 1.32 to 2.88 , PR=87.32%) compared to multiparous cows(PR= 77.69%).
Critical to TAI programs is protocol compliance, semen handling and insemination technique. Pregnancy rates of 37 % to first TAI and 30 % to the re-synchronization (second service) of open cows indicate an acceptable response in a large commercial dairy implementing a TAI program for the first time. We attribute the good reproductive performance obtained in the NS group to stringent bull management practices employed: bull selection, periodic breeding soundness evaluation of all bulls, removal and replacement of unsound bulls, allowing for a two week cow exposure followed by a two week rest period, and BCR of 1:20.
Economic comparison NS vs. timed AI
An attempt was made to compare the costs and revenues of the NS program to the timed AI program in the direct comparison study. Labor costs and pregnancy rates in both programs were assumed equal based upon experimental results. The net cost of the NS program was $92.29per slot per year. For the timed AI program, the net cost was $51.61 per slot per year. Therefore, the NS program cost $40.68 per slot per year more, including an opportunity cost of $13.67 for less genetic progress. Without considering genetic progress, the NS program would cost $27.02 more per slot per year.
Overton (2005) calculated an extra cost of $10.27 per slot per year for a NS program compared to an AI program that included 30% timed AI in large western Holstein dairy farms. He also assumed equal pregnancy rates. Overton assumed that for every 2 NS bulls, 1 extra cow could enter the herd. Thus, his AI program allowed for more cows than the NS program. When the number of cows in both programs was assumed equal, the extra cost per slot per year for a NS program was reduced to $3.61. Differences in pregnancy rates could be easily incorporated into the partial budgets. An increase in pregnancy rate of 1 percentage point (e.g. 15% to 16%) is worth approximately $15 to $20 per slot per year with lower values at higher pregnancy rates (De Vries, 2007).
Cows sired by proven AI sires produced 1400 kg more herd lifetime actual milk and were $148 more profitable when compared to daughters of non-AI sires (Cassel et al., 2002). Despite economic advantages of AI over NS, NS remains popular in many dairy herds. Reproductive performance studies do not show a clear advantage for either NS or AI. In herds with low PR related to poor estrous detection, TAI or NS are viable options. Both breeding systems require strict attention to management compliance to optimize reproductive performance. Natural service breeding programs are expensive when both direct and indirect costs are considered. Economic analysis in this study found TAI to be less expensive than NS, as well as allowing for immediate submission and a more rapid PR of all animals at the designated waiting period.
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